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lt1912 1 1912fa typical application applications 36v, 2a, 500khz step-down switching regulator the lt ? 1912 is an adjustable frequency (200khz to 500khz) monolithic step-down switching regulator that accepts input voltages up to 36v. a high efficiency 0.25 switch is included on the die along with a boost schottky diode and the necessary oscillator, control and logic circuitry. current mode topology is used for fast transient response and good loop stability. the lt1912 allows the use of ceramic capacitors resulting in low output ripple while keeping total solution size to a minimum. the low current shutdown mode reduces input supply current to less than 1a while a resistor and capacitor on the run/ss pin provide a controlled output voltage ramp (soft-start). the lt1912 is available in 10-pin msop and 3mm 3mm dfn packages with exposed pads for low thermal resistance. automotive battery regulation set-top box distributed supply regulation industrial supplies wall transformer regulation 3.3v step-down converter sw fb v c r t v in bd v in 4.5v to 36v v out 3.3v 2a 4.7f 0.47f 470pf 47f 100k 20k 68.1k 6.8h 316k gnd off on lt1912 1912 ta01 run/ss boost sync efficiency load current (a) 0 efficiency (%) 50 0.5 1.0 1.5 2 1912 ta01b 60 100 90 80 70 v in = 12v l = 6.8f f = 500khz v out = 3.3v v out = 5v wide input range: operation from 3.6v to 36v 2a maximum output current adjustable switching frequency: 200khz to 500khz low shutdown current: i q < 1a integrated boost diode synchronizable between 250khz to 500khz saturating switch design: 0.25 on-resistance 0.790v feedback reference voltage output voltage: 0.79v to 20v soft-start capability small 10-pin thermally enhanced msop and (3mm 3mm) dfn packages features description l , lt, ltc, ltm, burst mode, linear technology and the linear logo are registered trademarks and thinsot is a trademark of linear technology corporation. all other trademarks are the property of their respective owners.
lt1912 2 1912fa pin configuration absolute maximum ratings electrical characteristics v in , run/ss voltage ................................................. 36v boost pin voltage ................................................... 56v boost pin above sw pin ......................................... 30v fb, rt, v c voltage ....................................................... 5v bd, sync voltage ..................................................... 30v (note 1) the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v in = 10v, v run/ss = 10v, v boost = 15v, v bd = 3.3v unless otherwise noted. (note 2) operating junction temperature range (note 2) lt1912e ............................................ C40c to 125c storage temperature range .................. C65c to 150c lead temperature (soldering, 10 sec) mse only .......................................................... 300c order information lead free finish tape and reel part marking package description temperature range lt1912edd#pbf lt1912emse#pbf lt1912edd#trpbf lt1912emse#trpbf ldjt ltdjs 10-lead (3mm 3mm) plastic dfn 10-lead plastic msop C40c to 125c C40c to 125c consult ltc marketing for parts specified with wider operating temperature ranges. consult ltc marketing for information on non-standard lead based finish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ top view dd package 10-lead (3mm 3mm) plastic dfn 10 9 6 7 8 4 5 3 11 gnd 2 1 r t v c fb n/c sync bd boost sw v in run/ss ja = 45c/w, jc = 10c/w exposed pad (pin 11) is gnd, must be soldered to pcb 1 2 3 4 5 bd boost sw v in run/ss 10 9 8 7 6 r t v c fb n/c sync top view mse package 10-lead plastic msop 11 gnd ja = 45c/w, jc = 10c/w exposed pad (pin 11) is gnd, must be soldered to pcb parameter conditions min typ max units minimum input voltage l 3 3.6 v quiescent current from v in v run/ss = 0.2v v bd = 3v, not switching v bd = 0, not switching l 0.01 450 1.3 0.5 600 1.7 a a ma quiescent current from bd v run/ss = 0.2v v bd = 3v, not switching v bd = 0, not switching l 0.01 0.9 1 0.5 1.3 5 a ma a minimum bias voltage (bd pin) 2.7 3 v
lt1912 3 1912fa parameter conditions min typ max units feedback voltage l 780 775 790 790 800 805 mv mv fb pin bias current (note 3) v fb = 0.8v, v c = 0.4v l 7 30 na fb voltage line regulation 4v < v in < 36v 0.002 0.01 %/v error amp g m 25 mho error amp gain 1000 v c source current 45 a v c sink current 45 a v c pin to switch current gain 3.5 a/v v c clamp voltage 2 v switching frequency r t = 187k 160 200 240 khz minimum switch off-time l 60 150 ns switch current limit duty cycle = 5% 3.2 3.7 4.2 a switch v cesat i sw = 2a 500 mv boost schottky reverse leakage v sw = 10v, v bd = 0v 0.02 2 a minimum boost voltage (note 4) l 1.5 2.1 v boost pin current i sw = 1a 22 35 ma run/ss pin current v run/ss = 2.5v 5 10 a run/ss input voltage high 2.5 v run/ss input voltage low 0.2 v sync low threshold 0.5 v sync high threshold 0.7 v sync pin bias current v sync = 0v 0.1 a note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the lt1912e is guaranteed to meet performance specifications from 0c to 125c. specifications over the C40c to 125c operating temperature range are assured by design, characterization and correlation with statistical process controls. note 3: bias current flows out of the fb pin. note 4: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. electrical characteristics the l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25c. v in = 10v, v run/ss = 10v v boost = 15v, v bd = 3.3v unless otherwise noted. (note 2)
lt1912 4 1912fa typical performance characteristics t a = 25c, unless otherwise noted. switch current (ma) 0 boost pin current (ma) 10 30 40 50 80 1912 g08 20 60 70 0 1500 500 1000 2000 2500 load current (a) 0 50 efficiency (%) 100 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.0 1.0 1912 g01 60 90 80 70 v in = 24v v in = 34v v in = 12v l: nec plc-0745-5r6 f: 500khz v out = 5v 0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2.0 1.0 load current (a) 50 efficiency (%) 90 1912 g02 60 65 55 85 80 70 75 v in = 12v v in = 7v l: nec plc-0745-5r6 f: 500khz v in = 24v v in = 34v v out = 3.3v input voltage (v) 5 load current (a) 15 1912 g03 2.5 10 20 1.5 1.0 4.0 3.5 3.0 2.0 25 30 typical minimum v out = 3.3v l = 4.7h f = 500khz switch current (ma) 0 400 500 700 1500 1912 g07 300 200 500 1000 2000 2500 100 0 600 voltage drop (mv) efficiency efficiency boost pin current switch voltage drop maximum load current duty cycle (%) 0 switch current limit(a) 40 1912 g05 2.5 20 60 1.5 1.0 4.0 3.5 3.0 2.0 80 100 input voltage (v) 5 load current (a) 15 1912 g04 2.5 10 20 1.5 1.0 3.5 3.0 2.0 25 30 typical minimum v out = 5v l = 4.7h f = 500khz temperature (c) switch current limit (a) 2.0 2.5 3.5 3.0 1912 g06 1.5 1.0 0 0.5 4.5 4.0 duty cycle = 10 % duty cycle = 90 % ?50 25 ?25 0 50 75 100 150125 switch current limit switch current limit maximum load current
lt1912 5 1912fa typical performance characteristics t a = 25c, unless otherwise noted. boost diode current (a) 0 boost diode v f (v) 0.8 1.0 1.2 2.0 1912 g15 0.6 0.4 0 0.5 1.0 1.5 0.2 1.4 run/ss pin voltage (v) 0 switch current limit (a) 3.5 1.5 1912 g13 2.0 1.0 0.5 1 2 0.5 0 4.0 3.0 2.5 1.5 2.5 3 3.5 fb pin voltage (mv) 0 switching frequency (normalized) 0.8 1.0 1.2 600 1912 g11 0.6 0.4 200 400 800 500 100 300 700 900 0.2 0 temperature (?c) minimum switch on time (ns) 80 100 120 1912 g12 60 40 20 0 140 ?50 25 ?25 0 50 75 100 150 125 run/ss pin voltage (v) 0 run/ss pin current (a) 8 10 12 15 25 1912 g14 6 4 5 10 20 30 35 2 0 temperature (c) feedback voltage (mv) 800 1912 g09 760 840 780 820 ?50 25 ?25 0 50 75 100 150 125 temperature (c) frequency (normalized) 1.00 1.10 1912 g10 0.90 0.80 1.20 0.95 1.05 0.85 1.15 ?50 25 ?25 0 50 75 100 150125 feedback voltage switching frequency frequency foldback minimum switch on-time soft-start run/ss pin current boost diode fb pin error voltage (v) ?200 ?50 v c pin current (a) ?20 0 20 0 200 50 1912 g16 ?40 ?100 100 40 10 ?10 30 ?30 error amp output current
lt1912 6 1912fa temperature (c) v c voltage (v) 1.50 2.00 2.50 1912 g19 1.00 0.50 0 current limit clamp switching threshold ?50 25 ?25 0 50 75 100 150125 1912 g21 i l 0.5a/div v sw 5v/div v out 10mv/div v in = 12v; v out = 3.3v i load = 110ma 2s/div 1912 g22 i l 1a/div v sw 5v/div v out 10mv/div v in = 12v; v out = 3.3v i load = 1a 2s/div v c voltages switching waveforms; discontinuous operation switching waveforms; continuous operation t a = 25c unless otherwise noted. typical performance characteristics
lt1912 7 1912fa bd (pin 1): this pin connects to the anode of the boost schottky diode. bd also supplies current to the internal regulator. boost (pin 2): this pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. sw (pin 3): the sw pin is the output of the internal power switch. connect this pin to the inductor, catch diode and boost capacitor. v in (pin 4): the v in pin supplies current to the lt1912s internal regulator and to the internal power switch. this pin must be locally bypassed. run/ss (pin 5): the run/ss pin is used to put the lt1912 in shutdown mode. tie to ground to shut down the lt1912. tie to 2.5v or more for normal operation. if the shutdown feature is not used, tie this pin to the v in pin. run/ss also provides a soft-start function; see the applications information section. sync (pin 6): this is the external clock synchronization input. ground this pin when sync function is not used. tie to a clock source for synchronization. clock edges should have rise and fall times faster than 1s. see synchronizing section in applications information. n/c (pin 7): this pin should be tied to ground. fb (pin 8): the lt1912 regulates the fb pin to 0.790v. connect the feedback resistor divider tap to this pin. v c (pin 9): the v c pin is the output of the internal error amplifier. the voltage on this pin controls the peak switch current. tie an rc network from this pin to ground to compensate the control loop. r t (pin 10): oscillator resistor input. connecting a resis - tor to ground from this pin sets the switching frequency. gnd (exposed pad pin 11): ground. the exposed pad must be soldered to pcb. + ? + ? oscillator 200khz?500khz v c clamp soft-start slope comp r v in v in run/ss boost sw switch latch v c v out c2 c3 c f l1 d1 c c r c bd r t r2 gnd error amp r1 fb r t c1 s q 1912 bd 4 5 10 1 2 3 9 11 8 6 internal 0.79v ref sync pin functions block diagram
lt1912 8 1912fa the lt1912 is a constant frequency, current mode step- down regulator. an oscillator, with frequency set by r t , enables an rs flip-flop, turning on the internal power switch. an amplifier and comparator monitor the current flowing between the v in and sw pins, turning the switch off when this current reaches a level determined by the voltage at v c . an error amplifier measures the output voltage through an external resistor divider tied to the fb pin and servos the v c pin. if the error amplifiers output increases, more current is delivered to the output; if it decreases, less current is delivered. an active clamp on the v c pin provides current limit. the v c pin is also clamped to the voltage on the run/ss pin; soft-start is implemented by generating a voltage ramp at the run/ss pin using an external resistor and capacitor. an internal regulator provides power to the control circuitry. the bias regulator normally draws power from the v in pin, but if the bd pin is connected to an external voltage higher than 3v bias power will be drawn from the external source (typically the regulated output voltage). this improves efficiency. the run/ss pin is used to place the lt1912 in shutdown, disconnecting the output and reducing the input current to less than 1a. the switch driver operates from either the input or from the boost pin. an external capacitor and diode are used to generate a voltage at the boost pin that is higher than the input supply. this allows the driver to fully saturate the internal bipolar npn power switch for efficient operation. the oscillator reduces the lt1912s operating frequency when the voltage at the fb pin is low. this frequency foldback helps to control the output current during start- up and overload. operation
lt1912 9 1912fa applications information fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the 1% resis - tors according to: r1 = r2 v out 0.79v ? 1 ? ? ? ? ? ? reference designators refer to the block diagram. setting the switching frequency the lt1912 uses a constant frequency pwm architecture that can be programmed to switch from 200khz to 500khz by using a resistor tied from the r t pin to ground. a table showing the necessary r t value for a desired switching frequency is in figure 1. switching frequency (khz) r t value (k) 200 300 400 500 187 121 88.7 68.1 figure 1. switching frequency vs r t value operating frequency trade-offs selection of the operating frequency is a trade-off between efficiency, component size, minimum dropout voltage, and maximum input voltage. the advantage of high frequency operation is that smaller inductor and capacitor values may be used. the disadvantages are lower efficiency, lower maximum input voltage, and higher dropout voltage. the highest acceptable switching frequency (f sw(max) ) for a given application can be calculated as follows: f sw max ( ) = v d + v out t on min ( ) v d + v in ? v sw ( ) where v in is the typical input voltage, v out is the output voltage, v d is the catch diode drop (~0.5v) and v sw is the internal switch drop (~0.5v at max load). this equation shows that slower switching frequency is necessary to safely accommodate high v in /v out ratio. also, as shown in the next section, lower frequency allows a lower dropout voltage. the reason input voltage range depends on the switching frequency is because the lt1912 switch has finite minimum on and off times. the switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. typical minimum on time at 25c is 80ns. this means that the minimum and maximum duty cycles are: dc min = f sw t on min ( ) dc max = 1? f sw t off min ( ) where f sw is the switching frequency, the t on(min) is the minimum switch on time (~150ns), and the t off(min) is the minimum switch off time (~150ns). these equations show that duty cycle range increases when switching frequency is decreased. a good choice of switching frequency should allow ad - equate input voltage range (see next section) and keep the inductor and capacitor values small. input voltage range the maximum input voltage for lt1912 applications depends on switching frequency, the absolute maximum ratings of the v in and boost pins, and the operating mode. while the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following equation. v in min ( ) = v out + v d 1? f sw t off min ( ) ? v d + v sw where v in(max) is the maximum operating input voltage, v out is the output voltage, v d is the catch diode drop (~0.5v), v sw is the internal switch drop (~0.5v at max load), f sw is the switching frequency (set by r t ), and t on(min) is the minimum switch on time (~150ns). note that a higher switching frequency will depress the maximum operating input voltage. conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. if the output is in regulation and no short-circuit, start- up, or overload events are expected, then input voltage transients of up to 36v are acceptable regardless of the switching frequency. in this mode, the lt1912 may enter
lt1912 10 1912fa pulse-skipping operation where some switching pulses are skipped to maintain output regulation. in this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. the minimum input voltage is determined by either the lt1912s minimum operating voltage of ~3.6v or by its maximum duty cycle (see equation in previous section). the minimum input voltage due to duty cycle is: v in max ( ) = v out + v d f sw t on min ( ) ? v d + v sw where v in(min) is the minimum input voltage, and t off(min) is the minimum switch off time (150ns). note that higher switching frequency will increase the minimum input voltage. if a lower dropout voltage is desired, a lower switching frequency should be used. inductor selection for a given input and output voltage, the inductor value and switching frequency will determine the ripple current. the ripple current i l increases with higher v in or v out and decreases with higher inductance and faster switch - ing frequency. a reasonable starting point for selecting the ripple current is: i l = 0.4(i out(max) ) where i out(max) is the maximum output load current. to guarantee sufficient output current, peak inductor current must be lower than the lt1912s switch current limit (i lim ). the peak inductor current is: i l(peak) = i out(max) + i l /2 where i l(peak) is the peak inductor current, i out(max) is the maximum output load current, and i l is the inductor ripple current. the lt1912s switch current limit (i lim ) is at least 3.5a at low duty cycles and decreases linearly to 2.5a at dc = 0.8. the maximum output current is a func - tion of the inductor ripple current: i out(max) = i lim C i l /2 be sure to pick an inductor ripple current that provides sufficient maximum output current (i out(max) ). the largest inductor ripple current occurs at the highest v in . to guarantee that the ripple current stays below the specified maximum, the inductor value should be chosen according to the following equation: l = v out + v d f sw i l ? ? ? ? ? ? 1? v out + v d v in(max) ? ? ? ? ? ? where v d is the voltage drop of the catch diode (~0.4v), v in(max) is the maximum input voltage, v out is the output voltage, f sw is the switching frequency (set by r t ), and l is in the inductor value. the inductors rms current rating must be greater than the maximum load current and its saturation current should be about 30% higher. for robust operation in fault conditions (start-up or short circuit) and high input voltage (>30v), the saturation current should be above 3.5a. to keep the efficiency high, the series resistance (dcr) should be less than 0.1, and the core material should be intended for high frequency applications. table 1 lists several vendors and suitable types. table 1. inductor vendors vendor url part series type murata www.murata.com lqh55d open tdk www.componenttdk.com slf7045 slf10145 shielded shielded toko www.toko.com d62cb d63cb d75c d75f shielded shielded shielded open sumida www.sumida.com cr54 cdrh74 cdrh6d38 cr75 open shielded shielded open of course, such a simple design guide will not always re - sult in the optimum inductor for your application. a larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. if your load is lower than 2a, then you can decrease the value of the inductor and operate with higher ripple current. this allows you to use a physically smaller inductor, or one applications information
lt1912 11 1912fa with a lower dcr resulting in higher efficiency. there are several graphs in the typical performance characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. for details of maximum output current and discontinuous mode opera - tion, see linear technology application note 44. finally, for duty cycles greater than 50% (v out /v in > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. see an19. input capacitor bypass the input of the lt1912 circuit with a ceramic capaci - tor of x7r or x5r type. y5v types have poor performance over temperature and applied voltage, and should not be used. a 4.7f to 10f ceramic capacitor is adequate to bypass the lt1912 and will easily handle the ripple current. note that larger input capacitance is required when a lower switching frequency is used. if the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. this can be provided with a lower performance electrolytic capacitor. step-down regulators draw current from the input sup - ply in pulses with very fast rise and fall times. the input capacitor is required to reduce the resulting voltage ripple at the lt1912 and to force this very high frequency switching current into a tight local loop, minimizing emi. a 4.7f capacitor is capable of this task, but only if it is placed close to the lt1912 and the catch diode (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt1912. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt1912 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt1912s voltage rating. this situation is easily avoided (see the hot plugging safely section). output capacitor and output ripple the output capacitor has two essential functions. along with the inductor, it filters the square wave generated by the lt1912 to produce the dc output. in this role it determines the output ripple, and low impedance at the switching frequency is important. the second function is to store energy in order to satisfy transient loads and stabilize the lt1912s control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. a good starting value is: c out = 100 v out f sw where f sw is in mhz, and c out is the recommended output capacitance in f. use x5r or x7r types. this choice will provide low output ripple and good transient response. transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. a lower value of output capacitor can be used to save space and cost but transient performance will suffer. see the frequency compensation section to choose an appropriate compensation network. when choosing a capacitor, look carefully through the data sheet to find out what the actual capacitance is under operating conditions (applied voltage and temperature). a physically larger capacitor, or one with a higher voltage rating, may be required. high performance tantalum or electrolytic capacitors can be used for the output capacitor. low esr is important, so choose one that is intended for use in switching regulators. the esr should be specified by the supplier, and should be 0.05 or less. such a capaci - tor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low esr. table 2 lists several capacitor vendors. applications information
lt1912 12 1912fa applications information vendor phone url part series commands panasonic (714) 373-7366 www.panasonic.com ceramic, polymer, tantalum eef series kemet (864) 963-6300 www.kemet.com ceramic, tantalum t494, t495 sanyo (408) 749-9714 www.sanyovideo.com ceramic, polymer, tantalum poscap murata (408) 436-1300 www.murata.com ceramic avx www.avxcorp.com ceramic, tantalum tps series taiyo yuden (864) 963-6300 www.taiyo-yuden.com ceramic table 2. capacitor vendors catch diode the catch diode conducts current only during switch off time. average forward current in normal operation can be calculated from: i d(avg) = i out (v in C v out )/v in where i out is the output load current. the only reason to consider a diode with a larger current rating than neces - sary for nominal operation is for the worst-case condition of shorted output. the diode current will then increase to the typical peak switch current. peak reverse voltage is equal to the regulator input voltage. use a schottky diode with a reverse voltage rating greater than the input voltage. table? 3 lists several schottky diodes and their manufacturers. table 3. diode vendors part number v r (v) i ave (a) v f at 1a (mv) v f at 2a (mv) on semicnductor mbrm120e mbrm140 20 40 1 1 530 550 595 diodes inc. b220 b230 dfls240l 20 30 40 2 2 2 500 500 500 international rectifier 10bq030 20bq030 30 30 1 2 420 470 470 ceramic capacitors a precaution regarding ceramic capacitors concerns the maximum input voltage rating of the lt1912. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt1912 circuit is plugged into a live supply, the input volt - age can ring to twice its nominal value, possibly exceeding the lt1912s rating. this situation is easily avoided (see the hot plugging safely section). frequency compensation the lt1912 uses current mode control to regulate the output. this simplifies loop compensation. in particular, the lt1912 does not require the esr of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. frequency compensation is provided by the components tied to the v c pin, as shown in figure 2. generally a capacitor (c c ) and a resistor (r c ) in series to ground are used. in addi - tion, there may be lower value capacitor in parallel. this capacitor (c f ) is not part of the loop compensation but is used to filter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high esr. loop compensation determines the stability and transient performance. designing the compensation network is a bit complicated and the best values depend on the application
lt1912 13 1912fa applications information and in particular the type of output capacitor. a practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the com- pensation network to optimize the performance. stability should then be checked across all operating conditions, including load current, input voltage and temperature. the lt1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stabil- ity using a transient load. figure 2 shows an equivalent circuit for the lt1912 control loop. the error amplifier is a transconductance amplifier with finite output impedance. the power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance amplifier generating an output current proportional to the voltage at the v c pin. note that the output capacitor integrates this current, and that the capacitor on the v c pin (c c ) integrates the error amplifier output current, resulting in two poles in the loop. in most cases a zero is required and comes from either the output capacitor esr or from a resistor r c in series with c c . this simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. a phase lead capacitor (c pl ) across the feedback divider may improve the transient response. figure 3 shows the transient response when the load current is stepped from 500ma to 1500ma and back to 500ma. boost and bias pin considerations capacitor c3 and the internal boost schottky diode (see the block diagram) are used to generate a boost volt - age that is higher than the input voltage. in most cases a 0.22f capacitor will work well. figure 2 shows three ways to arrange the boost circuit. the boost pin must be more than 2.3v above the sw pin for best efficiency. for outputs of 3v and above, the standard circuit (figure 4a) is best. for outputs between 2.8v and 3v, use a 1f boost capacitor. a 2.5v output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. for reliable boost pin operation with 2.5v outputs use a good external schottky diode (such as the on semi mbr0540), and a 1f boost capacitor (see figure 4b). for lower output voltages the boost diode can be tied to the input (figure 4c), or to another supply greater than 2.8v. tying bd to v in reduces the maximum input voltage to 30v. the circuit in figure 4a is more efficient because the boost pin current and bd pin quiescent current comes from a lower voltage source. you must also be sure that the maximum voltage ratings of the boost and bd pins are not exceeded. the minimum operating voltage of an lt1912 application is limited by the minimum input voltage (3.6v) and by the maximum duty cycle as outlined in a previous section. for proper startup, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, then the boost capacitor may not be fully charged. because ? + 0.8v sw v c g m = 420mho gnd 3meg lt1912 1912 f02 r1 output esr c f c c r c error amplifier fb r2 c1 c1 current mode power stage g m = 3.5mho + polymer or tantalum ceramic c pl figure 2. model for loop response figure 3. transient load response of the lt1912 front page application as the load current is stepped from 500ma to 1500ma. v out = 3.3v 1912 f03 i l 0.5a/div v out 100mv/div 10s/div v in = 12v; front page application
lt1912 14 1912fa the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. this minimum load will depend on input and output volt - ages, and on the arrangement of the boost circuit. the minimum load generally goes to zero once the circuit has started. figure 5 shows a plot of minimum load to start and to run as a function of input voltage. in many cases the discharged output capacitor will present a load to the switcher, which will allow it to start. the plots show the worst-case situation where v in is ramping very slowly. for lower start-up voltage, the boost diode can be tied to v in ; however, this restricts the input range to one-half of the absolute maximum rating of the boost pin. at light loads, the inductor current becomes discontinu - ous and the effective duty cycle can be very high. this reduces the minimum input voltage to approximately 300mv above v out . at higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the lt1912, requiring a higher input voltage to maintain regulation. soft-start the run/ss pin can be used to soft-start the lt1912, reducing the maximum input current during start-up. the run/ss pin is driven through an external rc filter to create a voltage ramp at this pin. figure 6 shows the start- up and shutdown waveforms with the soft-start circuit. by choosing a large rc time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. choose the value of the resistor so that it can supply 20a when the run/ ss pin reaches 2.5v. synchronization synchronizing the lt1912 oscillator to an external fre - quency can be done by connecting a square wave (with 20% to 80% duty cycle) to the sync pin. the square wave amplitude should have valleys that are below 0.3v and peaks that are above 0.8v (up to 6v). the lt1912 may be synchronized over a 250khz to 500khz range. the r t resistor should be chosen to set the lt1912 switching frequency 20% below the lowest synchronization v in boost sw bd v in v out 4.7f c3 gnd lt1912 v in boost sw bd v in v out 4.7f c3 d2 gnd lt1912 v in boost sw bd v in v out 4.7f c3 gnd lt1912 1912 fo4 (4a) for v out > 2.8v (4b) for 2.5v < v out < 2.8v (4c) for v out < 2.5v; v in(max) = 30v figure 4. three circuits for generating the boost voltage input. for example, if the synchronization signal will be 250khz and higher, the r t should be chosen for 200khz. to assure reliable and safe operation the lt1912 will only synchronize when the output voltage is near regulation. it is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the r t resistor. see inductor selection section. it is also important to note that slope compensation is set by the r t value: when the sync frequency is much higher than the one set by r t , the slope compensation will be significantly reduced which may require a larger inductor value to prevent subharmonic oscillation. applications information
lt1912 15 1912fa shorted and reversed input protection if the inductor is chosen so that it wont saturate exces - sively, an lt1912 buck regulator will tolerate a shorted output. there is another situation to consider in systems where the output will be held high when the input to the lt1912 is absent. this may occur in battery charging ap - plications or in battery backup systems where a battery or some other supply is diode or-ed with the lt1912s output. if the v in pin is allowed to float and the run/ss pin is held high (either by a logic signal or because it is tied to v in ), then the lt1912s internal circuitry will pull its quiescent current through its sw pin. this is fine if your system can tolerate a few ma in this state. if you ground the run/ss pin, the sw pin current will drop to essentially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt1912 can pull large currents from the output through the sw pin and the v in pin. figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 8 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents flow in the lt1912s v in and sw pins, the catch diode (d1) and the input capacitor (c1). the loop formed by these components should be as small as possible. these components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. place a local, unbroken ground plane below these components. figure 5. the minimum input voltage depends on output voltage, load current and boost circuit 1912 f05 load current (a) 1 input voltage (v) 4.0 4.5 5.0 10000 3.5 3.0 2.0 10 100 1000 1 10000 10 100 1000 2.5 6.0 5.5 to start (worst case) to run load current (a) input voltage (v) 5.0 6.0 7.0 4.0 2.0 3.0 8.0 to run v out = 3.3v t a = 25c l = 8.2h f = 500khz v out = 5v t a = 25c l = 8.2h f = 500khz to start (worst case) figure 6. to soft-start the lt1912, add a resisitor and capacitor to the run/ss pin 1912 f06 i l 1a/div v run/ss 2v/div v out 2v/div run/ss gnd run 15k 0.22f 2ms/div figure 7. diode d4 prevents a shorted input from discharging a backup battery tied to the output. it also protects the circuit from a reversed input. the lt1912 runs only when the input is present v in boost gnd fb run/ss v c sw d4 mbrs140 v in lt1912 1912 f07 v out backup applications information
lt1912 16 1912fa applications information the sw and boost nodes should be as small as possible. finally, keep the fb and v c nodes small so that the ground traces will shield them from the sw and boost nodes. the exposed pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. to keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the lt1912 to additional ground planes within the circuit board and on the bottom side. hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt1912 circuits. however, these capaci - tors can cause problems if the lt1912 is plugged into a live supply (see linear technology application note 88 for a complete discussion). the low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the v in pin of the lt1912 can ring to twice the nominal input voltage, possibly exceeding the lt1912s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt1912 into an energized supply, the input network should be designed to prevent this overshoot. figure 9 shows the waveforms that result when an lt1912 circuit is connected to a 24v supply through six feet of 24-gauge twisted pair. the first plot is the response with a 4.7f ceramic capacitor at the input. the input voltage rings as high as 50v and the input current peaks at 26a. a good solution is shown in figure 9b. a 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). a 0.1f capacitor improves high frequency filtering. for high input voltages its impact on efficiency is minor, reducing efficiency by 1.5 percent for a 5v output at full load operating from 24v. vias to local ground plane vias to v out vias to run/ss vias to v in outline of local ground plane 1912 f08 l1 c2 r rt r c r2 c c v out d1 c1 gnd vias to sync r1 figure 8. a good pcb layout ensures proper, low emi operation high temperature considerations the pcb must provide heat sinking to keep the lt1912 cool. the exposed pad on the bottom of the package must be soldered to a ground plane. this ground should be tied to large copper layers below with thermal vias; these lay - ers will spread the heat dissipated by the lt1912. place additional vias can reduce thermal resistance further. with these steps, the thermal resistance from die (or junction) to ambient can be reduced to ja = 35c/w or less. with 100 lfpm airflow, this resistance can fall by another 25%. further increases in airflow will lead to lower thermal re - sistance. because of the large output current capability of the lt1912, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of
lt1912 17 1912fa applications information 125c. when operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125c. power dissipation within the lt1912 can be estimated by calculating the total power loss from an efficiency measure - ment and subtracting the catch diode loss and inductor loss. the die temperature is calculated by multiplying the lt1912 power dissipation by the thermal resistance from junction to ambient. other linear technology publications application notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. the lt1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. design note 100 shows how to generate a bipolar output supply using a buck regulator. figure 9. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the lt1912 is connected to a live supply + lt1912 4.7f v in 20v/div i in 10a/div 20s/div v in closing switch simulates hot plug i in (9a) (9b) low impedance energized 24v supply stray inductance due to 6 feet (2 meters) of twisted pair + lt1912 4.7f 0.1f 0.7 v in 20v/div i in 10a/div 20s/div danger ringing v in may exceed absolute maximum rating (9c) + lt1912 4.7f 22f 35v ai.ei. 1912 f09 v in 20v/div i in 10a/div 20s/div +
lt1912 18 1912fa typical applications 5v step-down converter 3.3v step-down converter sw fb v c r t v in bd v in 6.8v to 36v v out 5v 2a 4.7f 0.47f 47f 100k f = 500khz d: diodes inc. dfls240l l: taiyo yuden np06dzb6r8m d 16.2k 68.1k l 6.8h 536k gnd 470pf on off lt1912 1912 ta02 run/ss boost sync sw fb v c r t v in bd v in 4.4v to 36v v out 3.3v 2a 4.7f 0.47f 47f 100k f = 500khz d: diodes inc. dfls240l l: taiyo yuden np06dzb4r7m d 20k 68.1k l 6.8h 316k gnd 470pf on off lt1912 1912 ta03 run/ss boost sync
lt1912 19 1912fa typical applications 2.5v step-down converter sw fb v c r t v in bd v in 4v to 36v v out 2.5v 2a 4.7f 1f 47f 100k f = 500khz d1: diodes inc. dfls240l d2: mbr0540 l: taiyo yuden np06dzb4r7m d1 20k 68.1k l 6.8h 215k gnd 330pf on off lt1912 d2 1912 ta04 run/ss boost sync
lt1912 20 1912fa 1.8v step-down converter 12v step-down converter sw fb v c r t v in bd v in 15v to 36v v out 12v 2a 10f 0.47f 22f 50k f = 500khz d: diodes inc. dfls240l l: nec/tokin plc-0755-100 d 26.1k 68.1khz l 10h 715k gnd 330pf on off lt1912 1912 ta06 run/ss boost sync sw fb v c r t v in bd v in 3.5v to 27v v out 1.8v 2a 4.7f 0.47f 47f 100k f = 500khz d: diodes inc. dfls240l l: taiyo yuden np06dzb3r3m d 18.2k 68.1k l 3.3h 127k gnd 330pf on off lt1912 1912 ta08 run/ss boost sync typical applications
lt1912 21 1912fa dd package 10-lead plastic dfn (3mm 3mm) (reference ltc dwg # 05-08-1699 rev c) package description 3.00 0.10 (4 sides) note: 1. drawing to be made a jedec package outline m0-229 variation of (weed-2). check the ltc website data sheet for current status of variation assignment 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom view?exposed pad 1.65 0.10 (2 sides) 0.75 0.05 r = 0.125 typ 2.38 0.10 (2 sides) 1 5 10 6 pin 1 top mark (see note 6) 0.200 ref 0.00 ? 0.05 (dd) dfn rev c 0310 0.25 0.05 2.38 0.05 (2 sides) recommended solder pad pitch and dimensions 1.65 0.05 (2 sides) 2.15 0.05 0.50 bsc 0.70 0.05 3.55 0.05 package outline 0.25 0.05 0.50 bsc pin 1 notch r = 0.20 or 0.35 45 chamfer
lt1912 22 1912fa package description msop (mse) 0910 rev g 0.53 0.152 (.021 .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 ? 0.27 (.007 ? .011) typ 0.86 (.034) ref 0.50 (.0197) bsc 1 2 3 4 5 4.90 0.152 (.193 .006) 0.497 0.076 (.0196 .003) ref 8910 10 1 7 6 3.00 0.102 (.118 .004) (note 3) 3.00 0.102 (.118 .004) (note 4) note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 6. exposed pad dimension does not include mold flash. mold flash on e-pad shall not exceed 0.254mm (.010") per side. 0.254 (.010) 0 ? 6 typ detail ?a? detail ?a? gauge plane 5.23 (.206) min 3.20 ? 3.45 (.126 ? .136) 0.889 0.127 (.035 .005) recommended solder pad layout 1.68 0.102 (.066 .004) 1.88 0.102 (.074 .004) 0.50 (.0197) bsc 0.305 0.038 (.0120 .0015) typ bottom view of exposed pad option 1.68 (.066) 1.88 (.074) 0.1016 0.0508 (.004 .002) detail ?b? detail ?b? corner tail is part of the leadframe feature. for reference only no measurement purpose 0.05 ref 0.29 ref mse package 10-lead plastic msop, exposed die pad (reference ltc dwg # 05-08-1664 rev g)
lt1912 23 1912fa 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 representa - tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. revision history rev date description page number a 11/10 changed units to ma for quiescent current from v in for v bd = 0, not switching in electrical characteristics 2
lt1912 24 1912fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2007 lt 1110 rev a ? printed in usa related parts typical application part number description comments lt1933 500ma (i out ), 500khz step-down switching regulator in sot-23 v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.6ma, i sd <1a, thinsot? package lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode v in : 3.3v to 80v, v out(min) = 1.25v, i q = 100a, i sd <1a, 10-pin 3mm 3mm dfn and 16-pin tssop packages lt1936 36v, 1.4a (i out ), 500khz high efficiency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.9ma, i sd <1a, ms8e package lt3493 36v, 1.2a (i out ), 750khz high efficiency step-down dc/dc converter v in : 3.6v to 40v, v out(min) = 0.8v, i q = 1.9ma, i sd <1a, 6-pin 2mm 3mm dfn package lt1976/lt1977 60v, 1.2a (i out ), 200khz/500khz, high efficiency step-down dc/dc converter with burst mode ? operation v in : 3.3v to 60v, v out(min) = 1.2v, i q = 100a, i sd <1a, 16-pin tssop package lt1767 25v, 1.2a (i out ), 1.1mhz, high efficiency step-down dc/dc converter v in : 3v to 25v, v out(min) = 1.2v, i q = 1ma, i sd <6a, ms8e package lt1940 dual 25v, 1.4a (i out ), 1.1mhz, high efficiency step-down dc/dc converter v in : 3.6v to 25v, v out(min) = 1.2v, i q = 3.8ma, i sd <30a, 16-pin tssop package lt1766 60v, 1.2a (i out ), 200khz, high efficiency step-down dc/dc converter v in : 5.5v to 60v, v out(min) = 1.2v, i q = 2.5ma, i sd = 25a, 16-pin tssop package lt3434/lt3435 60v, 2.4a (i out ), 200/500khz, high efficiency step-down dc/dc converter with burst mode operation v in : 3.3v to 60v, v out(min) = 1.2v, i q = 100a, i sd <1a, 16-pin tssop package lt3480 38v, 2a (i out ), 2.4mhz, high efficiency step-down dc/dc converter with burst mode operation v in : 3.6v to 38v, v out(min) = 0.79v, i q = 70a, i sd <1a, 10-pin 3mm 3mm dfn and 10-pin msop packages lt3481 36v, 2a (i out ), 2.8mhz, high efficiency step-down dc/dc converter with burst mode operation v in : 3.6v to 34v, v out(min) = 1.26v, i q = 50a, i sd <1a, 10-pin 3mm 3mm dfn and 10-pin msop packages lt3684 36v, 2a (i out ), 2.8mhz, high efficiency step-down dc/dc converter v in : 3.6v to 34v, v out(min) = 1.26v, i q = 1.5ma, i sd <1a, 10-pin 3mm 3mm dfn and 10-pin msop packages lt3685 38v, 2a(i out ) 2.4mhz step-down dc/dc converter with 60v transient protection v in : 3.6v to 38v, v out(min) = 0.79v, i q = 450a, i sd < 1a, 3mm 3mm dfn, msop-10 packages sw fb v c r t v in bd v in 3.6v to 27v v out 1.2v 2a 4.7f 0.47f 47f f = 500khz d: diodes inc. dfls240l l: taiyo yuden np06dzb3r3m d 16.2k 68.1k l 3.3h gnd 330pf on off lt1912 1912 ta09 run/ss boost sync 100k 52.3k 1.2v step-down converter

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