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lt3990/lt3990-3.3/lt3990-5 1 3990fa typical application description 62v, 350ma step-down regulator with 2.5a quiescent current and integrated diodes the lt ? 3990 is an adjustable frequency monolithic buck switching regulator that accepts a wide input voltage range up to 62v, 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 accurate programmable undervoltage lockout feature 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 lt3990 is available in small, thermally enhanced 16-pin msop and 3mm 3mm dfn packages. 5v step-down converter features applications 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 62v 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, 5v 2a i q at 12v in n accurate programmable undervoltage lockout n fmea fault tolerant (msop package) output stays at or below regulation voltage during adjacent pin short or when a pin is left floating n low shutdown current: i q = 0.7a n internal sense limits catch diode current n power good flag n small, thermally enhanced 16-pin msop and (3mm 3mm) dfn packages n automotive battery regulation n power for portable products n industrial supplies power loss 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. load current (ma) 0.001 0.01 power loss (mw) 10 100 1000 0.01 0.1 1 10 100 3990 ta01b 1 0.1 v in = 12v v in boost lt3990-5 sw en/uvlo pg rt 0.22f 22f 2.2f v in 6.5v to 62v v out 5v 350ma 374k f = 400khz 33h bd v out gnd off on 3990 ta01a
lt3990/lt3990-3.3/lt3990-5 2 3990fa v in , en/uvlo voltage ............................................... 62v boost pin voltage ................................................... 75v boost pin above sw pin ......................................... 30v fb/v out , rt voltage .................................................... 6v pg, bd voltage ......................................................... 30v (note 1) order information lead free finish tape and reel part marking* package description temperature range lt3990edd#pbf lt3990edd#trpbf lfwj 10-lead (3mm 3mm) plastic dfn C40c to 125c lt3990idd#pbf lt3990idd#trpbf lfwj 10-lead (3mm 3mm) plastic dfn C40c to 125c lt3990emse#pbf lt3990emse#trpbf 3990 16-lead plastic msop C40c to 125c lt3990imse#pbf lt3990imse#trpbf 3990 16-lead plastic msop C40c to 125c lt3990hmse#pbf lt3990hmse#trpbf 3990 16-lead plastic msop C40c to 150c lt3990emse-3.3#pbf lt3990emse-3.3#trpbf 399033 16-lead plastic msop C40c to 125c lt3990imse-3.3#pbf lt3990imse-3.3#trpbf 399033 16-lead plastic msop C40c to 125c lt3990hmse-3.3#pbf lt3990hmse-3.3#trpbf 399033 16-lead plastic msop C40c to 150c lt3990emse-5#pbf lt3990emse-5#trpbf 39905 16-lead plastic msop C40c to 125c lt3990imse-5#pbf lt3990imse-5#trpbf 39905 16-lead plastic msop C40c to 125c lt3990hmse-5#pbf lt3990hmse-5#trpbf 39905 16-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/ absolute maximum ratings top view 11 gnd dd package 10-lead (3mm 3mm) plastic dfn 10 9 6 7 8 4 5 3 2 1 rt pg bd boost sw fb en/uvlo v in gnd gnd ja = 45c/w, jc = 10c/w exposed pad (pin 11) is gnd, must be soldered to pcb 1 2 3 4 5 6 7 8 fb/v out * fb/v out * nc en/uvlo nc v in nc gnd 16 15 14 13 12 11 10 9 rt nc pg bd nc boost nc sw top view 17 gnd mse package 16-lead plastic msop ja = 40c/w, jc = 10c/w exposed pad (pin 17) is gnd, must be soldered to pcb *fb for lt3990, v out for lt3990-3.3, lt3990-5 pin configuration operating junction temperature range (note 2) lt3990e/lt3990e-x ........................... C40c to 125c lt3990i/lt3990i-x ............................ C40c to 125c lt3990h/lt3990h-x .......................... C40c to 150c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) ms only ............................................................ 300c
lt3990/lt3990-3.3/lt3990-5 3 3990fa electrical characteristics 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 lt3990e 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 lt3990i is guaranteed over the full C40c to 125c operating junction temperature range. the lt3990h 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. 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/uvlo low v en/uvlo high v en/uvlo high l 0.7 1.9 1.2 2.8 4 a a a lt3990 feedback voltage l 1.195 1.185 1.21 1.21 1.225 1.235 v v lt3990-3.3 output voltage l 3.26 3.234 3.3 3.3 3.34 3.366 v v lt3990-5 output voltage l 4.94 4.9 5 5 5.06 5.1 v v lt3990 fb pin bias current (note 3) 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 1.84 672 168 2.3 840 210 2.76 1008 252 mhz khz khz switch current limit v in = 5v, v fb = 0v 535 700 865 ma catch schottky current limit v in = 5v 360 450 540 ma switch v cesat i sw = 200ma 210 mv switch leakage current 0.05 2 a catch schottky forward voltage i sch = 100ma, v in = v bd = nc 725 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 900 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 8.5 12 ma en/uvlo pin current v en/uvlo = 12v 1 30 na en/uvlo voltage threshold en/uvlo rising, v in 4.2v l 1.14 1.19 1.28 v en/uvlo voltage hysteresis 35 mv pg threshold offset from feedback voltage v fb rising 6.5 10 13.5 % 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 115 ns minimum switch off-time v in = 10v l 100 160 ns 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.
lt3990/lt3990-3.3/lt3990-5 4 3990fa typical performance characteristics lt3990-3.3 output voltage lt3990-5 output voltage no-load supply current no-load supply current maximum load current maximum load current effciency, v out = 3.3v effciency, v out = 5v lt3990 feedback voltage t a = 25c, unless otherwise noted. load current (ma) 50 efficiency (%) 60 70 80 90 0.01 101 100 3990 g01 10 20 40 30 0.1 v in = 12v v in = 48v front page application v out = 3.3v r1 = 1m r2 = 576k v in = 24v v in = 36v load current (ma) 50 efficiency (%) 60 70 80 90 0.01 101 100 3990 g02 20 40 30 0.1 v in = 12v v in = 48v front page application v in = 36v v in = 24v temperature (c) ?50 feedback voltage (v) 1.210 1.215 1.220 25 75 150 3990 g03 1.205 1.200 1.195 ?25 0 50 100 125 temperature (c) ?50 output voltage (v) 3.30 3.31 3.32 25 75 150 3990 g04 3.29 3.28 3.27 ?25 0 50 100 125 temperature (c) ?50 output voltage (v) 5.00 5.02 5.04 25 75 150 3990 g05 4.98 4.96 4.94 ?25 0 50 100 125 input voltage (v) 5 1.5 supply level (a) 2.0 2.5 3.0 3.5 4.0 15 25 35 45 3990 g06 55 front page application v out = 3.3v r1 = 1m r2 = 576k lt3990-3.3 temperature (c) ?50 supply current (a) 9 12 15 25 75 150 3990 g07 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) 400 450 500 650 600 550 15 25 35 45 3990 g08 55 front page application v out = 3.3v typical minimum input voltage (v) 5 350 load current (ma) 400 450 500 650 600 550 15 25 35 45 3990 g09 55 front page application v out = 5v typical minimum
lt3990/lt3990-3.3/lt3990-5 5 3990fa typical performance characteristics minimum switch on-time/switch off-time switch v cesat (i sw = 200ma) vs temperature switch v cesat boost pin current switch current limit maximum load current load regulation switch current limit switching frequency t a = 25c, unless otherwise noted. temperature (c) ?50 0 load current (a) 100 200 300 400 0 50 100 150 3990 g10 500 600 ?25 25 75 125 limited by current limit h-grade limited by maximum junction temperature ja = 45c/w front page application v in = 12v v out = 5v load current (ma) 0 load regulation (%) 0.15 150 3990 g11 0 ?0.10 50 100 200 ?0.15 ?0.20 0.25 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 3990 g12 100 switch peak current limit catch diode valley current limit temperature (c) ?50 300 switch current limit (ma) 400 500 600 700 0 50 100 150 3990 g13 800 900 ?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 3990 g14 100 125 150 2.0 0.2 0.6 1.0 1.4 2.2 1.8 temperature (c) ?50 0 switch on-time/switch off-time (ns) 25 75 100 125 250 175 0 50 75 3990 g15 50 200 225 150 ?25 25 100 125 150 load current = 175ma minimum on-time minimum off-time temperature (c) ?50 150 switch v cesat (mv) 200 250 300 ?25 0 25 50 3990 g16 75 100 125 150 switch current (ma) 0 0 switch current v cesat (mv) 100 200 300 400 500 600 100 200 300 400 3990 g17 500 switch current (ma) 0 15 18 21 400 3990 g18 12 9 100 200 300 500 6 3 0 boost pin current (ma)
lt3990/lt3990-3.3/lt3990-5 6 3990fa typical performance characteristics power good threshold en/uvlo threshold transient load response; load current is stepped from 100ma to 200ma boost diode forward voltage catch diode forward voltage catch diode leakage t a = 25c, unless otherwise noted. transient load response; load current is stepped from 10ma (burst mode operation) to 110ma minimum input voltage, v out = 3.3v minimum input voltage, v out = 5v load current (ma) 0 50 2.5 input voltage (v) 3.5 5.0 100 200 250 3990 g19 3.0 4.5 4.0 150 300 350 to start to run front page application v out = 3.3v load current (ma) 0 50 4.0 input voltage (v) 5.0 6.5 100 200 250 3990 g20 4.5 6.0 5.5 150 300 350 to start to run front page application f = 600khz 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 3990 g21 ?50c 25c 125c 150c catch diode current (ma) 0 catch diode, v f (v) 0.4 0.6 400 3990 g22 0.2 0 100 200 300 1.0 0.8 ?50c 25c 125c 150c temperature (c) ?50 catch diode leakage (a) 8 12 15 25 75 150 3990 g23 6 3 0 ?25 0 50 100 125 v r = 12v temperature (c) ?50 88 threshold (%) 89 90 91 92 ?25 0 25 50 3990 g24 75 100 125 150 temperature (c) ?50 1.140 threshold voltage (v) 1.165 1.190 1.215 1.240 ?25 0 25 50 3990 g25 75 100 125 150 v out 100mv/div i l 100ma/div 100s/div front page application v in = 12v v out = 5v 3990 g26 v out 100mv/div i l 100ma/div 100s/div front page application v in = 12v v out = 5v 3990 g27
lt3990/lt3990-3.3/lt3990-5 7 3990fa typical performance characteristics switching waveforms, burst mode operation t a = 25c, unless otherwise noted. switching waveforms, full frequency continuous 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 f = 600khz 3990 g28 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 f = 600khz 3990 g29 pin functions fb (pin 1/pins 1, 2 lt3990 only): the lt3990 regulates the fb pin to 1.21v. connect the feedback resistor divider tap to this pin. the two fb pins on the mse package are connected internally and provide a redundant path for the feedback divider. tie the divider to both pins. v out (pins 1, 2, lt3990-x only): the lt3990-3.3 and lt3990-5 regulate the v out pin to 3.3v and 5v, respec - tively. this pin connects to the internal feedback divider that programs the fxed output voltage. the two v out pins are connected internally and provide a redundant path to the output. tie the output to both pins. en/uvlo (pin 2/pin 4): the part is in shutdown when this pin is low and active when this pin is high. the threshold voltage is 1.19v going up with 35mv of hysteresis. tie to v in if shutdown feature is not used. the en/uvlo threshold is accurate only when v in is above 4.2v. if v in is lower than 4.2v, ground en/uvlo to place the part in shutdown. v in (pin 3/pin 6): the v in pin supplies current to the lt3990s internal circuitry and to the internal power switch. this pin must be locally bypassed. gnd (pins 4, 5, exposed pad pin 11/pin 8, exposed pad pin 17): ground. the exposed pad must be soldered to the pcb. sw (pin 6/pin 9): the sw pin is the output of an internal power switch. connect this pin to the inductor. boost (pin 7/pin 11): this pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. bd (pin 8/pin 13): this pin connects to the anode of the boost diode. this pin also supplies current to the lt3990s internal regulator when bd is above 3.2v. pg (pin 9/pin 14): 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/uvlo is high. rt (pin 10/pin 16): a resistor is tied between rt and ground to set the switching frequency. nc (pins 3, 5, 7, 10, 12, 15, msop only): no connects. these pins are not connected to internal circuitry and must be left foating to ensure fault tolerance. (dfn, msop)
lt3990/lt3990-3.3/lt3990-5 8 3990fa block 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/uvlo 1.19v c1 v in internal 1.21v ref ? + rt r t pg fb *lt3990-3.3: r1 = 12.65m, r2 = 7.35m lt3990-5: r1 = 15.15m, r2 = 4.85m lt3990 only lt3990-x only* r2 r1 r1 r2 v c v out gnd v in bd sw v out c2 c3 3990 bd l1
lt3990/lt3990-3.3/lt3990-5 9 3990fa operation the lt3990 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 450ma 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/uvlo pin is low, the lt3990 is shut down and draws 0.7a from the input. when the en/uvlo pin ex - ceeds 1.19v, 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 lt3990 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 1.8a. the lt3990 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 lt3990 is enabled and v in is above 4.2v.
lt3990/lt3990-3.3/lt3990-5 10 3990fa applications information fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the 1% resis - tors according to: r1 = r2 v out 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 lt3990 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 2.2 787 511 374 287 232 169 127 102 84.5 69.8 59 51.1 44.2 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 fre - quency because the lt3990 switch has fnite minimum on and off times. the switch can turn off for a minimum of ~160ns but the minimum on-time is a strong function of temperature. use the minimum switch on-time curve (see typical performance characteristics) to design for an applications maximum temperature, while adding about 30% for part-to-part variation. the minimum and maximum duty cycles that can be achieved taking these on and off times into account 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, 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 lt3990s 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.
lt3990/lt3990-3.3/lt3990-5 11 3990fa applications information if a lower dropout voltage is desired, a lower switching frequency should be used. 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. 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 800ma. 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 applications. 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 lt3990 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 lt3990 and will easily handle the ripple current. note that larger input capacitance is required when a lower switching frequency is used 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
lt3990/lt3990-3.3/lt3990-5 12 3990fa applications information (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 lt3990 and to force this very high frequency switch - ing 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 lt3990 (see the pcb layout section). a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt3990. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3990 circuit is plugged into a live supply, the input volt - age can ring to twice its nominal value, possibly exceeding the lt3990s 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 lt3990s 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 lt3990 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 3990 f01 4 20 60 12 14 16 front page application f = 600khz 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 lt3990 due to their piezoelectric nature. when in burst mode operation, the lt3990s switching frequency depends on the load current, and at very light loads the lt3990 can excite the ceramic capacitor at audio frequencies, generating audible noise. since the lt3990 figure 1. worst-case output ripple across full load range
lt3990/lt3990-3.3/lt3990-5 13 3990fa operates at a lower current limit during burst mode op - eration, 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 lt3990. as pre - viously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt3990 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt3990s rating. this situation is easily avoided (see the hot plugging safely section). low ripple burst mode operation to enhance effciency at light loads, the lt3990 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 lt3990 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 lt3990 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 lt3990 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 ~35ma for the front page application) the lt3990 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 lt3990 application is limited by the minimum input voltage (4.2v) and by the maximum duty cycle as outlined in a previous section. for proper start-up, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, the boost capacitor may not be fully charged. because applications information figure 2. burst mode operation figure 3. switching frequency in burst mode operation load current (ma) 0 50 0 switching frequency (khz) 200 500 100 200 250 3990 f03 100 400 300 150 300 350 front page application 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 f = 600khz 3990 g28
lt3990/lt3990-3.3/lt3990-5 14 3990fa the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. this minimum load will depend on input and output volt - ages, and on the arrangement of the boost circuit. the minimum load generally goes to zero once the circuit has started. figure 5 shows a plot of minimum load to start and to run as a function of input voltage. in many cases the discharged output capacitor will present a load to the switcher, which will allow it to start. the plots show the worst-case situation where v in is ramping very slowly. for lower start-up voltage, the boost diode can be tied to v in ; however, this restricts the input range to one-half of the absolute maximum rating of the boost pin. enable and undervoltage lockout the lt3990 is in shutdown when the en/uvlo pin is low and active when the pin is high. the rising threshold of the en/uvlo comparator is 1.19v, with a 35mv hysteresis. this threshold is accurate when v in is above 4.2v. if v in is lower than 4.2v, tie en/uvlo pin to gnd to place the part in shutdown. figure 6 shows how to add undervoltage lockout (uvlo) to the lt3990. typically, uvlo is used in situations where the input supply is current limited, or has a relatively high applications information figure 4. two circuits for generating the boost voltage figure 5. the minimum input voltage depends on output voltage, load current and boost circuit source resistance. a switching regulator draws constant power from the source, so source current increases as source voltage drops. this looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. uvlo prevents the regulator from operating at source voltages where the problems might occur. the uvlo threshold can be adjusted by setting the values r3 and r4 such that they satisfy the following equation: v uvlo = r3 + r4 r4 ? 1.19v where switching should not start until v in is above v uvlo . note that due to the comparators hysteresis, switching will not stop until the input falls slightly below v uvlo . undervoltage lockout is functional only when v uvlo is greater than 5v. bd lt3990 (4a) for v out 2.2v boost v in v in c3 v out sw gnd bd lt3990 (4b) for v out < 2.2v; v in < 30v boost v in v in c3 3990 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 to start to run front page application v out = 3.3v load current (ma) 0 50 4.0 input voltage (v) 5.0 6.5 100 200 250 3990 f05 4.5 6.0 5.5 150 300 350 to start to run front page application v out = 5v, f = 600khz
lt3990/lt3990-3.3/lt3990-5 15 3990fa applications information shorted and reversed input protection if the inductor is chosen so that it wont saturate exces - sively, a lt3990 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 lt3990 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 lt3990s output. if the v in pin is allowed to foat and the en/uvlo pin is held high (either by a logic signal or because it is tied to v in ), then the lt3990s 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/uvlo 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/uvlo, parasitic diodes inside the lt3990 can pull current from the output through the sw pin and the v in pin. figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 8 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents fow in the lt3990s v in and sw pins, the internal catch diode and the input capacitor. the loop formed by these components should be as small as pos - sible. these components, along with the inductor and figure 7. diode d4 prevents a shorted input from discharging a backup battery tied to the output. it also protects the circuit from a reversed input. the lt3990 runs only when the input is present figure 8. a good pcb layout ensures proper, low emi operation figure 6. undervoltage lockoout + ? 1.19v shdn 3990 f06 lt3990 en/uvlo v in v in r3 r4 bd lt3990 boost v in en/uvlo v in v out backup 3990 f07 sw d4 fb gnd + 6 8 7 9 10 5 4 2 3 1 vias to local ground plane vias to v out en/uvlo gnd gnd pg v out gnd v in 3990 f08 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 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 lt3990 to additional ground planes within the circuit board and on the bottom side.
lt3990/lt3990-3.3/lt3990-5 16 3990fa applications information hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt3990 circuits. however, these ca - pacitors can cause problems if the lt3990 is plugged into a live supply. the low loss ceramic capacitor, combined with stray inductance in series with the power source, forms an under damped tank circuit, and the voltage at the v in pin of the lt3990 can ring to twice the nominal input voltage, possibly exceeding the lt3990s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt3990 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 temperature considerations for higher ambient temperatures, care should be taken in the layout of the pcb to ensure good heat sinking of the lt3990. the exposed pad on the bottom 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 lt3990. placing additional vias can reduce thermal resistance further. the maximum load current should be derated as the ambient temperature approaches the maximum junction rating. power dissipation within the lt3990 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 lt3990 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 lt3990 converter. fault tolerance the lt3990 regulator in the msop package is designed to tolerate single fault conditions. shorting any two adjacent pins together or leaving any one single pin foating does not raise v out above the programmed value or cause damage to the part. the nc pins are not connected to internal circuitry and must be left foating to ensure fault tolerance. 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.
lt3990/lt3990-3.3/lt3990-5 17 3990fa typical applications 3.3v step-down converter 3.3v step-down converter v in boost lt3990 sw en/uvlo pg rt c3 0.22f 22pf c2 22f c1 2.2f v in 4.2v to 62v v out 3.3v 350ma r1 1m r2 576k 374k f = 400khz l1 33h bd fb gnd off on 3990 ta02 5v step-down converter 5v step-down converter 2.5v step-down converter v in boost lt3990 sw en/uvlo pg rt c3 0.22f 22pf 374k f = 400khz c2 22f c1 2.2f v in 6.5v to 62v v out 5v 350ma r1 1m r2 316k l1 33h bd fb gnd off on 3990 ta03 v in boost lt3990 sw en/uvlo pg rt c3 0.47f 47pf 511k f = 300khz c2 47f c1 2.2f v in 4.2v to 62v v out 2.5v 350ma r1 1m r2 931k l1 33h bd fb gnd off on 3990 ta04 1.8v step-down converter v in boost lt3990 sw en/uvlo bd pg rt c3 0.22f 47pf 374k f = 400khz c2 47f c1 2.2f v in 4.2v to 30v v out 1.8v 350ma r1 487k r2 1m l1 22h fb gnd off on 3990 ta05 v in boost lt3990-3.3 sw en/uvlo pg rt 0.22f 22f 2.2f v in 4.2v to 62v v out 3.3v 350ma 374k f = 400khz 33h bd v out gnd off on 3990 ta10 v in boost lt3990-5 sw en/uvlo pg rt 0.22f 22f 2.2f v in 6.5v to 62v v out 5v 350ma 374k f = 400khz 33h bd v out gnd off on 3990 ta11
lt3990/lt3990-3.3/lt3990-5 18 3990fa typical applications 12v step-down converter 5v, 2mhz step-down converter v in boost lt3990 sw en/ulvo pg rt c3 0.1f 22pf 127k f = 1mhz c2 22f c1 2.2f v in 15v to 62v v out 12v 350ma r1 1m r2 113k l1 33h bd fb gnd off on 3990 ta06 v in boost lt3990 sw en/uvlo pg rt 51.1k f = 2mhz c3 0.1f 22pf c2 10f c1 1f v in 8.5v to 16v transients to 62v v out 5v 350ma r1 1m r2 316k l1 10h bd fb gnd off on 3990 ta07 5v step-down converter with undervoltage lockout v in boost lt3990 sw en/uvlo pg rt 0.22f 22pf 22f 2.2f v in 6.5v to 62v v out 5v 350ma 1m 316k 374k 5.6m k f = 400khz 33h bd fb gnd 3990 ta08a 1.3m + ? input current during start-up input voltage (v) 0 ?0.5 input current (ma) 0.5 1.5 2.5 2 4 6 8 3990 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 uvlo programmed to 6.5v start-up from high impedance input source v out 2v/div v in 5v/div 5ms/div front page application v in = 12v v out = 5v 1k input source resistance 2.5ma load 3990 ta08c uvlo programmed to 6.5v
lt3990/lt3990-3.3/lt3990-5 19 3990fa package description please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. 3.00 0.10 (4 sides) note: 1. drawing to be made a jedec package outline m0-229 variation of (weed-2). check the ltc website data sheet for current status of variation assignment 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.40 0.10 bottom view?exposed pad 1.65 0.10 (2 sides) 0.75 0.05 r = 0.125 typ 2.38 0.10 (2 sides) 1 5 10 6 pin 1 top mark (see note 6) 0.200 ref 0.00 ? 0.05 (dd) dfn rev c 0310 0.25 0.05 2.38 0.05 (2 sides) recommended solder pad pitch and dimensions 1.65 0.05 (2 sides) 2.15 0.05 0.50 bsc 0.70 0.05 3.55 0.05 package outline 0.25 0.05 0.50 bsc dd package 10-lead plastic dfn (3mm 3mm) (reference ltc dwg # 05-08-1699 rev c) pin 1 notch r = 0.20 or 0.35 45 chamfer
lt3990/lt3990-3.3/lt3990-5 20 3990fa package description please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. msop (mse16) 0911 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 16 16151413121110 1 2 3 4 5 6 7 8 9 9 1 8 note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 6. exposed pad dimension does include mold flash. mold flash on e-pad shall not exceed 0.254mm (.010") per side. 0.254 (.010) 0 ? 6 typ detail ?a? detail ?a? gauge plane 5.23 (.206) min 3.20 ? 3.45 (.126 ? .136) 0.889 0.127 (.035 .005) recommended solder pad layout 0.305 0.038 (.0120 .0015) typ 0.50 (.0197) bsc bottom view of exposed pad option 2.845 0.102 (.112 .004) 2.845 0.102 (.112 .004) 4.039 0.102 (.159 .004) (note 3) 1.651 0.102 (.065 .004) 1.651 0.102 (.065 .004) 0.1016 0.0508 (.004 .002) 3.00 0.102 (.118 .004) (note 4) 0.280 0.076 (.011 .003) ref 4.90 0.152 (.193 .006) detail ?b? detail ?b? corner tail is part of the leadframe feature. for reference only no measurement purpose 0.12 ref 0.35 ref mse package 16-lead plastic msop, exposed die pad (reference ltc dwg # 05-08-1667 rev e)
lt3990/lt3990-3.3/lt3990-5 21 3990fa information furnished by linear technology corporation is believed to be accurate and reliable. however, no responsibility is assumed for its use. linear technology corporation makes no representa - tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. revision history rev date description page number b 08/12 title, features, typical application clarifed to add fxed output versions clarifed absolute maximum ratings, added h-grade option clarifed pinout for fxed voltage options, clarifed ordering information for fxed output and h-grades clarifed electrical characteristics table clarifed typical performance characteristics clarifed pin functions and block diagram clarifed en/uvlo text and formula clarifed typical applications 1 2 2 3 4, 6 7, 8 14, 15 17 (revision history begins at rev b)
lt3990/lt3990-3.3/lt3990-5 22 3990fa linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax : (408) 434-0507 www.linear.com ? linear technology corporation 2010 lt 0812 rev a ? printed in usa related parts part number description comments lt3970/lt3970-3.3/ lt3970-5 40v, 350ma, 2.2mhz high effciency micropower step-down dc/dc converter with i q = 2.5a v in : 4.2v to 40v, v out(min) = 1.21v, i q = 2.5a, i sd < 1a, 3mm 2mm dfn-10, msop-10 lt3971 38v, 1.2a, 2.2mhz high effciency micropower step-down dc/dc converter with i q = 2.8a v in : 4.3v to 38v, v out(min) = 1.2v, i q = 2.8a, i sd < 1a, 3mm 3mm dfn-10, msope-10 lt3991 55v, 1.2a, 2.2mhz high effciency micropower step-down dc/dc converter with i q = 2.8a v in : 4.3v to 55v, v out(min) = 1.2v, i q = 2.8a, i sd < 1a, 3mm 3mm dfn-10, msope-10 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 typical application 1.21v step-down converter v in boost lt3990 sw en/uvlo bd pg rt c3 0.22f c2 47f c1 2.2f v in 4.2v to 30v v out 1.2v 350ma 374k f = 400khz l1 15h fb gnd off on 3990 ta09

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