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lt3970 series 1 3970fb typical a pplica t ion descrip t ion 40v, 350ma step-down regulator with 2.5a quiescent current and integrated diodes the lt ? 3970 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 40v, and consumes only 2.5a of quiescent current. a high effciency switch is included on the die along with the catch diode, boost diode, and the neces- sary oscillator, control and logic circuitry. low ripple burst mode operation maintains high effciency at low output currents while keeping the output ripple below 5mv in a typical application. current mode topology is used for fast transient response and good loop stability. a catch diode current limit provides protection against shorted outputs and overvoltage conditions. an enable pin with accurate threshold is available, producing a low shutdown current of 0.7a. a power good fag signals when v out reaches 90% of the programmed output voltage. the lt3970 is available in small 10-pin msop and 3mm 2mm dfn packages. l , lt, ltc, ltm, burst mode, linear technology and the linear logo are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. 5v step-down converter fea t ures a pplica t ions n low ripple burst mode ? operation 2.5a i q at 12v in to 3.3v out output ripple < 5mv p-p n wide input voltage range: 4.2v to 40v operating n adjustable switching frequency: 200khz to 2.2mhz n integrated boost and catch diodes n 350ma output current n fixed output voltages: 3.3v , 3.42v, 5v 1.8a i q at 12v in n accurate 1v enable pin threshold n low shutdown current: i q = 0.7a n internal sense limits catch diode current n power good flag n output voltage: 1.21v to 25v n internal compensation n small 10-pin msop and (3mm 2mm) dfn packages n automotive battery regulation n power for portable products n industrial supplies v in boost lt3970 sw en pg rt 0.22f 22pf 22f 2.2f v in 6v to 40v v out 5v 350ma 1m 316k 226k f = 600khz 22h bd fb gnd off on 3490 ta01a effciency load current (ma) 50 efficiency (%) power loss (mw) 70 90 40 60 80 0.01 1 10 100 3970 ta01b 1 10 100 1000 0.01 0.1 0.1 v in = 12v
lt3970 series 2 3970fb v in , en voltage ......................................................... 40v boost pin voltage ................................................... 55v boost pin above sw pin ......................................... 30v fb/v out , rt voltage .................................................... 6v pg, bd voltage ......................................................... 30v (note 1) o r d er i n f orma t ion lead free finish tape and reel part marking* package description temperature range lt3970eddb#pbf lt3970eddb#trpbf lfcz 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970iddb#pbf lt3970iddb#trpbf lfcz 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970ems#pbf lt3970ems#trpbf ltfdb 10-lead plastic msop C40c to 125c lt3970ims#pbf lt3970ims#trpbf ltfdb 10-lead plastic msop C40c to 125c lt3970eddb-3.3#pbf lt3970eddb-3.3#trpbf lfqh 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970iddb-3.3#pbf lt3970iddb-3.3#trpbf lfqh 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970ems-3.3#pbf lt3970ems-3.3#trpbf ltfqg 10-lead plastic msop C40c to 125c lt3970ims-3.3#pbf lt3970ims-3.3#trpbf ltfqg 10-lead plastic msop C40c to 125c lt3970hms-3.3#pbf lt3970hms-3.3#trpbf ltfqg 10-lead plastic msop C40c to 150c lt3970eddb-3.42#pbf lt3970eddb-3.42#trpbf lggg 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970eddb-5#pbf lt3970eddb-5#trpbf lfqf 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970iddb-5#pbf lt3970iddb-5#trpbf lfqf 10-lead (3mm 2mm) plastic dfn C40c to 125c lt3970ems-5#pbf lt3970ems-5#trpbf ltfqd 10-lead plastic msop C40c to 125c lt3970ims-5#pbf lt3970ims-5#trpbf ltfqd 10-lead plastic msop C40c to 125c lt3970hms-5#pbf lt3970hms-5#trpbf ltfqd 10-lead plastic msop C40c to 150c consult ltc marketing for parts specifed with wider operating temperature ranges. *the temperature grade is identifed by a label on the shipping container. consult ltc marketing for information on non-standard lead based fnish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel specifcations, go to: http://www.linear.com/tapeandreel/ a bsolu t e m aximum r a t ings top view 11 gnd ddb package 10-lead (3mm 2mm) plastic dfn *fb/v out en v in gnd gnd rt pg bd boost sw 6 8 7 9 10 5 4 2 3 1 ja = 76c/w exposed pad (pin 11) is gnd, must be soldered to pcb 1 2 3 4 5 *fb/v out en v in gnd gnd 10 9 8 7 6 rt pg bd boost sw top view ms package 10-lead plastic msop ja = 100c/w *fb for lt3970, v out for lt3970-3.3, lt3970-3.42, lt3970-5. p in c on f igura t ion operating junction temperature range (note 2) e-, i-grade .......................................... C40c to 125c h-grade .............................................. C40c to 150c storage t emperature range ................... C65c to 150c lead t emperature (soldering, 10 sec) ms only ............................................................ 300c
lt3970 series 3 3970fb e lec t rical c harac t eris t ics the l denotes the specifcations which apply over the full operating temperature range, otherwise specifcations are at t a = 25c. v in = 12v, v bd = 3.3v unless otherwise noted. (note 2) parameter conditions min typ max units minimum input voltage l 4 4.2 v quiescent current from v in v en low v en high v en high, C40c to 125c v en high, C40c to 150c l l 0.7 1.7 1.2 2.7 3.5 4 a a a a lt3970 feedback voltage l 1.195 1.185 1.21 1.21 1.225 1.235 v v lt3970-3.3 output voltage C40c to 125c C40c to 150c l l 3.26 3.234 3.217 3.3 3.3 3.3 3.34 3.366 3.366 v v v lt3970-3.42 output voltage C40c to 125c l 3.379 3.352 3.42 3.42 3.461 3.488 v v lt3970-5 output voltage C40c to 125c C40c to 150c l l 4.94 4.9 4.875 5 5 5 5.06 5.1 5.1 v v v lt3970 fb pin bias current (note 3) v fb = 1.21v l 0.1 20 na fb/output voltage line regulation 4.2v < v in < 40v 0.0002 0.01 %/v switching frequency r t = 41.2k, v in = 6v r t = 158k, v in = 6v r t = 768k, v in = 6v 1.76 640 160 2.25 800 200 2.64 960 240 mhz khz khz switch current limit v in = 5v, v fb = 0v 535 700 865 ma catch schottky current limit v in = 5v 350 400 500 ma switch v cesat i sw = 200ma 175 mv switch leakage current 0.05 2 a catch schottky forward voltage i sch = 100ma, v in = v bd = nc 650 mv catch schottky reverse leakage v sw = 12v 0.05 2 a boost schottky forward voltage i sch = 50ma, v in = nc, v boost = 0v 875 mv boost schottky reverse leakage v reverse = 12v 0.02 2 a minimum boost voltage (note 4) v in = 5v l 1.4 1.8 v boost pin current i sw = 200ma, v boost = 15v 7 10 ma en pin current v en = 12v 1 30 na lt3970 en voltage threshold en rising, v in 4.2v l 0.94 1 1.06 v lt3970-x en voltage threshold en rising, v in 4.2v l 0.93 1 1.07 v en voltage hysteresis 30 mv lt3970 pg threshold offset from feedback voltage v fb rising 80 120 160 mv lt3970 pg hysteresis 12 mv lt3970-x pg threshold offset from output voltage v fb rising 6.5 10 13.5 % lt3970-x pg hysteresis as % of output voltage 1.0 % pg leakage v pg = 3v 0.01 1 a pg sink current v pg = 0.4v l 30 80 a minimum switch on-time 90 ns minimum switch off-time v in = 10v l 100 160 ns
lt3970 series 4 3970fb typical p er f ormance c harac t eris t ics effciency, v out = 3.3v effciency, v out = 5v lt3970 feedback voltage t a = 25c, unless otherwise noted. load current (ma) 50 efficiency (%) 70 90 40 60 80 0.01 1 10 100 3970 g01 30 20 0.1 v in = 12v front page application v out = 3.3v r1 = 1m r2 = 576k v in = 36v v in = 24v load current (ma) 50 efficiency (%) 70 90 40 60 80 0.01 1 10 100 3970 g02 30 0.1 v in = 12v front page application v in = 36v v in = 24v temperature (c) ?50 feedback voltage (v) 1.210 1.215 1.220 25 75 150 3970 g03 1.205 1.200 1.195 ?25 0 50 100 125 e lec t rical c harac t eris t ics 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 lt3970e is guaranteed to meet performance specifcations from 0c to 125c junction temperature. specifcations over the C40c to 125c operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. the lt3970i is guaranteed over the full C40c to 125c operating junction temperature range. the lt3970h is guaranteed over the full C40c to 150c operating junction temperature range. high junction temperatures degrade operating lifetimes. operating lifetime is derated at junction temperatures greater than 125c. note 3: bias current fows into the fb pin. note 4: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. lt3970-5 output voltage lt3970-3.3 output voltage lt3970-3.42 output voltage temperature (c) ?50 output voltage (v) 3.30 3.31 3.32 25 75 150 3970 g04 3.29 3.28 3.27 ?25 0 50 100 125 temperature (c) ?50 output voltage (v) 3.42 3.43 3.44 25 75 125 3970 g05 3.41 3.40 3.39 ?25 0 50 100 temperature (c) ?50 output voltage (v) 5.00 5.02 5.04 25 75 150 3970 g06 4.98 4.96 4.94 ?25 0 50 100 125
lt3970 series 5 3970fb no-load supply current no-load supply current maximum load current maximum load current maximum load current load regulation typical p er f ormance c harac t eris t ics switch current limit switch current limit switching frequency t a = 25c, unless otherwise noted. input voltage (v) supply current (a) 1.5 2.0 2.5 3.0 10 20 30 40 3970 g07 3.5 4.0 5 15 25 35 front page application v out = 3.3v r1 = 1m r2 = 576k lt3970-5 lt3970-3.3 temperature (c) ?50 supply current (a) 9 12 15 25 75 150 3970 g08 6 3 0 ?25 0 50 100 125 front page application v in = 12v v out = 3.3v r1 = 1m r2 = 576k input voltage (v) 5 350 load current (ma) 400 450 500 550 10 15 20 25 3870 g09 30 35 40 front page application v out = 3.3v typical minimum input voltage (v) 5 350 load current (ma) 400 450 500 600 550 10 15 20 25 3870 g10 30 35 40 front page application v out = 5v typical minimum temperature (c) ?50 load current (ma) 300 400 500 600 25 75 3970 g11 200 100 0 ?25 0 50 100 150125 limited by maximum junction temperature; ja = 76c/w front page application v in = 12v v out = 5v limited by current limit h grade load current (ma) 0 load regulation (%) 0.15 150 3970 g12 0 ?0.10 50 100 200 ?0.15 ?0.20 0.20 0.10 0.05 ?0.05 250 300 350 front page application referenced from v out at 100ma load duty cycle (%) 0 200 switch current limit (ma) 300 400 500 600 700 800 20 40 60 80 3970 g13 100 switch peak current limit catch diode valley current limit temperature (c) ?50 200 switch current limit (ma) 300 400 500 600 0 50 100 150 3970 g14 700 800 ?25 25 75 125 switch peak current limit catch diode valley current limit temperature (c) ?50 0 frequency (mhz) 0.4 0.8 1.2 1.6 2.4 ?25 0 25 50 75 3970 g15 100 125 150 2.0 0.2 0.6 1.0 1.4 2.2 1.8
lt3970 series 6 3970fb typical p er f ormance c harac t eris t ics boost diode forward voltage catch diode forward voltage catch diode leakage t a = 25c, unless otherwise noted. boost pin current minimum input voltage, v out = 3.3v minimum input voltage, v out = 5v switch v cesat (i sw = 200ma) vs temperature switch v cesat minimum switch on-time/switch off-time temperature (c) ?50 0 switch on-time/switch off-time (ns) 20 60 80 100 200 140 0 50 75 3970 g16 40 160 180 120 ?25 25 100 125 150 minimum off-time load current = 175ma minimum on-time temperature (c) ?50 100 switch v cesat (mv) 150 200 250 ?25 0 25 50 3970 g17 75 100 125 150 switch current (ma) 0 switch v cesat (mv) 300 400 400 3970 g18 200 100 0 100 200 300 500 switch current (ma) 0 10 12 14 16 400 3970 g19 8 6 100 200 300 500 4 2 boost pin current (ma) load current (ma) 0 50 2.5 input voltage (v) 3.5 5.0 100 200 250 3970 g20 3.0 4.5 4.0 150 300 350 front page application v out = 3.3v to start/run load current (ma) 0 50 4.0 input voltage (v) 5.0 6.5 100 200 250 3970 g21 4.5 6.0 5.5 150 300 350 front page application v out = 5v to start to run boost diode current (ma) 0 0 boost diode v f (v) 0.2 0.4 0.6 0.8 1.0 1.2 50 100 150 200 3970 g22 ?50c 25c 125c 150c catch diode current (ma) 0 catch diode v f (v) 0.6 0.8 1.0 400 3970 g23 0.4 0.2 0 100 200 300 ?50c 25c 125c 150c temperature (c) ?50 catch diode leakage (a) 12 16 20 25 75 150 3970 g24 8 4 0 ?25 0 50 100 125 v r = 12v
lt3970 series 7 3970fb power good threshold en threshold transient load response; load current is stepped from 100ma to 200ma switching waveforms, burst mode operation transient load response; load current is stepped from 10ma (burst mode operation) to 110ma switching waveforms, full frequency continuous operation typical p er f ormance c harac t eris t ics t a = 25c, unless otherwise noted. temperature (c) ?50 88 threshold (%) 89 90 91 92 ?25 0 25 50 3970 g25 75 100 125 150 temperature (c) ?50 0.950 threshold voltage (v) 0.975 1.000 1.025 1.050 ?25 0 25 50 3970 g26 75 100 125 150 v out 100mv/div i l 100ma/div 100s/div front page application v in = 12v v out = 5v 3970 g27 v out 100mv/div i l 100ma/div 100s/div front page application v in = 12v v out = 5v 3970 g28 v out 5mv/div v sw 5v/div i l 100ma/div 2s/div front page application v in = 12v v out = 5v i load = 10ma 3970 g29 v out 5mv/div v sw 5v/div i l 200ma/div 1s/div front page application v in = 12v v out = 5v i load = 350ma 3970 g30
lt3970 series 8 3970fb p in func t ions fb (pin 1, lt3970 only): the lt3970 regulates the fb pin to 1.21v. connect the feedback resistor divider tap to this pin. v out (pin 1, lt3970-x only): the lt3970-3.3, lt3970-3.42 and lt3970-5 regulate the v out pin to 3.3v, 3.42v and 5v respectively. this pin connects to the internal feedback divider that programs the fxed output voltage. en (pin 2): the part is in shutdown when this pin is low and active when this pin is high. the hysteretic threshold voltage is 1v going up and 0.97v going down. tie to v in if shutdown feature is not used. the en threshold is ac- curate only when v in is above 4.2v. if v in is lower than 4.2v, ground en to place the part in shutdown. v in (pin 3): the v in pin supplies current to the lt3970s internal circuitry and to the internal power switch. this pin must be locally bypassed. gnd (pins 4, 5, exposed pad (pin 11, dfn only)): ground. must be soldered to pcb. sw (pin 6): the sw pin is the output of an internal power switch. connect this pin to the inductor. boost (pin 7): this pin is used to provide a drive volt- age, higher than the input voltage, to the internal bipolar npn power switch. bd (pin 8): this pin connects to the anode of the boost diode. this pin also supplies current to the lt3970s internal regulator when bd is above 3.2v. pg (pin 9): the pg pin is the open-drain output of an internal comparator. pg remains low until the fb pin is within 10% of the fnal regulation voltage. pg is valid when v in is above 4.2v and en is high. rt (pin 10): a resistor is tied between rt and ground to set the switching frequency. b lock diagram r switch latch d boost d catch boost oscillator 200khz to 2.2mhz slope comp s q ? + ? + ? + burst mode detect error amp 1.09v shdn en 1v c1 v in internal 1.21v ref ? + 2 rt r t 10 pg 9 fb lt3970 only * lt3970-3.3: r1 = 12.65m, r2 = 7.35m lt3970-3.42: r1 = 12.65m, r2 = 6.93m lt3970-5: r1 = 15.15m, r2 = 4.85m r2 r1 r2 r1 v c 1 gnd (4, 5) v in 3 7 bd 8 sw v out 6 c2 c3 3990 bd l1 v out lt3970-x only* 1
lt3970 series 9 3970fb o pera t ion the lt3970 is a constant frequency, current mode step- down regulator. an oscillator, with frequency set by rt, sets an rs fip-fop, turning on the internal power switch. an amplifer and comparator monitor the current fowing between the v in and sw pins, turning the switch off when this current reaches a level determined by the voltage at v c (see block diagram). an error amplifer measures the output voltage through an external resistor divider tied to the fb pin and servos the v c node. if the error amplifers output increases, more current is delivered to the output; if it decreases, less current is delivered. another comparator monitors the current fowing through the catch diode and reduces the operating frequency when the current exceeds the 400ma bottom current limit. this foldback in frequency helps to control the output current in fault conditions such as a shorted output with high input voltage. maximum deliverable current to the output is therefore limited by both switch current limit and catch diode current limit. an internal regulator provides power to the control cir - cuitry. the bias regulator normally draws power from the v in pin, but if the bd pin is connected to an external voltage higher than 3.2v, bias power will be drawn from the external source (typically the regulated output voltage). this improves effciency. if the en pin is low, the lt3970 is shut down and draws 0.7a from the input. when the en pin exceeds 1v, the switching regulator will become active. the switch driver operates from either v in or from the boost pin. an external capacitor is 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 effcient operation. to further optimize effciency, the lt3970 automatically switches to burst mode operation in light load situations. between bursts, all circuitry associated with controlling the output switch is shut down reducing the input supply current to 1.7a. the lt3970 contains a power good comparator which trips when the fb pin is at 90% of its regulated value. the pg output is an open-drain transistor that is off when the output is in regulation, allowing an external resistor to pull the pg pin high. power good is valid when the lt3970 is enabled and v in is above 4.2v.
lt3970 series 10 3970fb a pplica t ions i n f orma t ion 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 1.21 ? 1 ? ? ? ? ? ? reference designators refer to the block diagram. note that choosing larger resistors will decrease the quiescent current of the application circuit. setting the switching frequency the lt3970 uses a constant frequency pwm architecture that can be programmed to switch from 200khz to 2.2mhz by using a resistor tied from the rt pin to ground. a table showing the necessary r t value for a desired switching frequency is in table 1. table 1. switching frequency vs r t value switching frequency (mhz) r t value (k) 0.2 0.3 0.4 0.5 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 768 499 357 280 226 158 124 100 80.6 68.1 57.6 49.9 42.2 operating frequency trade-offs selection of the operating frequency is a trade-off between effciency, 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 effciency, 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 out + v d t on(min) v in ? v sw + v d ( ) where v in is the typical input voltage, v out is the output voltage, v d is the integrated catch diode drop (~0.7v), and v sw is the internal switch drop (~0.5v at max load). this equation shows that slower switching frequency is necessary to accommodate high v in /v out ratio. lower frequency also allows a lower dropout voltage. the input voltage range depends on the switching frequency because the lt3970 switch has fnite minimum on and off times. the switch can turn on for a minimum of ~150ns and turn off for a minimum of ~160ns (note that the minimum on-time is a strong function of temperature). this means that the minimum and maximum duty cycles are: dc min = f sw ? t on(min) dc max = 1 C 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 (~160ns). 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 minimum input voltage is determined by either the lt3970s minimum operating voltage of 4.2v or by its maximum duty cycle (as explained in previous section). the minimum input voltage due to duty cycle is: v in(min) = v out + v d 1? f sw ? t off(min) ? v d + v sw where v in(min) is the minimum input voltage, v out is the output voltage, v d is the catch diode drop (~0.7v), v sw is the internal switch drop (~0.5v at max load), f sw is the switching frequency (set by rt), and t off(min) is the minimum switch off-time (160ns). note that higher switch- ing frequency will increase the minimum input voltage. if a lower dropout voltage is desired, a lower switching frequency should be used.
lt3970 series 11 3970fb a pplica t ions i n f orma t ion the highest allowed v in during normal operation (v in(op - max) ) is limited by minimum duty cycle and can be calculated by the following equation: v in(op-max) = v out + v d f sw ? t on(min) ? v d + v sw where t on(min) is the minimum switch on-time (~150ns). however, the circuit will tolerate inputs up to the absolute maximum ratings of the v in and boost pins, regardless of chosen switching frequency. during such transients where v in is higher than v in(op-max) , the switching frequency will be reduced below the programmed frequency to prevent damage to the part. the output voltage ripple and inductor current ripple may also be higher than in typical operation, however the output will still be in regulation. inductor selection for a given input and output voltage, the inductor value and switching frequency will determine the ripple current. the ripple current increases with higher v in or v out and decreases with higher inductance and faster switching frequency. a good starting point for selecting the induc - tor value is: l = 3 v out + v d f sw where v d is the voltage drop of the catch diode (~0.7v), l is in h and f sw is in mhz. 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 500ma. to keep the effciency high, the series resistance (dcr) should be less than 0.1, and the core material should be intended for high frequency ap- plications. table 2 lists several vendors and suitable types. this simple design guide will not always result in the optimum inductor selection for a given application. as a general rule, lower output voltages and higher switching frequency will require smaller inductor values. if the ap- plication requires less than 350ma load current, then a lesser inductor value may be acceptable. this allows use of a physically smaller inductor, or one with a lower dcr resulting in higher effciency. 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 for several popular output voltages. low inductance may result in discontinuous mode operation, which is acceptable but reduces maximum load current. for details of maximum output current and discontinu- ous mode operation, 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 application note 19. input capacitor bypass the input of the lt3970 circuit with a ceramic capacitor of x7r or x5r type. y5v types have poor performance over temperature and applied voltage, and should not be used. a 1f to 4.7f ceramic capacitor is adequate to bypass the lt3970 and will easily handle the ripple current. note that larger input capacitance is required table 2. inductor vendors vendor url coilcraft www.coilcraft.com sumida www.sumida.com toko www.tokoam.com wrth elektronik www.we-online.com coiltronics www.cooperet.com murata www.murata.com
lt3970 series 12 3970fb a pplica t ions i n f orma t ion when a lower switching frequency is used (due to longer on-times). if the input power source has high impedance, or there is signifcant inductance due to long wires or cables, additional bulk capacitance may be necessary. this can be provided with a low 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 lt3970 and to force this very high frequency switching current into a tight local loop, minimizing emi. a 1f capacitor is capable of this task, but only if it is placed close to the lt3970 (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt3970. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3970 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, pos - sibly exceeding the lt3970s 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. it stores energy in order to satisfy transient loads and stabilize the lt3970s control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. a good starting value is: c out = 50 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 combined with a phase lead capacitor (typically 22pf) between the output and the feedback pin. a lower value of output capacitor can be used to save space and cost but transient performance will suffer. the second function is that the output capacitor, along with the inductor, flters the square wave generated by the lt3970 to produce the dc output. in this role it determines the output ripple, so low impedance (at the switching frequency) is important. the output ripple decreases with increasing output capacitance, down to approximately 1mv. see figure 1. note that a larger phase lead capacitor should be used with a large output capacitor. c out (f) 0 0 worst-case output ripple (mv) 2 6 8 10 40 80 100 18 3970 f01 4 20 60 12 14 16 front page application c lead = 47pf for c out 47f v in = 24v v in = 12v when choosing a capacitor, look carefully through the data sheet to fnd 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. table 3 lists several capacitor vendors. table 3. recommended ceramic capacitor vendors manufacturer website avx www.avxcorp.com murata www.murata.com taiyo yuden www.t-yuden.com vishay siliconix www.vishay.com tdk www.tdk.com ceramic capacitors ceramic capacitors are small, robust and have very low esr. however, ceramic capacitors can cause problems when used with the lt3970 due to their piezoelectric nature. when in burst mode operation, the lt3970s switching frequency depends on the load current, and at very light loads the lt3970 can excite the ceramic capacitor at audio frequencies, generating audible noise. since the lt3970 operates at a lower current limit during burst mode figure 1. worst-case output ripple across full load range
lt3970 series 13 3970fb operation, the noise is typically very quiet to a casual ear. if this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. a fnal precaution regarding ceramic capacitors concerns the maximum input voltage rating of the lt3970. as pre - viously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3970 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt3970s rating. this situation is easily avoided (see the hot plugging safely section). low ripple burst mode operation to enhance effciency at light loads, the lt3970 operates in low ripple burst mode operation which keeps the output capacitor charged to the proper voltage while minimizing the input quiescent current. during burst mode opera- tion, the lt3970 delivers single cycle bursts of current to the output capacitor followed by sleep periods where the output power is delivered to the load by the output capaci- tor. because the lt3970 delivers power to the output with single, low current pulses, the output ripple is kept below 5mv for a typical application. see figure 2. as the load current decreases towards a no load condition, the percentage of time that the lt3970 operates in sleep mode increases and the average input current is greatly reduced resulting in high effciency even at very low loads. note that during burst mode operation, the switching frequency will be lower than the programmed switching frequency. see figure 3. at higher output loads (above ~45ma for the front page application) the lt3970 will be running at the frequency programmed by the r t resistor, and will be operating in standard pwm mode. the transition between pwm and low ripple burst mode is seamless, and will not disturb the output voltage. boost and bd pin considerations capacitor c3 and the internal boost schottky diode (see the block diagram) are used to generate a boost voltage that is higher than the input voltage. in most cases a 0.22f capacitor will work well. figure 4 shows two ways to ar - range the boost circuit. the boost pin must be more than 1.9v above the sw pin for best effciency. for outputs of 2.2v and above, the standard circuit (figure 4a) is best. for outputs between 2.2v and 2.5v, use a 0.47f boost capacitor. for output voltages below 2.2v, the boost diode can be tied to the input (figure 4b), or to another external supply greater than 2.2v. however, the circuit in figure 4a is more effcient because the boost pin current and bd pin quiescent current come from a lower voltage source. also, be sure that the maximum voltage ratings of the boost and bd pins are not exceeded. the minimum operating voltage of an lt3970 application is limited by the minimum input voltage (4.2v) and by the maximum duty cycle as outlined in a previous section. for output voltages greater than 3.4v, the minimum input volt - age is also limited by the boost circuit for proper start-up. a pplica t ions i n f orma t ion figure 2. burst mode operation figure 3. switching frequency in burst mode operation v out 5mv/div v sw 5v/div i l 100ma/div 2s/div front page application v in = 12v v out = 5v i load = 10ma 3970 f02 load current (ma) 0 400 500 700 150 250 3070 f03 300 200 50 100 200 300 350 100 0 600 switching frequency (khz) front page application
lt3970 series 14 3970fb if the input voltage is ramped slowly, the boost capacitor may not be fully charged. because 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 voltages, 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. enable pin the lt3970 is in shutdown when the en pin is low and active when the pin is high. the rising threshold of the en comparator is 1v, with a 30mv hysteresis. this threshold is accurate when v in is above 4.2v. if v in is lower than 4.2v, tie en pin to gnd to place the part in shutdown. a pplica t ions i n f orma t ion figure 4. two circuits for generating the boost voltage figure 5. the minimum input voltage depends on output voltage, load current and boost circuit adding a resistor divider from v in to en programs the lt3970 to regulate the output only when v in is above a desired voltage (see figure 6). this threshold voltage, v in(en) , can be adjusted by setting the values r3 and r4 such that they satisfy the following equation: v in(en) = r3 + r4 r4 ? 1v where output regulation should not start until v in is above v in(en) . note that due to the comparators hysteresis, regulation will not stop until the input falls slightly below v in(en) . bd lt3970 (4a) for v out 2.2v boost v in v in c3 v out sw gnd bd lt3970 (4b) for v out < 2.2v; v in < 27v boost v in v in c3 3970 f04 v out sw gnd load current (ma) 0 50 2.5 input voltage (v) 3.5 5.0 100 200 250 3.0 4.5 4.0 150 300 350 front page application v out = 3.3v to start/run load current (ma) 0 50 4.0 input voltage (v) 5.0 6.5 100 200 250 3970 f05 4.5 6.0 5.5 150 300 350 front page application v out = 5v to start to run
lt3970 series 15 3970fb a pplica t ions i n f orma t ion be aware that while v in is below 4.2v, the input current may rise up to several hundred a and the part may begin to switch while the internal circuitry starts up. figure 7 shows the startup behavior of a typical application with different programmed v in(en) . shorted and reversed input protection if the inductor is chosen so that it wont saturate exces- sively, a lt3970 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 lt3970 is absent. this may occur in battery charging ap - plications or in battery backup systems where a battery or some other supply is diode ored with the lt3970s output. if the v in pin is allowed to foat and the en pin is held high (either by a logic signal or because it is tied to v in ), then the lt3970s internal circuitry will pull its quiescent current through its sw pin. this is fne if the system can tolerate a few a in this state. if the en pin is grounded, the sw pin current will drop to 0.7a. however, if the v in pin is grounded while the output is held high, regardless of en, parasitic diodes inside the lt3970 can pull current from the output through the sw pin and the v in pin. figure 8 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. figure 8. 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 lt3970 runs only when the input is present 160 120 input current (a) 80 40 0 4 input voltage (v) 3 output voltage (v) 2 1 0 160 120 input current (a) 80 40 0 4 3 output voltage (v) 2 1 0 3970 f07 v in(en) = 6v r3 = 5m r4 = 1m v in(en) = 12v r3 = 11m r4 = 1m 0 1 2 3 4 5 6 7 8 input voltage (v) 0 2 4 6 8 10 12 14 figure 7. v in start-up of front page application with v out = 3.3v, no-load current, and v in(en) programmed as in figure 6. bd lt3970 boost v in en v in v out backup 3970 f08 sw d4 mbrs140 fb gnd + figure 6. enable + ? 1v shdn 3970 f06 lt3970 en v in v in r3 r4
lt3970 series 16 3970fb a pplica t ions i n f orma t ion pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 9 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents fow in the lt3970s v in and sw pins, the internal catch diode and the input capacitor. 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. the sw and boost nodes should be as small as possible. finally, keep the fb nodes small so that the ground traces will shield them from the sw and boost nodes. the exposed pad on the bottom of the dfn 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 lt3970 to additional ground planes within the circuit board and on the bottom side. figure 9. a good pcb layout ensures proper, low emi operation 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 lt3970 can ring to twice the nominal input voltage, possibly exceeding the lt3970s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt3970 into an energized supply, the input network should be designed to prevent this overshoot. see linear technology application note 88 for a complete discussion. high t emperature considerations for higher ambient temperatures, care should be taken in the layout of the pcb to ensure good heat sinking of the lt3970. the exposed pad on the bottom of the dfn package must be soldered to a ground plane. this ground should be tied to large copper layers below with thermal vias; these layers will spread the heat dissipated by the lt3970. placing additional vias can reduce thermal resistance further. in the msop package, the copper lead frame is fused to gnd (pin 5) so place thermal vias near this pin. the maximum load current should be derated as the ambient temperature approaches the maximum junction rating. power dissipation within the lt3970 can be estimated by calculating the total power loss from an effciency measure- ment and subtracting inductor loss. the die temperature is calculated by multiplying the lt3970 power dissipation by the thermal resistance from junction to ambient. finally, be aware that at high ambient temperatures the internal schottky diode will have signifcant leakage current (see typical performance characteristics) increasing the quiescent current of the lt3970 converter. 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. 6 8 7 9 10 5 4 2 3 1 vias to local ground plane vias to v out en gnd gnd pg v out gnd v in 3970 f09 hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt3970 circuits. however, these ca - pacitors can cause problems if the lt3970 is plugged into a live supply. the low loss ceramic capacitor, combined
lt3970 series 17 3970fb typical a pplica t ions 3.3v step-down converter v in boost lt3970 sw en pg rt c3 0.22f 22pf c2 22f c1 2.2f v in 4.2v to 40v v out 3.3v 350ma r1 1m r2 576k 226k f = 600khz l1 22h bd fb gnd off on 3490 ta02 5v step-down converter 2.5v step-down converter v in boost lt3970 sw en pg rt c3 0.22f 22pf 226k f = 600khz c2 22f c1 2.2f v in 6v to 40v v out 5v 350ma r1 1m r2 316k l1 22h bd fb gnd off on 3490 ta03 v in boost lt3970 sw en pg rt c3 0.47f 47pf 226k f = 600khz c2 47f c1 2.2f v in 4.2v to 40v v out 2.5v 350ma r1 1m r2 931k l1 15h bd fb gnd off on 3490 ta04 1.8v step-down converter v in boost lt3970 sw en bd pg rt c3 0.22f 47pf 226k f = 600khz c2 47f c1 2.2f v in 4.2v to 27v v out 1.8v 350ma r1 487k r2 1m l1 10h fb gnd off on 3490 ta05 3.3v step-down converter 5v step-down converter v in boost lt3970-3.3 sw en pg rt 0.22f 226k f = 600khz 22f 2.2f v in 4.2v to 40v v out 3.3v 350ma 22h bd v out gnd off on 3490 ta03a v in boost lt3970-5 sw en pg rt 0.22f 226k f = 600khz 22f 2.2f v in 6v to 40v v out 5v 350ma 22h bd v out gnd off on 3490 ta03b
lt3970 series 18 3970fb t ypical applica t ions 12v step-down converter 5v, 2mhz step-down converter v in boost lt3970 sw en pg rt c3 0.1f 22pf 226k f = 600khz c2 22f c1 2.2f v in 14v to 40v v out 12v 350ma r1 1m r2 113k l1 33h bd fb gnd off on 3490 ta06 v in boost lt3970 sw en pg rt 49.9k f = 2mhz c3 0.1f 22pf c2 10f c1 1f v in 8.5v to 16v transients to 40v v out 5v 350ma r1 1m r2 316k l1 10h bd fb gnd off on 3490 ta07 5v step-down converter with reduced input current during start-up v in boost lt3970 sw en pg rt 0.22f 22pf 22f 2.2f v in 6v to 40v v out 5v 350ma 1m 316k 226k 5m k f = 600khz 22h bd fb gnd 3490 ta08a 1m + ? input current during start-up input voltage (v) 0 ?0.5 input current (ma) 0.5 1.5 2.5 2 4 6 8 3970 ta08b 10 3.5 4.5 0 1.0 2.0 3.0 4.0 12 input current dropout conditions front page application front page application with en programmed to 6v start-up from high impedance input source v out 2v/div v in 5v/div 5ms/div front page application v out = 5v 1k input source resistance 2.5ma load 3970 ta08c en programmed to 6v
lt3970 series 19 3970fb p ackage descrip t ion ddb package 10-lead plastic dfn (3mm 2mm) (reference ltc dwg # 05-08-1722 rev ?) 2.00 0.10 (2 sides) note: 1. drawing conforms to version (wecd-1) in jedec package outline m0-229 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 0.64 0.05 (2 sides) 0.75 0.05 r = 0.115 typ r = 0.05 typ 2.39 0.05 (2 sides) 3.00 0.10 (2 sides) 1 5 10 6 pin 1 bar top mark (see note 6) 0.200 ref 0 ? 0.05 (ddb10) dfn 0905 rev ? 0.25 0.05 2.39 0.05 (2 sides) recommended solder pad pitch and dimensions 0.64 0.05 (2 sides) 1.15 0.05 0.70 0.05 2.55 0.05 package outline 0.25 0.05 0.50 bsc pin 1 r = 0.20 or 0.25 45 chamfer 0.50 bsc please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
lt3970 series 20 3970fb p ackage descrip t ion msop (ms) 0307 rev e 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 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 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 0.305 0.038 (.0120 .0015) typ 0.50 (.0197) bsc 0.1016 0.0508 (.004 .002) ms package 10-lead plastic msop (reference ltc dwg # 05-08-1661 rev e) please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
lt3970 series 21 3970fb 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. r evision h is t ory rev date description page number a 5/10 added lt3970-3.3 and lt3970-5 1 - 22 b 3/12 title and features clarifed to add 3.42v fxed output version. clarifed the absolute maximum ratings section, added 3.42v output option in the order information section. added 3.42v output option in the electrical characteristics table. added 3.42v output voltage vs temperature graph. clarifed v out pin function and block diagram. 1 2 3 4 8
lt3970 series 22 3970fb linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2009 lt 0312 rev b ? printed in usa r ela t e d p ar t s part number description comments lt3971 38v, 1.2a (i out ), i q = 2.8a high effciency step-down dc/dc converter v in = 4.3v to 38v, v out(min) = 1.2v, i q = 2.8a, i sd < 1a, msop-10e, 3mm 3mm dfn-10 LT3991 55v, 1.2a (i out ), i q = 2.8a high effciency step-down dc/dc converter v in = 4.3v to 55v, v out(min) = 1.2v, i q = 2.8a, i sd < 1a, msop-10e, 3mm 3mm dfn-10 lt3689 36v, 60v transient protection, 800ma, 2.2mhz high effciency micropower step-down dc/dc converter with por reset and watchdog timer v in = 3.6v to 36v, transient to 60v, v out(min) = 0.8v, i q = 75a, i sd < 1a, 3mm 3mm qfn-16 lt3682 36v, 60v max , 1a, 2.2mhz high effciency micropower step-down dc/dc converter v in = 3.6v to 36v, v out(min) = 0.8v, i q = 75a, i sd < 1a, 3mm 3mm dfn-12 lt3480 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high effciency step-down dc/dc converter with burst mode ? operation v in = 3.6v to 38v, v out(min) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn-10, msop-10e lt3685 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high effciency step-down dc/dc converter v in = 3.6v to 38v, v out(min) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn-10, msop-10e lt3481 34v with transient protection to 36v, 2a (i out ), 2.8mhz, high effciency 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, 3mm 3mm dfn-10, msop-10e lt1976/lt1977 60v, 1.2a (i out ), 200/500khz, high effciency step-down dc/dc converter with burst mode operation v in = 3.3v to 60v, v out(min) = 1.20v, i q = 100a, i sd < 1a, tssop-16e

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