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lt3570 1 3570fb typical application features applications description 1.5a buck converter, 1.5a boost converter and ldo controller the lt ? 3570 is a buck and boost converter with internal power switches and ldo controller. each converter is designed with a 1.5a current limit and an input range from 2.5v to 36v, making the lt3570 ideal for a wide variety of applications. switching frequencies up to 2.1mhz are programmed with an external timing resistor and the oscillator can be synchronized to an external clock up to 2.75mhz. the lt3570 features a programmable soft-start function that limits the feedback voltage during start-up helping prevent overshoot and limiting inrush current. the ldo controller is capable of delivering up to 10ma of base current to an external npn transistor. ef? ciency n 2.5v to 36v input voltage range n programmable switching frequency from 500khz to 2.1mhz n synchronizable up to 2.75mhz n v out(min) : 0.8v n independent soft-start for each converter n separate v in supplies for each converter n duty cycle range: 0% to 90% at 1mhz n available in 24-lead (4mm 4mm) qfn and 20-lead tssop packages n cable and satellite set-top boxes n automotive systems n telecom systems n dying gasp systems n tft lcd displays v in1 shdn1 shdn2 shdn3 shdn1 shdn2 shdn3 sw1 fb1 100nf 1nf 10nf 22f 2.2f v out3 2.5v 100ma 32.4k v out2 3.3v 1a 22k 143k v out1 12v 275ma v in 5v 22k 10.0k 10.2k 22.1k 10.2k 3570 ta01a 15.8k 10f 10f 1nf 10nf 3.3h 6.8h d2 d3 d1 boost sw2 fb2 ss2 npn_drv fb3 v c2 ss1 v c1 v in2 v in3 bias r t sync lt3570 gnd q1 l , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. i out2 (a) 0 70 efficiency (%) 75 80 85 90 95 100 0.2 0.4 0.6 0.8 3570 ta01b 1.0 f sw = 1.2mhz v in = 5v v out1 = 12v v out2 = 3.3v v out3 = 2.5v i out1 = 275ma i out3 = 100ma
lt3570 2 3570fb absolute maximum ratings v in1 , v in2 , v in3 , v bias voltage ..................................40v boost voltage .........................................................60v boost pin above sw2 .............................................25v npn_drv voltage .......................................................8v sw1 voltage .............................................................40v shdn1 , shdn2 , shdn3 voltage ..............................40v sync, r t voltage ........................................................3v ss1, ss2 voltage ........................................................3v fb1, fb2, fb3 voltage ...............................................10v (note 1) v c1 , v c2 voltage ..........................................................3v maximum junction temperature........................... 125c operating temperature range (note 2).. C40c to 125c storage temperature range tssop ............................................... C65c to 150c qfn .................................................... C65c to 125c lead temperature (soldering, 10 sec) tssop only ...................................................... 300c 24 23 22 21 20 19 7 8 9 top view 25 uf package 24-lead (4mm s 4mm) plastic qfn 10 11 12 6 5 4 3 2 1 13 14 15 16 17 18 v in2 v in2 sw2 sw1 gnd gnd v c2 ss2 gnd r t sync shdn3 boost bias v in3 npn_drv fb3 fb2 v in1 ss1 v c1 fb1 shdn1 shdn2 t jmax = 125c, ja = 37c/w exposed pad (pin 25) is gnd, must be soldered to pcb fe package 20-lead plastic tssop 1 2 3 4 5 6 7 8 9 10 top view 20 19 18 17 16 15 14 13 12 11 fb1 shdn1 shdn2 shdn3 sync r t ss2 v c2 fb2 fb3 v c1 ss1 v in1 gnd sw1 sw2 v in2 boost v in3 npn_drv 21 t jmax = 125c, ja = 38c/w exposed pad (pin 21) is gnd, must be soldered to pcb pin configuration order information lead free finish tape and reel part marking* package description temperature range lt3570euf#pbf lt3570euf#trpbf 3570 24-lead (4mm 4mm) plastic qfn C40c to 125c lt3570iuf#pbf lt3570iuf#trpbf 3570 24-lead (4mm 4mm) plastic qfn C40c to 125c lt3570efe#pbf lt3570efe#trpbf lt3570fe 20-lead plastic tssop C40c to 125c lt3570ife#pbf lt3570ife#trpbf lt3570fe 20-lead plastic tssop C40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/
lt3570 3 3570fb electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in1,2,3 = 12v, v shdn1,2,3 = 12v unless otherwise noted. parameter conditions min typ max units minimum operating voltage (v in1 ) (note 3) l 2.1 2.5 v minimum operating voltage (v in2 ) (note 3) l 2.1 2.5 v shutdown current (note 4) v shdn1,2,3 = 0v 0 1.5 a v in1 quiescent current v shdn1 = 12v, v shdn2,3 = 0v, v c1 = 0.4v (not switching) v shdn1 = 0v, v shdn2,3 = 12v 3.2 65 4.5 150 ma a v in2 quiescent current v shdn1,3 = 0v, v shdn2 = 12v, v c2 = 0.4v (not switching) v shdn1,3 = 12v, v shdn2 = 0v 3.5 3.5 4.5 4.5 ma ma v in3 quiescent current v shdn1,2 = 0v, v shdn3 = 12v v shdn1,2 = 12v, v shdn3 = 0v 700 0 950 1.5 a a bias quiescent current v bias = 2.5v 2.3 3.1 ma v shdn1,2,3 pin threshold i vin2 > 100a 0.3 1.4 v v shdn1,2,3 pin uvlo l 1.1 1.25 1.4 v v shdnx pin current v shdnx = 12v, v shdny,z = 0v (note 5) v shdn1,2,3 = 0v 30 0.1 50 1.5 a a switching frequency r t = 44.2k r t = 7.87k 450 1900 500 2100 550 2300 khz khz maximum duty cycle r t = 44.2k r t = 7.87k 95 80 % % synchronous frequency threshold 0.3 1.5 v synchronous frequency range 650 2750 khz synchronous frequency minimum on/off time 50 ns fb1,2,3 pin voltage l 772 788 804 mv fb1,2,3 pin voltage line regulation v vin1,2,3 = 2.5v to 36v, v c1,2 = 1v 0.01 %/v fb1,2 pin bias current v fb1,2 = 800mv, v c1,2 = 1v (note 6) 30 200 na fb3 pin bias current v fb3 = 800mv (note 6) 30 200 na ss1,2 pin source current v ss1,2 = 500mv 4.5 a v c1,2 pin source current v fb1,2 = 600mv 12 a v c1,2 pin sink current v fb1,2 = 1v 12 a sw1 error ampli? er 1 transconductance 190 mho error ampli? er 1 voltage gain 100 v/v v c1 pin switching threshold 750 mv v c1 to sw1 current gain 5.9 a/v sw1 current limit (note 7) 1.5 2.4 3.1 a sw1 v cesat i sw1 = 1a (note 7) 240 mv sw1 leakage current sw1 = 40v, v shdn1 = 0v 0.2 5 a
lt3570 4 3570fb parameter conditions min typ max units sw2 error ampli? er 2 transconductance 195 mho error ampli? er 2 voltage gain 100 v/v v c2 pin switching threshold 700 mv v c2 to sw2 current gain 5.4 a/v sw2 current limit (note 7) 1.5 2.4 3.1 a sw2 v cesat i sw2 = 1a (note 7) 240 mv sw2 leakage current sw2 = 0v, v in2 = 40v, v shdn2 = 0v 0.2 5 a boost pin current i sw2 = 0.5a i sw2 = 1.5a 15 30 ma ma ldo ldo maximum output current v fb3 = 600mv 10 20 ma electrical characteristics the l denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in1,2,3 = 12v, v shdn1,2,3 = 12v unless otherwise noted. 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 lt3570e is guaranteed to meet performance speci? cations from 0c to 125c junction temperature. speci? cations over the C40c to 125c operating junction temperature range are assured by design, characterization and correlation with statistical process controls. the lt3570i is guaranteed over the full C40c to 125c operating junction temperature range. note 3: v in2 supplies power for the part. v in1 supplies power only to the boost converter. v in3 supplies power only to the ldo controller. note 4: shutdown current is for each individual input current. note 5: current ? ows into the pin. note 6: current ? ows out of the pin. note 7: switch current limit and switch v cesat guaranteed by design and/or correlation to static test. note 8: this ic includes overtemperature protection that is intended to protect the device during momentary overload conditions. junction temperature will exceed the maximum operating junction temperature range when overtemperature protection is active. continuous operation above the speci? ed maximum operating junction temperature may impair device reliability. typical performance characteristics feedback voltage vs temperature frequency vs temperature soft-start current vs temperature temperature (c) C50 0.770 voltage (v) 0.775 0.780 0.785 0.790 050 100 150 3570 g01 0.795 0.800 C25 25 75 125 temperature (c) C50 frequency (khz) 1500 2000 2500 25 75 150 3570 g02 1000 500 0 C25 0 50 100 125 r t = 7.87k r t = 20k r t = 44.2k temperature (c) C50 current (a) 4.4 4.6 4.8 25 50 75 100 125 3570 g03 4.2 4.0 C25 0 150 3.8 3.6 5.0
lt3570 5 3570fb typical performance characteristics v in1 quiescent current vs temperature v in2 quiescent current vs temperature v in3 quiescent current vs temperature bias pin current vs temperature shdn pin uvlo vs temperature shdn pin current vs voltage sw1 current limit vs duty cycle sw1 saturation voltage vs sw1 current temperature (c) C50 current (ma) 2.0 2.5 3.0 25 50 75 100 125 3570 g04 1.5 1.0 C25 0 150 0.5 0 3.5 temperature (c) C50 current (ma) 2.0 3.0 150 3570 g05 1.0 0 0 50 100 C25 25 75 125 4.0 1.5 2.5 0.5 3.5 temperature (c) C50 current (a) 500 600 700 150 3570 g06 400 300 0 0 50 100 C25 25 75 125 200 100 900 800 temperature (c) C50 0 current (ma) 0.5 1.0 1.5 2.0 050 100 150 3570 g07 2.5 3.0 C25 25 75 125 temperature (c) C50 0 shdn pin voltage (v) 0.25 0.50 0.75 1.00 050 100 150 3570 g08 1.25 1.50 C25 25 75 125 voltage (v) 0 current (a) 30 40 50 15 25 40 3570 g09 20 10 0 510 20 30 35 duty cycle (%) 0 current (a) 1.0 2.0 3.0 0.5 1.5 2.5 20 40 60 80 3570 g10 100 10 030507090 t j = 125c t j = 25c t j = C40c current (a) 0 0 voltage (mv) 50 150 200 250 350 0.1 0.5 0.7 3570 g11 100 300 0.4 0.9 1.0 0.2 0.3 0.6 0.8 t j = 125c t j = 25c t j = C40c
lt3570 6 3570fb typical performance characteristics sw2 saturation voltage vs sw2 current boost pin current vs switch current npn_drv output current vs v in3 sw2 current limit vs duty cycle duty cycle (%) 0 current (a) 1.0 2.0 3.0 0.5 1.5 2.5 20 40 60 80 3570 g12 100 10 030507090 t j = 125c t j = 25c t j = C40c current (a) 0 0 voltage (mv) 50 150 200 250 350 0.1 0.5 0.7 3570 g13 100 300 0.4 0.9 1.0 0.2 0.3 0.6 0.8 t j = 125c t j = 25c t j = C40c voltage (v) 1 current (ma) 10 12 14 40 3570 g15 8 6 0 10 20 30 5 15 25 35 4 2 18 16 t j = 25c t j = 125c t j = C40c current (a) 0 0 current (ma) 5 10 15 20 25 30 0.2 0.4 0.6 0.8 3570 g14 1.0 t j = C40c t j = 125c t j = 25c
lt3570 7 3570fb pin functions (qfn/tssop) v in2 (pins 1,2/pin 14): input voltage for the buck regulator. this pin also supplies the current to the internal circuitry of the lt3570. this pin must be locally bypassed with a capacitor. sw2 (pin 3/pin15): switch node. this pin connects to the emitter of an internal npn power switch. connect a diode, inductor and boost capacitor to this pin to form the buck regulator. sw1 (pin 4/pin16): switch node. this pin connects to the collector of an internal npn power switch. connect a diode and inductor to this pin to form the boost regulator. gnd (pins 5, 6, 16, 25/pins 17, 21): ground. the exposed pad of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. the exposed pad must be soldered to the circuit board for proper operation. v in1 (pin 7/pin18): input voltage for the boost regulator. this pin supplies current to drive the boost npn transistor of the lt3570. this pin must be locally bypassed with a capacitor. ss1 (pin 8/pin 19): soft-start pin. place a soft-start capacitor here. upon start-up, a current charges the capacitor to 2v. this pin ramps the reference voltage of the boost switcher. v c1 (pin 9/pin 20): control voltage and compensation pin for the internal error ampli? er. connect a series rc from this pin to ground to compensate the switching regulator loop for the boost regulator. fb1 (pin 10/pin 1): feedback pin. the lt3570 regulates this pin to 788mv. connect the feedback resistors to this pin to set the output voltage for the boost switching regulator. shdn1 (pin 11/pin 2): shutdown pin. tie to 1.4v or more to enable the boost switcher. tie all shdn pins to 0.3v or less to shutdown the part. shdn2 (pin 12/pin 3): shutdown pin. tie to 1.4v or more to enable the buck switcher. tie all shdn pins to 0.3v or less to shutdown the part. shdn3 (pin13/pin 4): shutdown pin. tie to 1.5v or more to enable the npn ldo. tie all shdn pins to 0.3v or less to shutdown the part. sync (pin 14/pin 5): synchronization pin. the sync pin is used to synchronize the internal oscillator to an exter- nal signal. the synchronizing range is equal to the initial operating frequency set by the r t pin up to 1.3 times the initial operating frequency. r t (pin 15/pin 6): frequency set pin. place a resistor to gnd to set the internal frequency. the range of oscillation is 500khz to 2mhz. ss2 (pin 17/pin 7): soft-start pin. place a soft-start capacitor here. upon start-up, a current charges the capacitor to 2v. this pin ramps the reference voltage of the buck switcher. v c2 (pin 18/pin 8): control voltage and compensation pin for the internal error ampli? er. connect a series rc from this pin to ground to compensate the switching regulator loop for the buck regulator. fb2 (pin 19/pin 9): feedback pin. the lt3570 regulates this pin to 788mv. connect the feedback resistors to this pin to set the output voltage for the buck switching regulator. fb3 (pin 20/pin 10): feedback pin. the lt3570 regulates this pin to 788mv. connect the feedback resistors to this pin to set the output voltage for the ldo controller. npn_drive (pin 21/pin 11): base drive for the external npn. this pin provides a bias current to drive the base of the npn. this base current is driven from the in3 supply voltage. v in3 (pin 22/pin 12): input voltage for the npn ldo. this pin supplies current to drive the base of the npn. this pin must be locally bypassed with a capacitor. bias (pin 23): qfn package only. this pin supplies current to the internal circuitry of the lt3570 if greater than 2.5v. this pin must be locally bypassed with a capacitor. boost (pin 24/pin 13): bias for the base drive of the npn switch for the buck regulator. this pin provides a bias voltage higher than v in2 . the voltage on this pin is charged up through an external schottky diode.
lt3570 8 3570fb block diagram figure 1. block diagram s r6 q1 l1 gnd d1 bias v in2 sync r t boost sw2 ss2 fb2 d2 c2 r2b c6b shdn1 shdn2 sw1 788mv fb1 c6a 3570 bd r2a ss1 shdn3 r 788mv 788mv q2 4.5a c5 l2 d3 q r5 C + v c2 v c1 a6 C + a2 a8 s r q a4 C + C + + C + + r4 a7 C + a3 C + a11 a9 oscillator a10 regulator a5 r1b q3 r1c a1 v in1 v in3 fb3 npn_drv r3a r1a c1 c3 r2c c4a r3b c4b 4.5a
lt3570 9 3570fb operation the lt3570 is a constant frequency, current mode, buck converter and boost converter with an npn ldo regula- tor. operation can be best understood by referring to the block diagram. if all of the shdn pins are held low, the lt3570 is shut down and draws zero quiescent current. when any of the pins exceed 1.4v the internal bias circuits turn on. each regulator will only begin regulating when its corresponding shdn pin is pulled high. each switching regulator controls the output voltage in a similar manner. the operation of the switchers can be understood by looking at the boost regulator. a pulse from the oscillator sets the rs ? ip-? op a4 and turns on the internal npn bipolar power switch q1. current in q1 and the external inductor l1 begins to increase. when this current exceeds a level determined by the voltage at v c1 , comparator a3 resets a4, turning off q1. the current in l1 ? ows through the external schottky diode d1 and begins to decrease. the cycle begins again at the next pulse from the oscillator. in this way, the voltage on the v c1 pin controls the current through the inductor to the output. the internal error ampli? er a1 regulates the output voltage by continually adjusting the v c1 pin voltage. the threshold for switching on the v c1 pin is approximately 750mv and an active clamp of 1.15v limits the output current. the soft-start capacitor c6a allows the part to slowly start up by ramping the internal reference. the driver for the buck regulator can operate from either v in2 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 saturate the internal bipolar npn power switch for ef? cient operation. the driver for the boost regulator is operated from v in1 . the bias pin allows the internal circuitry to draw its current from a lower voltage supply than the input. this reduces power dissipation and increases ef? ciency. if the voltage on the bias pin falls below 2.5v, then the lt3570 quiescent current will ? ow from v in2 .
lt3570 10 3570fb applications information fb resistor network the output voltage is programmed with a resistor divider (refer to the block diagram) between the output and the fb pin. choose the resistors according to: r1 = r2 v out 788mv ?1 ? ? ? ? ? ? buck inductor selection and maximum output current a good ? rst choice for the inductor value is l = v out2 + v f 0.75 ? f for sw2 where v f is the voltage drop of the catch diode (~0.4v) and f is the switching frequency. with this inductance value or greater, the maximum load current will be 1a, independent of input voltage. the inductors rms current rating must be greater than the maximum load current and its saturation current should be at least 30% higher. for highest ef? ciency, the series resistance (dcr) should be less than 0.1. table 1 lists several vendors and types that are suitable. table 1. inductors part number value (h) i sat (a) dcr ( ) height (mm) sumida cdrh4d28-3r3 3.3 1.57 0.049 3.0 cdrh4d28-4r7 4.7 1.32 0.072 3.0 cdc5d23-2r2 2.2 2.50 0.03 2.5 cr43-3r3 3.3 1.44 0.086 3.5 cdrh5d28-100 10 1.3 0.048 3.0 coilcraft do1608c-332 3.3 2.00 0.080 2.9 do1608c-472 4.7 1.50 0.090 2.9 mos6020-332 3.3 1.8 0.046 2.0 d03314-103 10 0.8 0.520 1.4 d03314-222 2.2 1.6 0.200 1.4 toko (d62f)847fy-2r4m 2.4 2.5 0.037 2.7 (d73lf)817fy-2r2m 2.2 2.7 0.03 3.0 coiltronics tp3-4r7 4.7 1.5 0.181 2.2 tp1-2r2 2.2 1.3 0.188 1.8 tp4-100 10 1.5 0.146 3.0 the optimum inductor for a given application may differ from the one indicated by this simple design guide. a larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. if your load is lower than the maximum load current, then you can relax the value of the inductor and operate with higher ripple current. this allows you to use a physically smaller inductor or one with a lower dcr resulting in higher ef? ciency. be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input voltage. in addition, low inductance may result in discontinuous mode operation, which further reduces maximum load current. for details of maximum output current and discontinuous mode operation, see linear technologys application note 44. finally, for duty cycles greater than 50% (v out2 /v in2 > 0.5) a minimum inductance is required to avoid subharmonic oscillations, see application note 19. the current in the inductor is a triangle wave with an average value equal to the load current. the peak switch current is equal to the output current plus half the peak-to-peak inductor ripple current. the lt3570 limits its switch cur- rent in order to protect itself and the system from overload faults. therefore, the maximum output current that the lt3570 will deliver depends on the switch current limit, the inductor value and the input and output voltages. when the switch is off, the potential across the inductor is the output voltage plus the catch diode drop. this gives the peak-to-peak ripple current in the inductor: i l2 = 1? dc2 () v out2 + v f () l?f where dc2 is the duty cycle and is de? ned as: dc2 = v out2 v in2 the peak inductor and switch current is: i swpk2 = i lpk2 = i out2 + i l2 2 to maintain output regulation, this peak current must be less than the lt3570s switch current limit i lim2 . i lim2 is at least 1.5a at low duty cycles and decreases linearly
lt3570 11 3570fb to 1.2a at dc2 = 0.8. the maximum output current is a function of the chosen inductor value: i out2(max) = i lim2 ? i l2 2 = 1.5 ? 1? 0.25 ? dc2 () ? i l2 2 choosing an inductor value so that the ripple current is small will allow a maximum output current near the switch current limit. one approach to choosing the inductor is to start with the simple rule given above, look at the available inductors and choose one to meet cost or space goals. then use these equations to check that the lt3570 will be able to deliver the required output current. note again that these equations assume that the inductor current is continu- ous. discontinuous operation occurs when i out2 is less than i l2 /2. boost inductor selection for most applications the inductor will fall in the range of 2.2h to 22h. 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 power switch, which has a 1.5a current limit. higher values also reduce input ripple voltage and reduce core loss. the following procedure is suggested as a way of choosing a more optimum inductor. assume that the average inductor current for a boost converter is equal to the load current times v out1 /v in1 and decide whether or not the inductor must withstand continuous overload conditions. if average inductor cur- rent at maximum load current is 0.5a, for instance, a 0.5a inductor may not survive a continuous 1.5a overload condition. also be aware that boost converters are not short-circuit protected, and that under short conditions, inductor current is limited only by the available current of the input supply. calculate peak inductor current at full load current to en- sure that the inductor will not saturate. peak current can be signi? cantly higher than output current, especially with smaller inductors and lighter loads, so dont omit this step. powdered iron cores are forgiving because they saturate softly, whereas ferrite cores saturate abruptly. other core materials fall somewhere in between. the following formula assumes continuous mode operation but it errs only slightly on the high side for discontinuous mode, so it can be used for all conditions. i peak1 = i out1 ?v out1 v in1 + v in1 v out1 ?v in1 () 2?f?l?v out1 make sure that i peak1 is less than the switch current i lim1 . i lim1 is at least 1.5a at low duty cycles and decreases linearly to 1.2a at dc1 = 0.8. the maximum switch current limit can be calculated by the following formula: i lim1 = 1.5 ? (1 C 0.25 ? dc1) where dc1 is the duty cycle and is de? ned as: dc1 = 1? v in1 v out1 remember also that inductance can drop signi? cantly with dc current and manufacturing tolerance. consideration should also be given to the dc resistance of the inductor as this contributes directly to the ef? ciency losses in the overall converter. table 1 lists several inductor vendors and types that are suitable. buck output capacitor selection for 5v and 3.3v outputs, a 10f, 6.3v ceramic capacitor (x5r or x7r) at the output results in very low output volt- age ripple and good transient response. for lower voltages, 10f is adequate for ripple requirements but increasing c out will improve transient performance. other types and values will also work; the following discusses tradeoffs in output ripple and transient performance. the output capacitor ? lters the inductor current to gener ate an output with low voltage ripple. it also stores energy in order to satisfy transient loads and stabilize the lt3570s control loop. because the lt3570 operates at a high frequency, minimal output capacitance is necessary. in addition, the control loop operates well with or without the presence of output capacitor series resistance (esr). ceramic capacitors, which achieve very low output ripple applications information
lt3570 12 3570fb and small circuit size, are therefore an option. you can estimate output ripple with the following equations: v ripple = i l2 8?f?c out for ceramic capacitors and v ripple = i l2 ? esr for electrolytic capacitors (tantalum and aluminum) the rms content of this ripple is very low so the rms current rating of the output capacitor is usually not of concern. it can be estimated with the formula: i c(rms) = i l2 12 another constraint on the output capacitor is that it must have greater energy storage than the inductor; if the stored energy in the inductor transfers to the output, the resulting voltage step should be small compared to the regulation voltage. for a 5% overshoot, this requirement indicates: c out > 10 ? l ? i lim2 v out2 ? ? ? ? ? ? 2 the low esr and small size of ceramic capacitors make them the preferred type for lt3570 applications. not all ceramic capacitors are the same, however. many of the higher value capacitors use poor dielectrics with high temperature and voltage coef? cients. in particular, y5v and z5u types lose a large fraction of their capacitance with applied voltage and at temperature extremes. because loop stability and transient response depend on the value of c out , this loss may be unacceptable. use x7r and x5r types. electrolytic capacitors are also an option. the esrs of most aluminum electrolytic capacitors are too large to deliver low output ripple. tantalum, as well as newer, lower esr organic electrolytic capacitors intended for power supply use are suitable. chose a capacitor with a low enough esr for the required output ripple. because the volume of the capacitor determines its esr, both the size and the value will be larger than a ceramic capacitor that would give similar ripple performance. one bene? t is that the larger capacitance may give better transient re sponse for large changes in load current. table 2 lists several capacitor vendors. table 2. low esr surface mount capacitors vendor type series taiyo yuden ceramic x5r, x7r avx ceramic tantalum x5r, x7r tps kemet tantalum ta organic al organic t491, t494, t495 t520 a700 sanyo ta or al organic poscap panasonic al organic sp cap tdk ceramic x5r, x7r boost output capacitor selection low esr capacitors should be used at the output to minimize the output ripple voltage. multilayer ceramic capacitors are the best choice, as they have a very low esr and are available in very small packages. always use a capacitor with a suf? cient voltage rating. boost regula- tors have large rms ripple current in the output capacitor, which must be rated to handle the current. the formula to calculate this is: i ripple(rms) = i out dc1 1? dc1 = i out1 v out1 ?v in1 v in1 and is largest when v in1 is at its minimum value if v out1 and i out1 are constant. with a 1.5a current limit, the maximum that the output current ripple can be is ~0.75a. table 2 lists several capacitor vendors. buck input capacitor selection bypass the input of the lt3570 circuit with a 10f or higher ceramic capacitor of x7r or x5r type. a lower value or a less expensive y5v type will work if there is additional bypassing provided by bulk electrolytic capaci- tors, or if the input source impedance is low. the following paragraphs describe the input capacitor considerations in more detail. step-down regulators draw current from the input supply in pulses with very fast rise and fall times. the input ca- pacitor is required to reduce the resulting voltage ripple at the lt3570 input and to force this switching current applications information
lt3570 13 3570fb into a tight local loop, minimizing emi. the input capaci- tor must have low impedance at the switching frequency to do this effectively and it must have an adequate ripple current rating. the rms input current is: i in2(rms) = i out2 ? v out2 v in2 ?v out2 () v in2 < i out2 2 and is largest when v in2 = 2 ? v out2 (50% duty cycle). considering that the maximum load current is ~1.5a, rms ripple current will always be less than 0.75a. the high frequency of the lt3570 reduces the energy storage requirements of the input capacitor, so that the capacitance required is often less than 10f. the combi- nation of small size and low impedance (low equivalent series resistance or esr) of ceramic capacitors makes them the preferred choice. the low esr results in very low voltage ripple. ceramic capacitors can handle larger mag nitudes of ripple current than other capacitor types of the same value. use x5r and x7r types. an alternative to a high value ceramic capacitor is a lower value along with a larger electrolytic capacitor, for ex ample a 1f ceramic capacitor in parallel with a low esr tantalum capacitor. for the electrolytic capacitor, a value larger than 10f will be required to meet the esr and ripple current requirements. because the input capacitor is likely to see high surge currents when the input source is applied, tantalum capacitors should be surge rated. the manu- facturer may also recommend operation below the rated voltage of the capacitor. be sure to place the 1f ceramic as close as possible to the v in2 and gnd pins on the ic for optimal noise immunity. a ? nal caution is in order regarding the use of ceramic capacitors at the input. a ceramic input capacitor can combine with stray inductance to form a resonant tank circuit. if power is applied quickly (for example by plug ging the circuit into a live power source), this tank can ring, doubling the input voltage and damaging the lt3570. the solution is to either clamp the input voltage or dampen the tank circuit by adding a lossy capacitor in parallel with the ceramic capacitor. for details, see application note 88. boost input capacitor selection the capacitor of a boost converter is less critical due to the fact that the input current waveform is triangular and does not contain large squarewave currents as found in the output capacitor. capacitors in the range of 10f to 100f with an esr of 0.3 or less work well up to the full 1.5a switch current. higher esr capacitors may be acceptable at low switch currents. input capacitor ripple current for boost converters is: i ripple = 0.3 ? v in1 ? v out1 ?v in1 f?l?v out1 buck diode selection the catch diode (d2 from figure 1) conducts current only during switch-off time. average forward current in normal operation can be calculated from: i d(avg) = i out1 ? v in1 ?v out1 v in1 the only reason to consider a diode with a larger current rating than necessary 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 volt age. use a diode with a reverse voltage rating greater than the input voltage. table 3 lists several schottky diodes and their manufacturers. table 3. schottky diodes part number v r (v) i ave (a) v f at 1a (mv) on semiconductor mbrm120e 20 1 530 mbrm140 40 1 550 diodes inc. b120 20 1 500 b130 30 1 500 international recti? er 10bq030 30 1 420 applications information
lt3570 14 3570fb applications information boost diode selection a schottky diode is recommended for use with the lt3570 inverter/boost regulator. the microsemi ups120 is a very good choice. where the input to output voltage differen- tial exceeds 20v, use the ups140 (a 40v diode). these diodes are rated to handle an average forward current of 1a. for applications where the average forward current of the diode is less than 0.5a, use an on semiconductor mbr0520l diode. boost pin considerations the capacitor and diode tied to the boost pin generate a voltage that is higher than the input voltage. in most cases, a 0.1f capacitor and fast switching diode (such as the cmdsh-3 or mmsd914lt1) will work well. fig- ure 2 shows three ways to arrange the boost circuit. the boost pin must be more than 2.5v above the sw pin for full ef? ciency. for outputs of 3.3v and higher, the standard circuit (figure 2a) is best. for outputs between 2.8v and 3.3v, use a small schottky diode (such as the bat-54). for lower output voltages, the boost diode can be tied to the input (figure 2b). the circuit in figure 2a is more ef? cient because the boost pin current comes from a lower voltage source. finally, as shown in figure 2c, the anode of the boost diode can be tied to another source that is at least 3v. for example, if you are generating 3.3v and 1.8v and the 3.3v is on whenever the 1.8v is on, the 1.8v boost diode can be connected to the 3.3v output. in any case, be sure that the maximum voltage at the boost pin is less than 60v and the voltage difference between the boost and sw2 pins is less than 25v. the minimum operating voltage of an lt3570 application is limited by the undervoltage lockout (2.5v) and by the maximum duty cycle. the boost circuit also limits the minimum input voltage for proper start-up. if the input voltage ramps slowly, or the lt3570 turns on when the output is already in regulation, the boost capacitor may not be fully charged. because the boost capacitor charges 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 voltages, and on the arrangement of the boost circuit. the minimum load current generally goes to zero once the circuit has started. even without an output load current, in many cases the discharged output capacitor will present a load to the switcher that will allow it to start. switcher frequency compensation the lt3570 uses current mode control to regulate the output. this simpli? es loop compensation. in particular, the lt3570 does not depend on the esr of the output capaci- tor for stability so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. to compensate the feedback loop of the lt3570, a series resistor-capacitor network should be connected from the v c pin to gnd. for most applications, a capacitor in the range of 500pf to 4.7nf will suf? ce. a good starting value for the compensation capacitor, c c , is 1nf. the v in boost gnd sw lt3570 (2a) d3 c3 d2 c2 v in boost gnd sw lt3570 (2b) d3 c5 d2 c2 d3 v in boost gnd sw lt3570 ( 2c ) c5 d2 c2 v ext 3570 f02 figure 2. boost pin con? gurations
lt3570 15 3570fb applications information compensation resistor, r c , is usually in the range of 5k to 50k. a good technique to compensate a new application is to use a 50k potentiometer in place of r c , and use a 1nf capacitor for c c . by adjusting the potentiometer while observing the transient response, the optimum value for r c can be found. figures 3a to 3c illustrate this process for the circuit of figure 1 with load current stepped from 100ma to 500ma for the buck converter. figure 3a shows the transient response with r c equal to 1.6k. the phase margin is poor as evidenced by the excessive ringing in the output voltage and inductor current. in figure 3b, the value of r c is increased to 5.75k, which results in a more damped response. figure 3c shows the result when r c is increased further to 25k. the transient response is nicely damped and the compensation procedure is complete. the same procedure is used to compensate the boost converter. soft-start the soft-start time is programmed with an external capaci- tor to ground on ss. an internal current source charges it with a nominal 4.5a. the voltage on the soft-start pin is used to control the feedback voltage. the soft-start time is determined by the equation: t ss = 0.2 ? c ss where c ss is in nf and t ss is in ms. in the event of a commanded shutdown, ulvo on the input or a thermal shutdown, the capacitor is discharged automatically. the soft-start will remain low and only charge back up after the fault goes away and the voltage on ss is less than approximately 100mv. figure 3a. transient response shows excessive ringing figure 3b. transient response is better figure 3c. transient response well damped i out 500ma/div v out 200s/div 3570 f03a i out 500ma/div v out 200s/div 3570 f03b i out 500ma/div v out 200s/div 3570 f03c
lt3570 16 3570fb oscillator the free-running frequency is set through a resistor from the r t pin to ground. the oscillator frequency vs r t can be seen in figure 4. the oscillator can be synchronized with an external clock applied to the sync pin. when synchronizing the oscillator, the free running frequency must be set approximately 10% lower than the desired synchronized frequency. ldo regulator the lt3570 ldo regulator is capable of delivering up to 10ma of base drive for an external npn transistor. for stable operation the total output capacitance can be from 1f up to 100f. the regulator has its own independent supply voltage which allows for the base of the npn to be driven from a higher voltage than its collector. this allows for the npn regulator to run more ef? ciently. the power dissipated in the external npn is equal to: p diss = (v col C v out3 ) ? i load where v col is the collector voltage of the npn. the maxi- mum output voltage is limited to: v in3 C 1.4v and v col C 0.2v or 8v the short-circuit protection of the npn regulator is set by the max output current of the npn_drv pin multiplied by the beta of the npn. thermal shutdown an internal temperature monitor will turn off the internal circuitry and prevent the switches from turning on when the die temperature reaches approximately 160c. when the die temperature has dropped below this value the part applications information figure 4. frequency vs r t resistance buck regulator minimum on-time as the input voltage is increased, the lt3570 is required to turn on for shorter periods of time. delays associated with turning off the power switch determine the minimum on-time that can be achieved and limit the minimum duty cycle. figure 5 shows the minimum duty cycle versus frequency for the lt3570. when the required on-time has decreased below the minimum on-time of the lt3570 the inductor current will increase, exceeding the current limit. if the current through the inductor exceeds the current limit of the lt3570, the switch is prevented from turning on for 10s allowing the inductor current to decrease. the 10s off-time limits the average current that can be delivered to the load. to return to normal switching frequency either the input voltage or load current must decrease. resistance (k) 5 frequency (khz) 1250 1750 45 3570 f04 750 250 15 25 35 10 20 30 40 2250 1000 1500 500 2000 frequency (khz) 500 0 minimum duty cycle (%) 5 10 15 20 25 750 1000 1250 1500 3570 f05 1750 2000 figure 5. minimum duty cycle vs frequency
lt3570 17 3570fb will be enabled again going through a soft-start cycle. note: overtemperature protection is intended to protect the device during momentary overload conditions. continuous operation above the speci? ed maximum operating junction temperature may result in device degradation or failure. pcb layout for proper operation and minimum emi, care must be taken during printed circuit board (pcb) layout. figure 6 shows the high current paths in the step-down regulator circuit. note that in the step-down regulator, large switched currents ? ow in the power switch, the catch diode and the input capacitor. figure 7 shows the high current paths in the step-up regulator. in the boost regulator, large switched currents ? ow through the power switch, the switching diode, and the output capacitor. the loop formed by these large switched current com- ponents should be as small as possible. place these components on the same side of the circuit board and connect them on that layer. place a local, unbroken ground plane below these components and tie this ground plane applications information figure 6. buck high speed switching path figure 7. boost high speed switching path to system ground at one location. addi tionally, keep the sw and boost nodes as small as possible. this is implemented in the suggested layout of figure 8 for the qfn package which shows the topside metal from the dc1106a demonstration board. thermal considerations to deliver the power that the lt3570 is capable of, it is imperative that a good thermal path be provided to dissipate the heat generated within the package. this can be accomplished by taking advantage of the large ther- mal pad on the underside of the ic. it is recommended that multiple vias in the printed circuit board be used to conduct heat away from the ic and into a copper plane with as much area as possible. related linear technology publications application notes 19, 35, 44, 76 and 88 contain more detailed descriptions and design information for buck regulators and other switching regulators. the lt1375 data sheet has a more extensive discussion of output ripple, loop compensation, and stability testing. figure 8. suggested layout 3570 f06 l2 d1 c out high frequency circulating path c in load lt3570 3750 f07 l2 lt3570 d1 c out c in high frequency switching path load
lt3570 18 3570fb typical applications dsl modem dying gasp system v in1 shdn1 shdn2 shdn3 shdn1 shdn2 shdn3 sw1 fb1 c8 100nf c7 1nf c5 10nf c2 22f c3 2.2f v out3 3.3v 500ma r3 118k v out2 5v r8 25k r1 105k v out1 8v 250ma v in 8v to 28v r7 25k r2 11.5k r4 22.1k r5 34.0k r6 10.7k 3570 ta02 r9 20.0k c9 10f c1 10f c6 1nf 10nf l2 10h l1 4.7h d2 d3 d1 boost sw2 fb2 ss2 npn_drv fb3 v c2 ss1 v c1 v in2 v in3 bias r t sync lt3570 gnd q1 v in1 shdn1 shdn2 shdn3 sw1 fb1 c8 100nf c7 1nf c5 10nf c2 22f c3 2.2f v out3 2.5v 200ma r3 205k v out2 3.3v 200ma r8 51k r1 442k v out1 34v v in 12v r7 20k r2 10.5k r4 64.9k r5 137k r6 64.9k 3570 ta03 r9 44.2k c9 10f c10 0.1f c1 10f c6 1nf c4 10nf l2 22h l1 47h d2 d3 d1 boost sw2 fb2 ss2 npn_drv fb3 v c2 ss1 v c1 v in2 v in3 bias r t sync lt3570 gnd q1
lt3570 19 3570fb 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. uf package 24-lead plastic qfn (4mm 4mm) (reference ltc dwg # 05-08-1697) package description fe package 20-lead plastic tssop (4.4mm) (reference ltc dwg # 05-08-1663) exposed pad variation cb 4.00 0.10 (4 sides) note: 1. drawing proposed to be made a jedec package outline mo-220 variation (wggd-x)to be approved 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, if present 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 pin 1 top mark (note 6) 0.40 0.10 24 23 1 2 bottom viewexposed pad 2.45 0.10 (4-sides) 0.75 0.05 r = 0.115 typ 0.25 0.05 0.50 bsc 0.200 ref 0.00 C 0.05 (uf24) qfn 0105 recommended solder pad pitch and dimensions 0.70 0.05 0.25 0.05 0.50 bsc 2.45 0.05 (4 sides) 3.10 0.05 4.50 0.05 package outline pin 1 notch r = 0.20 typ or 0.35 45 chamfer fe20 (cb) tssop 0204 0.09 ? 0.20 (.0035 ? .0079) 0 ? 8 0.25 ref recommended solder pad layout 0.50 ? 0.75 (.020 ? .030) 4.30 ? 4.50* (.169 ? .177) 134 5 6 7 8910 11 12 14 13 6.40 ? 6.60* (.252 ? .260) 3.86 (.152) 2.74 (.108) 20 1918 17 16 15 1.20 (.047) max 0.05 ? 0.15 (.002 ? .006) 0.65 (.0256) bsc 0.195 ? 0.30 (.0077 ? .0118) typ 2 2.74 (.108) 0.45 0.05 0.65 bsc 4.50 0.10 6.60 0.10 1.05 0.10 3.86 (.152) 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 3. drawing not to scale see note 4 4. recommended minimum pcb metal size for exposed pad attachment 6.40 (.252) bsc
lt3570 20 3570fb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2008 lt 0309 rev b ? printed in usa related parts typical application pda core part number description comments lt1767 1.5a, 1.25mhz step-down switching regulator 3v to 25v input, v ref = 1.2v, synchronizable up to 2mhz, msop package lt1930/lt1930a 1a (i sw ), 1.2mhz/2.2mhz, high ef? ciency step-up dc/dc converter v in : 2.6v to 16v, v out(max) = 34v, i q = 4.2ma/5.5ma, i sd < 1a, thinsot? package lt1939 25v, 2.4mhz step-down dc/dc converter and ldo controller v in : 3v to 40v, v out(min) = 0.8v, i q = 2ma, i sd < 1a, 3mm 3mm dfn lt1943 quad output, 2.6a buck, 2.6a boost, 0.3a boost, 0.4a inverter 1.2mhz tft dc/dc converter v in : 4.5v to 22v, v out(max) = 40v, i q = 10ma, i sd < 35a, tssop28e package lt1945 dual output pos/neg 350ma (i sw ), constant off-time, high ef? ciency step-up dc/dc converter v in : 1.2v to 15v, v out(max) = 34v, i q = 20a, i sd < 1a, 10-pin ms package lt3463 dual output pos/neg 250ma (i sw ), constant off-time, high ef? ciency step-up dc/dc converter with integrated schottkys v in : 2.4v to 15v, v out(max) = 40v, i q = 40a, i sd < 1a, 3mm 3mm dfn10 package lt3467 1.1a, 1.3mhz step up dc/dc converter with integrated soft-start v in : 2.4v to 16v, v out(max) = 40v, i sd < 1 a, low pro? le (1mm) sot-23 package lt3500 40v, 2a, 2.4mhz step-down dc/dc converter and ldo controller v in : 3v to 40v, v out(min) = 0.8v, i q = 2ma, i sd < 1a, 3mm 3mm dfn lt3507 36v, 2.5mhz triple (2.4a, 1.5a, 1.5a) step-down dc/dc converter and ldo controller v in : 4v to 36v, v out(min) = 0.8v, i q = 7ma, i sd < 1a, 5mm 7mm qfn38 package thinsot is a trademark of linear technology corporation. v in1 shdn1 shdn2 shdn3 shdn1 shdn2 shdn3 sw1 fb1 c8 100nf c7 1nf c5 10nf c2 22f c3 4.7f v out3 1.8v 500ma r3 34k v out2 3.3v 500ma r8 25k r1 191k v out1 15v 200ma v in 4v to 12v r7 25k r2 10.7k r4 10.7k r5 13.7k r6 10.7k 3570 ta04 r9 20k c9 10f c1 10f c6 1nf c4 10nf l2 8.2h l1 12h d2 d3 d1 boost sw2 fb2 ss2 npn_drv fb3 v c2 ss1 v c1 v in2 v in3 bias r t sync lt3570 gnd q1

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