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1 LT3430 sn3430 3430is high voltage, 3a, 200khz step-down switching regulator august 2002 n wide input range: 5.5v to 60v n 3a peak switch current n small thermally enhanced 16-pin tssop package n constant 200khz switching frequency n saturating switch design: 0.1 w n peak switch current maintained over full duty cycle range n effective supply current: 2.5ma n shutdown current: 30 m a n 1.2v feedback reference voltage n easily synchronizable n cycle-by-cycle current limiting the lt ? 3430 is a 200khz monolithic buck switching regu- lator that accepts input voltages up to 60v. a high efficiency 3a, 0.1 w switch is included on the die along with all the nec- essary oscillator, control and logic circuitry. a current mode architecture provides fast transient response and excellent loop stability. special design techniques and a new high voltage process achieve high efficiency over a wide input range. efficiency is maintained over a wide output current range by using the output to bias the circuitry and by utilizing a supply boost capacitor to saturate the power switch. patented circuitry maintains peak switch current over the full duty cycle range. a shutdown pin reduces supply current to 30 m a and a sync pin can be externally synchronized from 228khz to 700khz with a logic level input. the LT3430 is available in a thermally enhanced 16-pin tssop package. n industrial and automotive power supplies n portable computers n battery chargers n distributed power systems , ltc and lt are registered trademarks of linear technology corporation. 5v, 2a buck converter efficiency vs load current features descriptio u applicatio s u typical applicatio u boost v in 6 2, 5 10 12 30bq060 15.4k v out 5v 2a 3, 4 15 14 11 220pf 0.022 f * ? for input voltages below 7.5v, some restrictions may apply united chemi-con 1, 8, 9, 16 LT3430 shdn sync sw bias fb v c gnd 0.68 f 100 f 10v solid tantalum 22 h mmsd914ti 4.99k 3430 ta01 4.7 f 100v ceramic ? on off v in 5.5v* to 60v + 3.3k load current (a) 0 efficiency (%) 80 90 100 2.0 3430 ta02 70 60 50 0.5 1.0 1.5 2.5 v in = 12v v in = 42v v out = 5v l = 27 h information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. final electrical specifications
2 LT3430 sn3430 3430is (note 1) input voltage (v in ) ................................................. 60v boost pin above sw ............................................ 35v boost pin voltage ................................................. 68v sync voltage ........................................................... 7v shdn voltage ........................................................... 6v bias pin voltage .................................................... 30v fb pin voltage/current .................................. 3.5v/2ma operating junction temperature range LT3430efe (note 8) ......................... C 40 c to 125 c LT3430ife (note 8) ......................... C 40 c to 125 c storage temperature range ................ C 65 c to 150 c lead temperature (soldering, 10 sec)................. 300 c order part number LT3430efe LT3430ife parameter conditions min typ max units reference voltage (v ref )v ol + 0.2 v c v oh C 0.2 1.204 1.219 1.234 v 5.5v v in 60v l 1.195 1.243 fb input bias current l C 0.2 C 1.5 m a error amp voltage gain (note 2) 200 400 v/v error amp g m dl (v c ) = 10 m a 1650 2200 3300 m mho l 1000 4200 m mho v c to switch g m 3.4 a/v ea source current fb = 1v l 125 225 450 m a ea sink current fb = 1.4v l 100 225 500 m a v c switching threshold duty cycle = 0 0.9 v v c high clamp shdn = 1v 2.1 v switch current limit v c open, boost = v in + 5v, fb = 1v C40 c t j 25 c 3 4.5 6 a t j = 125 c (note 9) 2.5 3.5 6 a switch on resistance i sw = 3a, boost = v in + 5v (note 7) 0.1 0.14 w l 0.18 w maximum switch duty cycle fb = 1v 93 96 % l 90 % switch frequency v c set to give dc = 50% 184 200 216 khz l 172 200 228 khz f sw line regulation 5.5v v in 60v l 0.05 0.15 %/v f sw shifting threshold df = 10khz 0.8 v the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t j = 25 c. v in = 15v, v c = 1.5v, shdn = 1v, boost = open circuit, sw = open circuit, unless otherwise noted. t jmax = 125 c, q ja = 45 c/ w, q jc = 10 c/ w exposed pad must be soldered to ground plane absolute axi u rati gs w ww u package/order i for atio uu w electrical characteristics fe package 16-lead plastic tssop 1 2 3 4 5 6 7 8 top view 16 15 14 13 12 11 10 9 gnd sw v in v in sw boost nc gnd gnd shdn sync nc fb v c bias gnd consult ltc marketing for parts specified with wider operating temperature ranges.
3 LT3430 sn3430 3430is parameter conditions min typ max units minimum input voltage (note 3) l 4.6 5.5 v minimum boost voltage (note 4) i sw 3a l 1.8 3 v boost current (note 5) boost = v in + 5v, i sw = 1a l 25 50 ma boost = v in + 5v, i sw = 3a l 75 120 ma input supply current (i vin ) (note 6) v bias = 5v 1.5 2.2 ma bias supply current (i bias ) (note 6) v bias = 5v 3.1 4.2 ma shutdown supply current shdn = 0v, v in 60v, sw = 0v, v c open 35 100 m a l 200 m a lockout threshold v c open C40 c t j < 25 c 2.2 2.48 v 25 c t j 125 c 2.3 2.38 2.48 v shutdown thresholds v c open, shutting down l 0.15 0.37 0.58 v v c open, starting up l 0.25 0.42 0.60 v minimum sync amplitude 1.5 v sync frequency range 228 700 khz sync input resistance 20 k w the l denotes specifications which apply over the full operating temperature range, otherwise specifications are at t j = 25 c. v in = 15v, v c = 1.5v, shdn = 1v, boost = open circuit, sw = open circuit, unless otherwise noted. electrical characteristics note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: gain is measured with a v c swing equal to 200mv above the low clamp level to 200mv below the upper clamp level. note 3: minimum input voltage is not measured directly, but is guaranteed by other tests. it is defined as the voltage where internal bias lines are still regulated so that the reference voltage and oscillator remain constant. actual minimum input voltage to maintain a regulated output will depend upon output voltage and load current. see applications information. note 4: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch. note 5: boost current is the current flowing into the boost pin with the pin held 5v above input voltage. it flows only during switch on time. note 6: input supply current is the quiescent current drawn by the input pin when the bias pin is held at 5v with switching disabled. bias supply current is the current drawn by the bias pin when the bias pin is held at 5v. total input referred supply current is calculated by summing input supply current (i vin ) with a fraction of bias supply current (i bias ): i total = i vin + (i bias )(v out /v in ) with v in = 15v, v out = 5v, i vin = 1.4ma, i bias = 2.9ma, i total = 2.4ma. note 7: switch on resistance is calculated by dividing v in to sw voltage by the forced current (3a). see typical performance characteristics for the graph of switch voltage at other currents. note 8: the LT3430efe is guaranteed to meet performance specifications from 0 c to 125 c junction temperature. specifications over the C40 c to 125 c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the LT3430ife is guaranteed over the full C40 c to 125 c operating junction temperature range. note 9: see peak switch current limit vs junction temperature graph in the typical performance characteristics section.
4 LT3430 sn3430 3430is typical perfor a ce characteristics uw switch peak current limit duty cycle (%) switch peak current (a) 6 5 4 3 2 20 40 60 80 3430 g01 100 0 guaranteed minimum t j = 25 c typical lockout and shutdown thresholds fb pin voltage and current shdn pin bias current junction temperature ( c) ?0 feedback voltage (v) current ( a) 1.224 1.229 1.234 25 75 3430 g02 1.219 1.214 ?5 0 50 100 125 1.209 1.204 1.5 2.0 1.0 0.5 0 voltage current junction temperature ( c) ?0 250 200 150 100 12 6 0 25 75 3430 g03 ?5 0 50 100 125 current ( m a) current required to force shutdown (flows out of pin). after shutdown, current drops to a few m a at 2.38v standby threshold (current flows out of pin) junction temperature ( c) ?0 shdn pin voltage (v) 50 100 3430 g04 0 25 75 2.4 2.0 1.6 1.2 0.8 0.4 0 25 125 lockout start-up shutdown shutdown supply current shutdown supply current error amplifier transconductance input voltage (v) 0 input supply current ( a) 3430 g05 10 20 30 40 50 60 40 35 30 25 20 15 10 5 0 v shdn = 0v shutdown voltage (v) 0 0 input supply current ( a) 50 100 150 200 250 300 0.1 0.2 0.3 0.4 3430 g06 0.5 v in = 60v v in = 15v junction temperature ( c) transconductance ( mho) 3430 g07 2500 2000 1500 1000 500 0 ?0 50 100 0 25 75 25 125 error amplifier transconductance frequency foldback frequency (hz) gain ( m mho) phase (deg) 3000 2500 2000 1500 1000 500 200 150 100 50 0 ?0 100 10k 100k 10m 3430 g08 1k 1m gain phase error amplifier equivalent circuit r out 200k c out 12pf v c r load = 50 w v fb 2 10 ? ) ( v fb (v) 0 switiching frequency (khz) or fb current ( a) 300 400 600 500 3430 g09 200 100 0 0.5 1.0 1.5 switching frequency fb pin current
5 LT3430 sn3430 3430is typical perfor a ce characteristics uw minimum input voltage with 5v output junction temperature ( c) ?0 frequency (khz) 50 100 3430 g10 0 25 75 230 220 210 200 190 180 170 25 125 load current (a) 0 input voltage (v) 3430 g11 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 7.5 7.0 6.5 6.0 5.5 5.0 minimum input voltage to start minimum input voltage to run switch current (a) 0123 boost pin current (ma) 3430 g12 90 80 70 60 50 40 30 20 10 0 switching frequency boost pin current v c pin shutdown threshold junction temperature ( c) ?0 1.5 1.7 2.1 25 75 3430 g13 1.3 1.1 ?5 0 50 100 125 0.9 0.7 1.9 threshold voltage (v) switch voltage drop switch current (a) 0123 switch voltage (mv) 3430 g14 450 400 350 300 250 200 150 100 50 0 t j = 125 c t j = 25 c t j = 40 c junction temperature ( c) ?0 switch minimum on time (ns) 400 500 600 25 75 3430 g15 300 200 ?5 0 50 100 125 100 0 switch minimum on time vs temperature junction temperature ( c) ?0 switch peak current limit (a) 50 100 3430 g16 0 25 75 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 25 125 switch peak current limit
6 LT3430 sn3430 3430is uu u pi fu ctio s gnd (pins 1, 8, 9, 16): the gnd pin connections act as the reference for the regulated output, so load regulation will suffer if the ground end of the load is not at the same voltage as the gnd pins of the ic. this condition will occur when load current or other currents flow through metal paths between the gnd pins and the load ground. keep the paths between the gnd pins and the load ground short and use a ground plane when possible. the fe package has an exposed pad that is fused to the gnd pins. the pad should be soldered to the copper ground plane under the device to reduce thermal resistance. (see applications informationlayout considerations.) sw (pins 2, 5): the switch pin is the emitter of the on-chip power npn switch. this pin is driven up to the input pin voltage during switch on time. inductor current drives the switch pin voltage negative during switch off time. nega- tive voltage is clamped with the external catch diode. maximum negative switch voltage allowed is C 0.8v. v in (pins 3, 4): this is the collector of the on-chip power npn switch. v in powers the internal control circuitry when a voltage on the bias pin is not present. high di/dt edges occur on this pin during switch turn on and off. keep the path short from the v in pin through the input bypass capacitor, through the catch diode back to sw. all trace inductance in this path creates voltage spikes at switch off, adding to the v ce voltage across the internal npn. boost (pin 6): the boost pin is used to provide a drive voltage, higher than the input voltage, to the internal bipo- lar npn power switch. without this added voltage, the typical switch voltage loss would be about 1.5v. the ad- ditional boost voltage allows the switch to saturate and voltage loss approximates that of a 0.1 w fet structure. nc (pins 7, 13): no connection. b ias (pin 10): the bias pin is used to improve efficiency when operating at higher input voltages and light load current. connecting this pin to the regulated output volt- age forces most of the internal circuitry to draw its oper- ating current from the output voltage rather than the input supply. this architecture increases efficiency especially when the input voltage is much higher than the output. minimum output voltage setting for this mode of operation is 3v. v c (pin 11) the v c pin is the output of the error amplifier and the input of the peak switch current comparator. it is normally used for frequency compensation, but can also serve as a current clamp or control loop override. v c sits at about 0.9v for light loads and 2.1v at maximum load. it can be driven to ground to shut off the regulator, but if driven high, current must be limited to 4ma. fb (pin 12): the feedback pin is used to set the output voltage using an external voltage divider that generates 1.22v at the pin for the desired output voltage. three additional functions are performed by the fb pin. when the pin voltage drops below 0.6v, switch current limit is reduced and the external sync function is disabled. below 0.8v, switching frequency is also reduced. see feedback pin functions in applications information for details. sync (pin 14): the sync pin is used to synchronize the internal oscillator to an external signal. it is directly logic compatible and can be driven with any signal between 10% and 90% duty cycle. the synchronizing range is equal to initial operating frequency up to 700khz. see synchronizing in applications information for details. shdn (pin 15): the shdn pin is used to turn off the regulator and to reduce input drain current to a few microamperes. this pin has two thresholds: one at 2.38v to disable switching and a second at 0.4v to force com- plete micropower shutdown. the 2.38v threshold func- tions as an accurate undervoltage lockout (uvlo); some- times used to prevent the regulator from delivering power until the input voltage has reached a predetermined level. if the shdn pin functions are not required, the pin can either be left open (to allow an internal bias current to lift the pin to a default high state) or be forced high to a level not to exceed 6v.
7 LT3430 sn3430 3430is block diagra w the LT3430 is a constant frequency, current mode buck converter. this means that there is an internal clock and two feedback loops that control the duty cycle of the power switch. in addition to the normal error amplifier, there is a current sense amplifier that monitors switch current on a cycle-by-cycle basis. a switch cycle starts with an oscilla- tor pulse which sets the r s flip-flop to turn the switch on. when switch current reaches a level set by the inverting input of the comparator, the flip-flop is reset and the switch turns off. output voltage control is obtained by using the output of the error amplifier to set the switch current trip point. this technique means that the error amplifier commands current to be delivered to the output rather than voltage. a voltage fed system will have low phase shift up to the resonant frequency of the inductor and output capacitor, then an abrupt 180 shift will occur. the current fed system will have 90 phase shift at a much lower frequency, but will not have the additional 90 shift until well beyond the lc resonant frequency. this makes it much easier to frequency compensate the feedback loop and also gives much quicker transient response. most of the circuitry of the LT3430 operates from an internal 2.9v bias line. the bias regulator normally draws power from the regulator input pin, but if the bias pin is connected to an external voltage equal to or higher than 3v, bias power will be drawn from the external source (typically the regulated output voltage). this will improve efficiency if the bias pin voltage is lower than regulator input voltage. high switch efficiency is attained by using the boost pin to provide a voltage to the switch driver which is higher than the input voltage, allowing switch to be saturated. this boosted voltage is generated with an external capacitor and diode. two comparators are connected to the shutdown pin. one has a 2.38v threshold for under- voltage lockout and the second has a 0.4v threshold for complete shutdown. figure 1. LT3430 block diagram + + + + s v in 3, 4 2.9v bias regulator 200khz oscillator frequency foldback 2, 5 fb sw gnd 1, 8, 9, 16 1766 f01 slope comp antislope comp bias internal v cc sync 0.4v 5.5 a current comparator r limit r sense error amplifier g m = 2000 m mho q2 foldback current limit clamp boost r s flip-flop driver circuitry s r q1 power switch 1.22v 10 14 shdn 15 6 12 11 v c lockout comparator shutdown comparator 2.38v 1 q3 v c(max) clamp
8 LT3430 sn3430 3430is applicatio s i for atio wu uu feedback pin functions the feedback (fb) pin on the LT3430 is used to set output voltage and provide several overload protection features. the first part of this section deals with selecting resistors to set output voltage and the second part talks about foldback frequency and current limiting created by the fb pin. please read both parts before committing to a final design. the suggested value for the output divider resistor (see figure 2) from fb to ground (r2) is 5k or less, and a formula for r1 is shown below. the output voltage error caused by ignoring the input bias current on the fb pin is less than 0.25% with r2 = 5k. a table of standard 1% values is shown in table 1 for common output voltages. please read the following if divider resistors are increased above the suggested values. r rv out 1 2122 122 = - () . . table 1 output r1 % error at output voltage r2 (nearest 1%) due to discreet 1% (v) (k w )(k w ) resistor steps 3 4.99 7.32 + 0.32 3.3 4.99 8.45 C 0.43 5 4.99 15.4 C 0.30 6 4.75 18.7 + 0.38 8 4.47 24.9 + 0.20 10 4.32 30.9 C 0.54 12 4.12 36.5 + 0.24 15 4.12 46.4 C 0.27 more than just voltage feedback the feedback pin is used for more than just output voltage sensing. it also reduces switching frequency and current limit when output voltage is very low (see the frequency foldback graph in typical performance characteristics). this is done to control power dissipation in both the ic and in the external diode and inductor during short-circuit conditions. a shorted output requires the switching regu- lator to operate at very low duty cycles, and the average current through the diode and inductor is equal to the short-circuit current limit of the switch (typically 4a for the LT3430, folding back to less than 2a). minimum switch on time limitations would prevent the switcher from attaining a sufficiently low duty cycle if switching frequency were maintained at 200khz, so frequency is reduced by about 5:1 when the feedback pin voltage drops below 0.8v (see frequency foldback graph). this does not affect operation with normal load conditions; one simply sees a gear shift in switching frequency during start-up as the output voltage rises. in addition to lower switching frequency, the LT3430 also operates at lower switch current limit when the feedback pin voltage drops below 0.6v. q2 in figure 2 performs this function by clamping the v c pin to a voltage less than its normal 2.1v upper clamp level. this foldback current limit greatly reduces power dissipation in the ic, diode and in- ductor during short-circuit conditions. external synchro- nization is also disabled to prevent interference with fold- back operation. again, it is nearly transparent to the user under normal load conditions. the only loads that may be affected are current source loads which maintain full load current with output voltage less than 50% of final value. in these rare situations the feedback pin can be clamped above 0.6v with an external diode to defeat foldback current limit. caution: clamping the feedback pin means that frequency shifting will also be defeated, so a combination of high in- put voltage and dead shorted output may cause the LT3430 to lose control of current limit. the internal circuitry which forces reduced switching frequency also causes current to flow out of the feedback pin when output voltage is low. the equivalent circuitry is shown in figure 2. q1 is completely off during normal operation. if the fb pin falls below 0.8v, q1 begins to conduct current and reduces frequency at the rate of approximately 1.4khz/ m a. to ensure adequate frequency foldback (under worst-case short-circuit conditions), the external divider thevinin resistance must be low enough to pull 115 m a out of the fb pin with 0.44v on the pin (r div 3.8k). the net result is that reductions in frequency and current limit are affected by output voltage divider imped- ance. although divider impedance is not critical, caution should be used if resistors are increased beyond the suggested values and short-circuit conditions will occur
9 LT3430 sn3430 3430is with high input voltage. high frequency pickup will in- crease and the protection accorded by frequency and current foldback will decrease. choosing the inductor for most applications, the output inductor will fall into the range of 5 m h to 47 m h. lower values are chosen to reduce physical size of the inductor. higher values allow more output current because they reduce peak current seen by the LT3430 switch, which has a 3a limit. higher values also reduce output ripple voltage. when choosing an inductor you will need to consider output ripple voltage, maximum load current, peak induc- tor current and fault current in the inductor. in addition, other factors such as core and copper losses, allowable component height, emi, saturation and cost should also be considered. the following procedure is suggested as a way of handling these somewhat complicated and con- flicting requirements. output ripple voltage figure 3 shows a comparison of output ripple voltage for the LT3430 using either a tantalum or ceramic output capacitor. it can be seen from figure 3 that output ripple voltage can be significantly reduced by using the ceramic output capacitor; the significant decrease in output ripple voltage is due to the very low esr of ceramic capacitors. + 1.2v buffer v sw l1 v c gnd to sync circuit 3430 f02 to frequency shifting r3 1k r4 2k r1 c1 r2 output 5v error amplifier fb 1.4v q1 LT3430 q2 + figure 2. frequency and current limit foldback applicatio s i for atio wu uu output ripple voltage is determined by ripple current (i lp-p ) through the inductor and the high frequency impedance of the output capacitor. at high frequencies, the impedance of the tantalum capacitor is dominated by its effective series resistance (esr). tantalum output capacitor the typical method for reducing output ripple voltage when using a tantalum output capacitor is to increase the inductor value (to reduce the ripple current in the induc- tor). the following equations will help in choosing the required inductor value to achieve a desirable output ripple voltage level. if output ripple voltage is of less figure 3. LT3430 output ripple voltage waveforms. ceramic vs tantalum output capacitors 20mv/div 20mv/div v out using 100 m f ceramic output capacitor v out using 100 m f, 0.08 w tantalum output capacitor v in = 40v 2 m s/div 3430 f03 v out = 5v l = 22 m h
10 LT3430 sn3430 3430is applicatio s i for atio wu uu importance, the subsequent suggestions in peak induc- tor and fault current and emi will additionally help in the selection of the inductor value. peak-to-peak output ripple voltage is the sum of a triwave (created by peak-to-peak ripple current (i lp-p ) times esr) and a square wave (created by parasitic inductance (esl) and ripple current slew rate). capacitive reactance is assumed to be small compared to esr or esl. v i esr esl di dt ripple lp p = ()() + () - s where: esr = equivalent series resistance of the output capacitor esl = equivalent series inductance of the output capacitor di/dt = slew rate of inductor ripple current = v in /l peak-to-peak ripple current (i lp-p ) through the inductor and into the output capacitor is typically chosen to be between 20% and 40% of the maximum load current. it is approximated by: i vvv vfl lp p out in out in - = ()( ) ()()() example: with v in = 40v, v out = 5v, l = 22 m h, esr = 0.080 w and esl = 10nh, output ripple voltage can be approximated as follows: ia di dt va mv ripple p- p p- p = () - () () ()() = == = ()() + ()() () =+= - - - 540 5 40 22 10 200 10 099 40 22 10 10 1 8 0 99 0 08 10 10 10 1 8 0 079 0 018 97 63 6 6 96 . . .. . .. s to reduce output ripple voltage further requires an in- crease in the inductor value with the trade-off being a physically larger inductor with the possibility of increased component height and cost. ceramic output capacitor an alternative way to further reduce output ripple voltage is to reduce the esr of the output capacitor by using a ceramic capacitor. although this reduction of esr re- moves a useful zero in the overall loop response, this zero can be replaced by inserting a resistor (r c ) in series with the v c pin and the compensation capacitor c c . (see ceramic capacitors in applications information.) peak inductor current and fault current to ensure that the inductor will not saturate, the peak inductor current should be calculated knowing the maxi- mum load current. an appropriate inductor should then be chosen. in addition, a decision should be made whether or not the inductor must withstand continuous fault conditions. if maximum load current is 1a, for instance, a 1a inductor may not survive a continuous 4a overload condition. dead shorts will actually be more gentle on the inductor because the LT3430 has frequency and current limit foldback. peak switch and inductor current can be significantly higher than output current, especially with smaller induc- table 2 vendor/ value i dc dcr height part no. ( m h) (amps) (ohms) (mm) sumida cdrh104r-150 15 3.6 0.050 4 cdrh104r-220 22 2.9 0.073 4 cdrh104r-330 33 2.3 0.093 4 cdrh124-220 22 2.9 0.066 4.5 cdrh124-330 33 2.7 0.097 4.5 cdrh127-330 33 3.0 0.065 8 cdrh127-470 47 2.5 0.100 8 cei122-220 22 2.3 0.085 3 coiltronics up3b-330 33 3 0.069 6.8 up3b-470 47 2.4 0.108 6.8 up4b-680 68 4.3 0.120 7.9 coilcraft do3316p-153 15 3 0.046 5.2 do5022p-683 68 3.5 0.130 7.1
11 LT3430 sn3430 3430is applicatio s i for atio wu uu tors and lighter loads, so dont omit this step. powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. other core mate- rials fall somewhere in between. the following formula assumes continuous mode of operation, but errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. ii i i vvv vfl peak out lp p out out in out in =+ =+ ()( ) ()( )()() - 22 emi decide if the design can tolerate an open core geometry like a rod or barrel, which have high magnetic field radiation, or whether it needs a closed core like a toroid to prevent emi problems. this is a tough decision because the rods or barrels are temptingly cheap and small and there are no helpful guidelines to calculate when the magnetic field radiation will be a problem. additional considerations after making an initial choice, consider additional factors such as core losses and second sourcing, etc. use the experts in linear technologys applications department if you feel uncertain about the final choice. they have experience with a wide range of inductor types and can tell you about the latest developments in low profile, surface mounting, etc. maximum output load current maximum load current for a buck converter is limited by the maximum switch current rating (i p ). the current rating for the LT3430 is 3a. unlike most current mode convert- ers, the LT3430 maximum switch current limit does not fall off at high duty cycles. most current mode converters suffer a drop off of peak switch current for duty cycles above 50%. this is due to the effects of slope compensa- tion required to prevent subharmonic oscillations in cur- rent mode converters. (for detailed analysis, see applica- tion note 19.) the LT3430 is able to maintain peak switch current limit over the full duty cycle range by using patented circuitry to cancel the effects of slope compensation on peak switch current without affecting the frequency compensation it provides. maximum load current would be equal to maximum switch current for an infinitely large inductor , but with finite inductor size, maximum load current is reduced by one-half peak-to-peak inductor current (i lp-p ). the follow- ing formula assumes continuous mode operation, imply- ing that the term on the right is less than one-half of i p . i out(max) = continuous mode i i 2 = i p lp-p p - + () - () ()()( ) vvvvv lfv out f in out f in 2 for v out = 5v, v in = 12v, v f(d1) = 0.52v, f = 200khz and l = 15 m h: i a out max () - =- + () - () ()() () =- = 3 5 0 52 12 5 0 52 2 15 10 200 10 12 30525 63 .. .. note that there is less load current available at the higher input voltage because inductor ripple current increases. at v in = 24v, duty cycle is 23% and for the same set of conditions: i a out max () .. .. =- + () - () ()() () =- = - 3 5 0 52 24 5 0 52 2 15 10 200 10 24 3 0 71 2 29 63 to cal- culate actual peak switch current with a given set of conditions, use: ii i vvvvv lfv sw peak out p out out f in out f in () =+ =+ +- () ()()( ) i 2 l-p () 2
12 LT3430 sn3430 3430is applicatio s i for atio wu uu reduced inductor value and discontinuous mode if the smallest inductor value is of most importance to a converter design, in order to reduce inductor size/cost, discontinuous mode may yield the smallest inductor solu- tion. the maximum output load current in discontinuous mode, however, must be calculated and is defined later in this section. discontinuous mode is entered when the output load current is less than one-half of the inductor ripple current (i lp-p ). in this mode, inductor current falls to zero before the next switch turn on (see figure 8). buck converters will be in discontinuous mode for output load current given by: i out discontinuous mode the inductor value in a buck converter is usually chosen large enough to keep inductor ripple current (i lp-p ) low; this is done to minimize output ripple voltage and maxi- mize output load current. in the case of large inductor values, as seen in the equation above, discontinuous mode will be associated with light loads. when choosing small inductor values, however, discon- tinuous mode will occur at much higher output load currents. the limit to the smallest inductor value that can be chosen is set by the LT3430 peak switch current (i p ) and the maximum output load current required, given by: i out(max) discontinuous mode example: for v in = 15v, v out = 5v, v f = 0.52v, f = 200khz and l = 4.7 m h. i out(max) discontinuous mode i out(max) = 1.21a discontinuous mode what has been shown here is that if high inductor ripple current and discontinuous mode operation can be toler- ated, small inductor values can be used. if a higher output load current is required, the inductor value must be increased. if i out(max) no longer meets the discontinuous mode criteria, use the i out(max) equation for continuous mode; the LT3430 is designed to operate well in both modes of operation, allowing a large range of inductor values to be used. short-circuit considerations the LT3430 is a current mode controller. it uses the v c node voltage as an input to a current comparator which turns off the output switch on a cycle-by-cycle basis as this peak current is reached. the internal clamp on the v c node, nominally 2v, then acts as an output switch peak current limit. this action becomes the switch current limit specification. the maximum available output power is then determined by the switch current limit. a potential controllability problem could occur under short-circuit conditions. if the power supply output is short circuited, the feedback amplifier responds to the low output voltage by raising the control voltage, v c , to its peak current limit value. ideally, the output switch would be turned on, and then turned off as its current exceeded the value indicated by v c . however, there is finite response time involved in both the current comparator and turnoff of the output switch. these result in a minimum on time t on(min) . when combined with the large ratio of v in to (v f + i ? r), the diode forward voltage plus inductor i ? r voltage drop, the potential exists for a loss of control. expressed mathematically the requirement to maintain control is: ft vir v on f in + where: f = switching frequency t on = switch minimum on time v f = diode forward voltage v in = input voltage i ? r = inductor i ? r voltage drop < + ()() ()( )()() vvvvv vfl out f in out f in 2 = = () + i iflv vvvvv p pin out f in out f 2 2 2 2 ()( ) ()() i lp-p = + - 3 200 10 4 7 10 15 25052155052 236 ( )( . )( ) (.)(.)
13 LT3430 sn3430 3430is applicatio s i for atio wu uu if this condition is not observed, the current will not be limited at i pk , but will cycle-by-cycle ratchet up to some higher value. using the nominal LT3430 clock frequency of 200khz, a v in of 40v and a (v f + i ? r) of say 0.7v, the maximum t on to maintain control would be approximately 90ns, an unacceptably short time. the solution to this dilemma is to slow down the oscilla- tor when the fb pin voltage is abnormally low thereby indicating some sort of short-circuit condition. oscillator fre quency is unaffected until fb voltage drops to about 2/3 of its normal value. below this point the oscillator fre- quency decreases roughly linearly down to a limit of about 40khz. this lower oscillator frequency during short-cir- cuit conditions can then maintain control with the effective minimum on time. it is recommended that for [v in /(v out + v f )] ratios > 10, a soft-start circuit should be used to control the output capacitor charge rate during start-up or during recovery from an output short circuit, thereby adding additional control over peak inductor current. see buck converter with adjustable soft-start later in this data sheet. output capacitor the output capacitor is normally chosen by its effective series resistance (esr), because this is what determines output ripple voltage. to get low esr takes volume , so physically smaller capacitors have high esr. the esr range for typical LT3430 applications is 0.05 w to 0.2 w . a typical output capacitor is an avx type tps, 100 m f at 10v, with a guaranteed esr less than 0.1 w . this is a d size surface mount solid tantalum capacitor. tps capacitors are specially constructed and tested for low esr, so they give the lowest esr for a given volume. the value in microfarads is not particularly critical, and values from 22 m f to greater than 500 m f work well, but you cannot cheat mother nature on esr. if you find a tiny 22 m f solid tantalum capacitor, it will have high esr, and output ripple voltage will be terrible. table 3 shows some typical solid tantalum surface mount capacitors. table 3. surface mount solid tantalum capacitor esr and ripple current e case size esr (max., w ) ripple current (a) avx tps, sprague 593d 0.1 to 0.3 0.7 to 1.1 d case size avx tps, sprague 593d 0.1 to 0.3 0.7 to 1.1 c case size avx tps 0.2 (typ) 0.5 (typ) many engineers have heard that solid tantalum capacitors are prone to failure if they undergo high surge currents. this is historically true, and type tps capacitors are specially tested for surge capability, but surge ruggedness is not a critical issue with the output capacitor. solid tantalum capacitors fail during very high turn-on surges, which do not occur at the output of regulators. high discharge surges, such as when the regulator output is dead shorted, do not harm the capacitors. unlike the input capacitor, rms ripple current in the output capacitor is normally low enough that ripple cur- rent rating is not an issue. the current waveform is triangular with a typical value of 250ma rms . the formula to calculate this is: output capacitor ripple current (rms): i vvv lfv ripple rms out in out in () = () - () ()()( ) 029 . ceramic capacitors higher value, lower cost ceramic capacitors are now becoming available. they are generally chosen for their good high frequency operation, small size and very low esr (effective series resistance). their low esr reduces output ripple voltage but also removes a useful zero in the loop frequency response, common to tantalum capaci- tors. to compensate for this, a resistor r c can be placed in series with the v c compensation capacitor c c . care must be taken however, since this resistor sets the high frequency gain of the error amplifier, including the gain at
14 LT3430 sn3430 3430is applicatio s i for atio wu uu the switching frequency. if the gain of the error amplifier is high enough at the switching frequency, output ripple voltage (although smaller for a ceramic output capacitor) may still affect the proper operation of the regulator. a filter capacitor c f in parallel with the r c /c c network is suggested to control possible ripple at the v c pin. an all ceramic solution is possible for the LT3430 by choosing the correct compensation components for the given application. example: for v in = 8v to 40v, v out = 5v at 2a, the LT3430 can be stabilized, provide good transient response and maintain very low output ripple voltage using the follow- ing component values: (refer to the first page of this data sheet for component references) c3 = 4.7 m f, r c = 3.3k, c c = 22nf, c f = 220pf and c1 = 100 m f. see application note 19 for further detail on techniques for proper loop compensation. input capacitor step-down regulators draw current from the input supply in pulses. the rise and fall times of these pulses are very fast. the input capacitor is required to reduce the voltage ripple this causes at the input of LT3430 and force the switching current into a tight local loop, thereby minimiz- ing emi. the rms ripple current can be calculated from: iivvvv ripple rms out out in out in () = () / 2 ceramic capacitors are ideal for input bypassing. at 200khz switching frequency, the energy storage requirement of the input capacitor suggests that values in the range of 4.7 m f to 20 m f are suitable for most applications. if opera- tion is required close to the minimum input required by the output of the LT3430, a larger value may be required. this is to prevent excessive ripple causing dips below the minimum operating voltage resulting in erratic operation. depending on how the LT3430 circuit is powered up you may need to check for input voltage transients. the input voltage transients may be caused by input voltage steps or by connecting the LT3430 converter to an already powered up source such as a wall adapter. the sudden application of input voltage will cause a large surge of current in the input leads that will store energy in the parasitic inductance of the leads. this energy will cause the input voltage to swing above the dc level of input power source and it may exceed the maximum voltage rating of input capacitor and LT3430. the easiest way to suppress input voltage transients is to add a small aluminum electrolytic capacitor in parallel with the low esr input capacitor. the selected capacitor needs to have the right amount of esr in order to critically dampen the resonant circuit formed by the input lead inductance and the input capacitor. the typical values of esr will fall in the range of 0.5 w to 2 w and capacitance will fall in the range of 5 m f to 50 m f. if tantalum capacitors are used, values in the 22 m f to 470 m f range are generally needed to minimize esr and meet ripple current and surge ratings. care should be taken to ensure the ripple and surge ratings are not exceeded. the avx tps and kemet t495 series are surge rated. avx recommends derating capacitor operating voltage by 2:1 for high surge applications. catch diode highest efficiency operation requires the use of a schottky type diode. dc switching losses are minimized due to its low forward voltage drop, and ac behavior is benign due to its lack of a significant reverse recovery time. schottky diodes are generally available with reverse voltage ratings of up to 60v and even 100v, and are price competitive with other types. the use of so-called ultrafast recovery diodes is gener- ally not recommended. when operating in continuous mode, the reverse recovery time exhibited by ultrafast diodes will result in a slingshot type effect. the power internal switch will ramp up v in current into the diode in an
15 LT3430 sn3430 3430is applicatio s i for atio wu uu attempt to get it to recover. then, when the diode has finally turned off, some tens of nanoseconds later, the v sw node voltage ramps up at an extremely high dv/dt, per- haps 5 to even 10v/ns ! with real world lead inductances, the v sw node can easily overshoot the v in rail. this can result in poor rfi behavior and if the overshoot is severe enough, damage the ic itself. the suggested catch diode (d1) is an international recti- fier 30bq060 schottky. it is rated at 3a average forward current and 60v reverse voltage. typical forward voltage is 0.52v at 3a. the diode conducts current only during switch off time. peak reverse voltage is equal to regulator input voltage. average forward current in normal opera- tion can be calculated from: i ivv v d avg out in out in () = () this formula will not yield values higher than 3a with maximum load current of 3a. boost pin for most applications, the boost components are a 0.68 m f capacitor and a mmsd914ti diode. the anode is typically connected to the regulated output voltage to generate a voltage approximately v out above v in to drive the output stage. however, the output stage discharges the boost capacitor during the on time of the switch. the output driver requires at least 3v of headroom throughout this period to keep the switch fully saturated. if the output voltage is less than 3.3v, it is recommended that an alternate boost supply is used. the boost diode can be connected to the input, although, care must be taken to prevent the 2 v in boost voltage from exceeding the boost pin absolute maximum rating. the additional voltage across the switch driver also increases power loss, reducing efficiency. if available, and independent supply can be used with a local bypass capacitor. a 0.68 m f boost capacitor is recommended for most appli- cations. almost any type of film or ceramic capacitor is suitable, but the esr should be <1 w to ensure it can be fully recharged during the off time of the switch. the capacitor value is derived from worst-case conditions of 4700ns on time, 80ma boost current and 0.7v discharge ripple. the boost capacitor value could be reduced under less demanding conditions, but this will not improve circuit operation or efficiency. under low input voltage and low load conditions, a higher value capacitor will reduce discharge ripple and improve start-up operation. shutdown function and undervoltage lockout figure 4 shows how to add undervoltage lockout (uvlo) to the LT3430. typically, uvlo is used in situations where the input supply is current limited , or has a relatively high source resistance. a switching regulator draws constant power from the source, so source current increases as source voltage drops. this looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. uvlo prevents the regulator from operating at source voltages where these problems might occur. threshold voltage for lockout is about 2.38v. a 5.5 m a bias current flows out of the pin at this threshold. the internally generated current is used to force a default high state on the shutdown pin if the pin is left open. when low shut- down current is not an issue, the error due to this current can be minimized by making r lo 10k or less. if shutdown current is an issue, r lo can be raised to 100k, but the error due to initial bias current and changes with temperature should be considered. rk r rv v vr a lo hi lo in lo = () = - () - () 10 238 238 55 to 100k 25k suggested . .. m v in = minimum input voltage keep the connections from the resistors to the shutdown pin short and make sure that interplane or surface capaci- tance to the switching nodes are minimized. if high
16 LT3430 sn3430 3430is applicatio s i for atio wu uu resistor values are used, the shutdown pin should be bypassed with a 1000pf capacitor to prevent coupling problems from the switch node. if hysteresis is desired in the undervoltage lockout point, a resistor r fb can be added to the output node. resistor values can be calcu- lated from: r rv vv v ra rrv v hi lo in out lo fb hi out = -+ () + [] - () = () () 238 1 238 55 ./ .. / dd d m 25k suggested for r lo v in = input voltage at which switching stops as input voltage descends to trip level d v = hysteresis in input voltage level example: output voltage is 5v, switching is to stop if input voltage drops below 12v and should not restart unless input rises back to 13.5v. d v is therefore 1.5v and v in = 12v. let r lo = 25k. r k ka k k rk k hi fb = -+ () + [] m () = () = = () = 25 12 238155 1 15 238 25 55 25 10 41 224 116 116 5 1 5 387 ../ . . . . . /. synchronizing the sync input must pass from a logic level low, through the maximum synchronization threshold with a duty cycle between 10% and 90%. the input can be driven directly from a logic level output. the synchronizing range is equal to initial operating frequency up to 700khz. this means that minimum practical sync frequency is equal to the worst-case high self-oscillating frequency (228khz), not the typical operating frequency of 200khz. caution should be used when synchronizing above 265khz because at higher sync frequencies the amplitude of the internal slope compensation used to prevent subharmonic switching is reduced. this type of subharmonic switching only occurs at input voltages less than twice output voltage. higher inductor values will tend to eliminate this problem. see frequency compensation section for a discussion of an entirely different cause of subharmonic switching before assuming that the cause is insufficient slope compensa- tion. application note 19 has more details on the theory of slope compensation. at power-up, when v c is being clamped by the fb pin (see figure 2, q2), the sync function is disabled. this allows the frequency foldback to operate in the shorted output con- dition. during normal operation, switching frequency is controlled by the internal oscillator until the fb pin reaches 0.6v, after which the sync pin becomes operational. if no synchronization is required, this pin should be connected to ground. + + 2.38v 0.4v gnd v sw LT3430 input r fb r hi 3430 f04 output c1 l1 shdn standby in total shutdown 5.5 m a r lo c2 + figure 4. undervoltage lockout
17 LT3430 sn3430 3430is applicatio s i for atio wu uu layout considerations as with all high frequency switchers, when considering layout, care must be taken in order to achieve optimal elec- trical, thermal and noise performance. for maximum effi- ciency, switch rise and fall times are typically in the nano- second range. to prevent noise both radiated and con- ducted, the high speed switching current path, shown in figure 5, must be kept as short as possible. this is imple- mented in the suggested layout of figure 6. shortening this path will also reduce the parasitic trace inductance of approximately 25nh/inch. at switch off, this parasitic in- ductance produces a flyback spike across the LT3430 switch. when operating at higher currents and input volt- ages, with poor layout, this spike can generate voltages across the LT3430 that may exceed its absolute maximum rating. a ground plane should always be used under the switcher circuitry to prevent interplane coupling and over- all noise. gnd gnd 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 shdn sync gnd boost v in v in sw sw place feedthrough around ground pins (4 corners) for good thermal conductivity LT3430 c3 c1 d1 c2 d2 r2 r1 3430 f06 c fb c f r c c c l1 minimize LT3430 c3-d1 loop v in pins 3 and 4 are shorted together. sw pins 2 and 5 are also shorted together (using available space underneath the device between pins and gnd plane) gnd gnd bias fb v c connect to ground plane kelvin sense v out keep fb and v c components away from high frequency, high current components solder the exposed pad to the entire copper ground plane underneath the device. note: the boost and bias copper traces are on a separate layer from the ground plane gnd v out v in gnd 1 2 3 4 5 6 boost v in v in sw sw LT3430 figure 6. suggested layout 3430 f05 5v l1 v in LT3430 d1 c3 c1 high frequency circulating path load figure 5. high speed switching path
18 LT3430 sn3430 3430is applicatio s i for atio wu uu the v c and fb components should be kept as far away as possible from the switch and boost nodes. the LT3430 pinout has been designed to aid in this. the ground for these components should be separated from the switch current path. failure to do so will result in poor stability or subharmonic like oscillation. board layout also has a significant effect on thermal resistance. pins 1, 8, 9 and 16, gnd, are a continuous copper plate that runs under the LT3430 die. this is an exposed pad and is the best thermal path for heat out of the package. soldering the exposed pad to the copper ground plane under the device will reduce die temperature and increase the power capability of the LT3430. adding multiple solder filled feedthroughs under and around the four corner pins to the ground plane will also help. similar treatment to the catch diode and coil terminations will reduce any additional heating effects. parasitic resonance resonance or ringing may sometimes be seen on the switch node (see figure 7). very high frequency ringing following switch rise time is caused by switch/diode/input capacitor lead inductance and diode capacitance. schot- tky diodes have very high q junction capacitance that can ring for many cycles when excited at high frequency. if total lead length for the input capacitor, diode and switch path is 1 inch, the inductance will be approximately 25nh. at switch off, this will produce a spike across the npn output device in addition to the input voltage. at higher currents this spike can be in the order of 10v to 20v or higher with a poor layout, potentially exceeding the abso lute max switch voltage. the path around switch, catch diode and input capacitor must be kept as short as possible to ensure reliable operation. when looking at this, a >100mhz oscilloscope must be used, and waveforms should be observed on the leads of the package. this switch off spike will also cause the sw node to go below ground. the LT3430 has special circuitry inside which mitigates this problem, but negative voltages over 0.8v lasting longer than 10ns should be avoided. note that 100mhz oscilloscopes are barely fast enough to see the details of the falling edge overshoot in figure 7. a second, much lower frequency ringing is seen during switch off time if load current is low enough to allow the inductor current to fall to zero during part of the switch off time (see figure 8). switch and diode capacitance reso- nate with the inductor to form damped ringing at 1mhz to 10mhz. this ringing is not harmful to the regulator and it has not been shown to contribute significantly to emi. any attempt to damp it with a resistive snubber will degrade efficiency. 50ns/div 3430 f07 2v/div sw rise sw fall figure 7. switch node resonance figure 8. discontinuous mode ringing 10v/div 0.2a/div switch node voltage inductor current at i out = 0.1a v in = 40v 1 m s/div 3430 f08 v out = 5v l = 22 m h
19 LT3430 sn3430 3430is thermal calculations power dissipation in the LT3430 chip comes from four sources: switch dc loss, switch ac loss, boost circuit current, and input quiescent current. the following formu- las show how to calculate each of these losses. these formulas assume continuous mode operation, so they should not be used for calculating efficiency at light load currents. switch loss: p ri v v tivf sw sw out out in eff out in = ()( ) + ()()() 2 12 (/ ) boost current loss: p vi v boost out out in = () 2 36 / quiescent current loss: pv v q in out = () + () 0 0015 0 003 .. r sw = switch resistance ( ? 0.15) hot t eff = effective switch current/voltage overlap time = (t r + t f + t ir + t if ) t r = (v in /1.2)ns t f = (v in /1.1)ns t ir = t if = (i out /0.05)ns f = switch frequency example: with v in = 40v, v out = 5v and i out = 2a: p w pw pw sw boost q = ( )()() + () () ()( ) () =+= = () () = =+= - 015 2 5 40 150 10 1 2 2 40 200 10 008 12 128 5236 40 004 40 0 0015 5 0 003 0 08 2 93 2 . / ... / . (. ) (. ) . total power dissipation in the ic is given by: p tot = p sw + p boost + p q = 1.28w + 0.04w + 0.08w = 1.4w thermal resistance for the LT3430 package is influenced by the presence of internal or backside planes. tssop (exposed pad) package: with a full plane under the tssop package, thermal resistance will be about 45 c/w. to calculate die temperature, use the proper thermal resistance number for the desired package and add in worst-case ambient temperature: t j = t a + ( q ja ? p tot ) when estimating ambient, remember the nearby catch diode and inductor will also be dissipating power: p vv v i v diode f in out load in = ( )( )( ) v f = forward voltage of diode (assume 0.52v at 2a) pw diode == (. )( )() . 052 40 5 2 40 091 notice that the catch diodes forward voltage contributes a significant loss in the overall system efficiency. a larger, lower v f diode can improve efficiency by several percent. p inductor = (i load )(l dcr ) l dcr = inductor dc resistance (assume 0.1 w ) p inductor (2)(0.1) = 0.2w typical thermal resistance of the board is 5 c/w. taking the catch diode and inductor power dissipation into ac- count and using the example calculations for LT3430 dissipation, the LT3430 die temperature will be estimated as: t j = t a + ( q ja ? p tot ) + [10 ? (p diode + p inductor )] with the tssop package ( q ja = 45 c/w), at an ambient temperature of 50 c: t j = 50 + (45 ? 1.4) + (5 ? 1.11) = 119 c die temperature can peak for certain combinations of v in , v out and load current. while higher v in gives greater switch ac losses, quiescent and catch diode losses, a lower v in may generate greater losses due to switch dc losses. in general, the maximum and minimum v in levels should be checked with maximum typical load current for applicatio s i for atio wu uu
20 LT3430 sn3430 3430is calculation of the LT3430 die temperature. if a more accurate die temperature is required, a measurement of the sync pin resistance (to gnd) can be used. the sync pin resistance can be measured by forcing a voltage no greater than 0.5v at the pin and monitoring the pin current over temperature in an oven. this should be done with minimal device power (low v in and no switching (v c = 0v)) in order to calibrate sync pin resistance with ambient (oven) temperature. note: some of the internal power dissipation in the ic, due to boost pin voltage, can be transferred outside of the ic to reduce junction temperature, by increasing the voltage drop in the path of the boost diode d2 (see figure 9). this reduction of junction temperature inside the ic will allow higher ambient temperature operation for a given set of conditions. boost pin circuitry dissipates power given by: p vi v v diss out sw c in boost pin = () / 36 2 typically v c2 (the boost voltage across the capacitor c2) equals v out . this is because diodes d1 and d2 can be considered almost equal, where: v c2 = v out C v fd2 C (Cv fd1 ) = v out hence the equation used for boost circuitry power dissipa- tion given in the previous thermal calculations section is stated as: p vi v v diss boost out sw in out () / = () 36 here it can be seen that boost power dissipation increases as the square of v out . it is possible, however, to reduce v c2 below v out to save power dissipation by increasing the voltage drop in the path of d2. care should be taken that v c2 does not fall below the minimum 3.3v boost voltage required for full saturation of the internal power switch. for output voltages of 5v, v c2 is approximately 5v. during switch turn on, v c2 will fall as the boost capacitor c2 is dicharged by the boost pin. in the previous boost pin section, the value of c2 was designed for a 0.7v droop in v c2 = v droop . hence, an output voltage as low as 4v would still allow the minimum 3.3v for the boost function using the c2 capacitor calculated. if a target output voltage of 12v is required, however, an excess of 8v is placed across the boost capacitor which is not required for the boost function but still dissipates additional power. what is required is a voltage drop in the path of d2 to achieve minimal power dissipation while still maintaining minimum boost voltage across c2. a zener, d4, placed in series with d2 (see figure 9), drops voltage to c2. example : the boost pin power dissipation for a 20v input to 12v output conversion at 2a is given by: pw boost = () = 12 2 36 12 20 04 / . if a 7v zener d4 is placed in series with d2, then power dissipation becomes : pw boost = () = 12 2 36 5 20 0 167 / . for an fe package with thermal resistance of 45 c/w, ambient temperature savings would be, t(ambient) sav- ings = 0.233w ? 45 c/w = 11 c. the 7v zener should be sized for excess of 0.233w operaton. the tolerances of the zener should be considered to ensure minimum v c2 exceeds 3.3v + v droop . applicatio s i for atio wu uu boost v in d1 r1 v out c f c c LT3430 shdn sync sw bias fb v c gnd c2 c1 l1 d2 r2 3430 f09 c3 v in d2 d4 + r c figure 9. boost pin, diode selection
21 LT3430 sn3430 3430is input voltage vs operating frequency considerations the absolute maximum input supply voltage for the LT3430 is specified at 60v. this is based solely on internal semi- conductor junction breakdown effects. due to internal power dissipation, the actual maximum v in achievable in a particular application may be less than this. a detailed theoretical basis for estimating internal power loss is given in the section, thermal considerations. note that ac switching loss is proportional to both operating frequency and output current. the majority of ac switch- ing loss is also proportional to the square of input voltage. for example, while the combination of v in = 40v, v out = 5v at 2a and f osc = 200khz may be easily achievable, simultaneously raising v in to 60v and f osc to 700khz is not possible. nevertheless, input voltage transients up to 60v can usually be accommodated, assuming the result- ing increase in internal dissipation is of insufficient time duration to raise die temperature significantly. a second consideration is controllability. a potential limi- tation occurs with a high step-down ratio of v in to v out , as this requires a correspondingly narrow minimum switch on time. an approximate expression for this (assuming continuous mode operation) is given as follows: min t vv vf on out f in osc = + () where: v in = input voltage v out = output voltage v f = schottky diode forward drop f osc = switching frequency a potential controllability problem arises if the LT3430 is called upon to produce an on time shorter than it is able to produce. feedback loop action will lower then reduce the v c control voltage to the point where some sort of cycle- skipping or odd/even cycle behavior is exhibited. in summary: 1. be aware that the simultaneous requirements of high v in , high i out and high f osc may not be achievable in practice due to internal dissipation. the thermal con- siderations section offers a basis to estimate internal power. in questionable cases a prototype supply should be built and exercised to verify acceptable operation. 2. the simultaneous requirements of high v in , low v out and high f osc can result in an unacceptably short minimum switch on time. cycle skipping and/or odd/ even cycle behavior will result although correct output voltage is usually maintained. frequency compensation before starting on the theoretical analysis of frequency response, the following should be rememberedthe worse the board layout, the more difficult the circuit will be to stabilize. this is true of almost all high frequency analog circuits, read the layout considerations section first. common layout errors that appear as stability problems are distant placement of input decoupling capacitor and/ or catch diode, and connecting the v c compensation to a ground track carrying significant switch current. in addi- tion, the theoretical analysis considers only first order non-ideal component behavior. for these reasons, it is important that a final stability check is made with produc- tion layout and components. the LT3430 uses current mode control. this alleviates many of the phase shift problems associated with the inductor. the basic regulator loop is shown in figure 10. the LT3430 can be considered as two g m blocks, the error amplifier and the power stage. figure 11 shows the overall loop response. at the v c pin, the frequency compensation components used are: r c = 3.3k, c c = 0.022 m f and c f = 220pf. the output capacitor used is a 100 m f, 10v tantalum capacitor with typical esr of 100m w . the esr of the tantalum output capacitor provides a useful zero in the loop frequency response for maintaining stabil- ity. this esr, however, contributes significantly to the ripple voltage at the output (see output ripple voltage in the applications information section). it is possible to reduce capacitor size and output ripple voltage by replac- ing the tantalum output capacitor with a ceramic output capacitor because of its very low esr. the zero provided by the tantalum output capacitor must now be reinserted back into the loop. alternatively, there may be cases applicatio s i for atio wu uu
22 LT3430 sn3430 3430is where, even with the tantalum output capacitor, an addi- tional zero is required in the loop to increase phase margin for improved transient response. a zero can be added into the loop by placing a resistor (r c ) at the v c pin in series with the compensation capacitor, c c , or by placing a capacitor (c fb ) between the output and the fb pin. when using r c , the maximum value has two limitations. first, the combination of output capacitor esr and r c may stop the loop rolling off altogether. second, if the loop gain is not rolled off sufficiently at the switching frequency, output ripple will perturb the v c pin enough to cause unstable duty cycle switching similar to subharmonic oscillations. if needed, an additional capacitor (c f ) can be added across the r c /c c network from the v c pin to ground to further suppress v c ripple voltage. with a tantalum output capacitor, the LT3430 already includes a resistor (r c ) and filter capacitor (c f ) at the v c pin (see figures 10 and 11) to compensate the loop over the entire v in range (to allow for stable pulse skipping for high v in -to-v out ratios 3 10). a ceramic output capacitor can still be used with a simple adjustment to the resistor r c for stable operation (see ceramic capacitors section for stabilizing LT3430). if additional phase margin is required, a capacitor (c fb ) can be inserted between the output and fb pin but care must be taken for high output voltage applications. sudden shorts to the output can create unacceptably large negative transients on the fb pin. applicatio s i for atio wu uu for v in -to-v out ratios < 10, higher loop bandwidths are possible by readjusting the frequency compensation com- ponents at the v c pin. when checking loop stability, the circuit should be oper- ated over the applications full voltage, current and tem- perature range. proper loop compensation may be obtained by empirical methods as described in application notes 19 and 76. converter with backup output regulator in systems with a primary and backup supply, for ex- ample, a battery powered device with a wall adapter input, the output of the LT3430 can be held up by the backup supply with the LT3430 input disconnected. in this condi- tion, the sw pin will source current into the v in pin. if the shdn pin is held at ground, only the shut down current of 25 m a will be pulled via the sw pin from the second supply. with the shdn pin floating, the LT3430 will consume its quiescent operating current of 1.5ma. the v in pin will also source current to any other components connected to the input line. if this load is greater than 10ma or the input could be shorted to ground, a series schottky diode must be added, as shown in figure 12. with these safeguards, the output can be held at voltages up to the v in absolute maximum rating. + 1.22v v sw v c LT3430 gnd 3430 f10 r1 output esr c f c c r c r o 200k error amplifier fb r2 c1 r load current mode power stage g m = 2mho g m = 2000 mho + tantalum esl c1 ceramic c fb figure 10. model for loop response frequency (hz) gain (db) 80 60 40 20 0 ?0 ?0 phase (deg) 180 150 120 90 60 30 0 3430 f11 gain phase 10 v in = 42v v out = 5v i load = 1a c out = 100 f, 10v, 0.1 1k 10k 1m 100 100k r c = 3.3k c c = 22nf c f = 220pf figure 11. overall loop response
23 LT3430 sn3430 3430is applicatio s i for atio wu uu output 5v 2a input 40v 3430 f13 c2 0.68 m f c1 100 m f 10v c ss 15nf c f 220pf d1 30bq060 or b250a c3 4.7 m f 50v cer d2 mmsd914ti l1 33 m h r1 15.4k r3 2k c c 0.022 f r2 4.99k r4 47k l1: cdrh104r-220m q1 boost bias v in LT3430 shdn sync sw fb v c gnd + r c 3.3k figure 13. buck converter with adjustable soft-start buck converter with adjustable soft-start large capacitive loads or high input voltages can cause high input currents at start-up. figure 13 shows a circuit that limits the dv/dt of the output at start-up, controlling the capacitor charge rate. the buck converter is a typical configuration with the addition of r3, r4, c ss and q1. as the output starts to rise, q1 turns on, regulating switch current via the v c pin to maintain a constant dv/dt at the output. output rise time is controlled by the current through c ss defined by r4 and q1s v be . once the output is in regulation, q1 turns off and the circuit operates normally. r3 is transient protection for the base of q1. rise time rc v v ss out be = ()( )( ) 4 using the values shown in figure 10, rise time ms = ()( ) () = 47 10 15 10 5 07 5 39 . the ramp is linear and rise times in the order of 100ms are possible. since the circuit is voltage controlled, the ramp rate is unaffected by load characteristics and maximum output current is unchanged. variants of this circuit can be used for sequencing multiple regulator outputs. 5v, 2a removable input c2 0.68 m f c f 220pf 54k d1 30bq060 3430 f12 c3 4.7 m f r c 3.3k c c 0.022 f d3 30bq060 d2 mmsd914ti 33 m h c1 100 f 10v alternate supply 25k r1 15.4k r2 4.99k boost v in LT3430 shdn sync sw bias fb v c gnd + figure 12. dual source supply with 25 m a reverse leakage
24 LT3430 sn3430 3430is applicatio s i for atio wu uu dual output sepic converter the circuit in figure 14 generates both positive and negative 5v outputs with a single piece of magnetics. the two inductors shown are actually just two windings on a standard coiltronics inductor. the topology for the 5v output is a standard buck converter. the C 5v topology would be a simple flyback winding coupled to the buck converter if c4 were not present. c4 creates a sepic (single-ended primary inductance converter) topology which improves regulation and reduces ripple current in l1. without c4, the voltage swing on l1b compared to l1a would vary due to relative loading and coupling losses. c4 provides a low impedance path to maintain an equal voltage swing in l1b, improving regulation. in a flyback converter, during switch on time, all the converters energy is stored in l1a only, since no current flows in l1b. at switch off, energy is transferred by magnetic coupling into l1b, powering the C 5v rail. c4 pulls l1b positive during switch on time, causing current to flow, and energy to build in l1b and c4. at switch off, the energy stored in both l1b and c4 supply the C5v rail. this reduces the current in l1a and changes l1b current waveform from square to triangular. for details on this circuit, including maximum output currents, see design note 100. positive-to-negative converter the circuit in figure 15 is a positive-to-negative topology using a grounded inductor. it differs from the standard approach in the way the ic chip derives its feedback signal because the LT3430 accepts only positive feedback sig- nals. the ground pin must be tied to the regulated negative output. a resistor divider to the fb pin then provides the proper feedback voltage for the chip. the following equation can be used to calculate maximum load current for the positive-to-negative converter: i i vv vvfl vv vv vv max p in out out in out in out in out f = + ? ? ++ ()( ) ()()() ()(.) (.)() 2 015 015 i p = maximum rated switch current v in = minimum input voltage v out = output voltage v f = catch diode forward voltage 0.15 = switch voltage drop at 3a example: with v in(min) = 5.5v, v out = 12v, l = 10 m h, v f = 0.52v, i p = 3a: i max = 0.6a. v out 5v v out ?v ? * l1 is a single core with two windings coiltronics #ctx25-4a ? if load can go to zero, an optional preload of 1k to 5k may be used to improve load regulation d1, d3: 30bq060 v in 7.5v to 60v gnd 3430 f14 c2 0.68 m f c f 220pf d1 c1 100 m f 10v tant c5 100 m f 10v tant c3 4.7 m f 100v ceramic c4 100 f 10v tant d2 mmsd914ti d3 l1a* 25 m h l1b* r1 15.4k r2 4.99k + + + r c 3.3k c c 0.022 f boost v in LT3430 shdn sync sw fb v c gnd figure 14. dual output sepic converter
25 LT3430 sn3430 3430is applicatio s i for atio wu uu inductor value the criteria for choosing the inductor is typically based on ensuring that peak switch current rating is not exceeded. this gives the lowest value of inductance that can be used, but in some cases (lower output load currents) it may give a value that creates unnecessarily high output ripple voltage. the difficulty in calculating the minimum inductor size needed is that you must first decide whether the switcher will be in continuous or discontinuous mode at the critical point where switch current reaches 3a. the first step is to use the following formula to calculate the load current above which the switcher must use continuous mode. if your load current is less than this, use the discontinuous mode formula to calculate minimum inductor needed. if load current is higher, use the continuous mode formula. output current where continuous mode is needed: i vi vv vv v cont in p in out in out f > +++ ()() ()( ) 22 4 minimum inductor discontinuous mode: l vi fi min out out p = 2 2 ()() ()( ) minimum inductor continuous mode: l vv fv v i i vv v min in out in out p out out f in = ++ + ? ? ? ? ? ? ()( ) ()( ) () 21 for a 40v to C12v converter using the LT3430 with peak switch current of 3a and a catch diode of 0.52v: ia cont = +++ = ()() ()( .) . 40 3 440124012052 1 148 22 for a load current of 0.5a, this says that discontinuous mode can be used and the minimum inductor needed is found from: lh min ==m 212 05 200 10 3 67 32 ()(.) ()() . in practice, the inductor should be increased by about 30% over the calculated minimum to handle losses and variations in value. this suggests a minimum inductor of 10 m h for this application. ripple current in the input and output capacitors positive-to-negative converters have high ripple current in the input capacitor. for long capacitor lifetime, the rms value of this current must be less than the high frequency ripple current rating of the capacitor. the following for- mula will give an approximate value for rms ripple cur- rent. this formula assumes continuous mode and large inductor value . small inductors will give somewhat higher ripple current, especially in discontinuous mode. the exact formulas are very complex and appear in application note 44, pages 29 and 30. for our purposes here i have simply added a fudge factor (ff). the value for ff is about 1.2 for higher load currents and l 3 15 m h. it increases to about 2.0 for smaller inductors at lower load currents. capacitor i ff i v v rms out out in = ()( ) ff = 1.2 to 2.0 the output capacitor ripple current for the positive-to- negative converter is similar to that for a typical buck regulatorit is a triangular waveform with peak-to-peak output** ?2v, 0.5a input ? 5.5v to 48v 3430 f15 c2 0.68 m f c c r c d1 30bq060 r1 36.5k c1 100 m f 16v tant c3 4.7 f 100v cer d2 ? mmsd914ti l1* 10 m h c f boost LT3430 v in v sw fb gnd v c r2 4.12k * increase l1 for higher current applications. see applications information ** maximum load current depends on minimum input voltage and inductor size. see applications information ? for v in > 44v and v out = ?2v, additional voltage drop in the path of d2 is required to ensure boost pin maximum rating is not exceeded. see applications information (boost pin voltage) + figure 15. positive-to-negative converter
26 LT3430 sn3430 3430is applicatio s i for atio wu uu value equal to the peak-to-peak triangular waveform of the inductor. the low output ripple design in figure 15 places the input capacitor between v in and the regulated negative output. this placement of the input capacitor significantly reduces the size required for the output capacitor (versus placing the input capacitor between v in and ground). the peak-to-peak ripple current in both the inductor and output capacitor (assuming continuous mode) is: i p-p p-p = == + ++ = dc v fl dc duty cycle vv vvv i rms i in out f out in f cout () 12 the output ripple voltage for this configuration is as low as the typical buck regulator based predominantly on the inductors triangular peak-to-peak ripple current and the esr of the chosen capacitor (see output ripple voltage in applications information). diode current average diode current is equal to load current. peak diode current will be considerably higher. peak diode current: continuous mode i vv v vv lfv v discontinuous mode iv lf out in out in in out in out out out = + + + = ()()() ()()( ) ()( ) ()() 2 2 keep in mind that during start-up and output overloads, average diode current may be much higher than with normal loads. care should be used if diodes rated less than 1a are used, especially if continuous overload conditions must be tolerated. boost pin voltage to ensure that the boost pin voltage does not exceed its absolute maximum rating of 68v with respect to device gnd pin voltage, care should be taken in the generation of boost voltage. for the conventional method of generating boost voltage, shown in figure 1, the voltage at the boost pin during switch on time is approximately given by: v boost (gnd pin) = (v in C v gndpin ) + v c2 where: v c2 = (d2+) C v d2 C (d1+) + v d1 = voltage across the boost capacitor for the positive-to-negative converter shown in figure 15, the conventional buck output node is grounded (d2+) = 0v and the catch diode (d1+) is connected to the negative output = v out = C12v. absolute maximum ratings should also be observed with the gnd pin now at C12v. it can be seen that for v d1 = v d2 : v c2 = (d2+) C (d1+) = |v out | = 12v the maximum v in voltage allowed for the device (gnd pin at C12v) is 48v. the maximum v in voltage allowed without exceeding the boost pin voltage absolute maximum rating is given by: v in(max) = boost (max) + (v gndpin ) C v c2 v in(max) = 68 + (C12) C 12 = 44v to increase usable v in voltage, v c2 must be reduced. this can be achieved by placing a zener diode v z1 (anode at c2+) in series with d2. note: a maximum limit on v z1 must be observed to ensure a minimum v c2 is maintained on the boost capacitor; referred to as v boost(min) in the electrical characteristics.
27 LT3430 sn3430 3430is u package descriptio fe package 16-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663, exposed pad variation bb) fe16 tssop 0802 0.09 ?0.20 (.0036 ?.0079) 0 ?8 0.45 ?0.75 (.018 ?.030) 4.30 ?4.50* (.169 ?.177) 6.40 bsc 134 5 6 7 8 10 9 4.90 ?5.10* (.193 ?.201) 16 1514 13 12 11 1.10 (.0433) max 0.05 ?0.15 (.002 ?.006) 0.65 (.0256) bsc 0.195 ?0.30 (.0077 ?.0118) 2 millimeters (inches) dimensions do not include mold flash. mold flash shall not exceed 0.150mm (.006") per side * note: 1. controlling dimension: millimeters 2. dimensions are in recommended solder pad layout 3. drawing not to scale for exposed pad dimensions, refer to table 0.45 0.05 0.65 bsc 4.50 0.10 6.60 0.10 1.05 0.10 3.58 (.141) 2.94 (.116)
28 LT3430 sn3430 3430is lt/tp 0802 1.5k ? printed in usa ? linear technology corporation 2002 related parts part number description comments lt1074/lt1076 step-down switching regulators 40v input, 100khz, 5a and 2a lt1082 1a high voltage efficiency switching voltage regulator 3v to 75v input, 60khz operation ltc ? 1149 high efficiency synchronous step-down switching regulator external fet switches, v in up to 48v lt1176 step-down switching regulator pdip lt1076, v in up to 35v lt1339 high power synchronous dc/dc controller external fet switches, v in up to 60v lt1370 high efficiency dc/dc converter 42v, 6a, 500khz switch lt1371 high efficiency dc/dc converter 35v, 3a, 500khz switch lt1372/lt1377 500khz and 1mhz high efficiency 1.5a switching regulators boost topology, v in up to 30v ltc1435/ltc1436 high efficiency step-down converter external switches, low noise, v in up to 36v lt1676 wide input range, high efficiency, 7.4v to 60v input, 100khz operation, 700ma internal switch step-down switching regulator lt1765 monolithic 3a, 1.25mhz step-down regulator v in : 3v to 25v; v ref = 1.2v; so-8, 16-lead ssop exposed pad lt1766 monolithic 1.5a, 200khz step-down regulator v in : 5.5v to 60v, v ref = 1.2v, gn16, 16-lead tssop exposed pad lt1767 monolithic 1.5a, 1.25mhz step-down regulator v in : 3v to 25v; v ref = 1.2v; 8-lead msop package lt1776 wide input range, high efficiency, 7.4v to 60v input, 200khz operation, 700ma internal switch step-down switching regulator lt1777 low noise buck regulator operation up to 48v, controlled voltage ltc1875 synchronous 1.5a, 550khz step-down regulator v in : 2.65v to 6v; v ref = 0.8v; pll; 8-lead msop package ltc1878 monolithic 600ma, 550khz step-down regulator v in : 2.65v to 6v; v ref = 0.8v; 8-lead msop package lt1956 monolithic 1.5a, 500khz step-down regulator v in : 5.5v to 60v, v ref = 1.2v, gn16, 16-lead tssop exposed pad ltc3404 synchronous 600ma, 1.4mhz step-down regulator v in : 2.65v to 6v; v ref = 0.8v; 8-lead msop package ltc3405 synchronous 300ma, 1.5mhz step-down regulator v in : 2.5v to 5.5v; v ref = 0.8v; thinsot tm package thinsot is a trademark of linear technology corporation. linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com

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