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lt1936 1 1936fd typical application features applications description 1.4a, 500khz step-down switching regulator the lt ? 1936 is a current mode pwm step-down dc/dc converter with an internal 1.9a power switch, packaged in a tiny, thermally enhanced 8-lead msop. the wide in- put range of 3.6v to 36v makes the lt1936 suitable for regulating power from a wide variety of sources, including automotive batteries, 24v industrial supplies and unregu- lated wall adapters. its high operating frequency allows the use of small, low cost inductors and ceramic capacitors, resulting in low, predictable output ripple. cycle-by-cycle current limit, frequency foldback and thermal shutdown provide protection against shorted outputs, and soft-start eliminates input current surge during start-up. transient response can be optimized by using external compensation components, or board space can be minimized by using internal compensation. the low current (<2a) shutdown mode enables easy power management in battery-powered systems. wide input range: 3.6v to 36v short-circuit protected over full input range 1.9a guaranteed minimum switch current 5v at 1.4a from 10v to 36v input 3.3v at 1.4a from 7v to 36v input 5v at 1.2a from 6.3v to 36v input 3.3v at 1.2a from 4.5v to 36v input output adjustable down to 1.20v 500khz fixed frequency operation soft-start uses small ceramic capacitors internal or external compensation low shutdown current: <2a thermally enhanced 8-lead msop package automotive battery regulation industrial control supplies unregulated wall adapters 3.3v step-down converter ef? ciency v in 4.5v to 36v on off 0.22f 10h 17.4k 22f 1936 ta01a 4.7f v out 3.3v1.2a v in boost v c gnd comp fb lt1936 shdn sw 10k load current (a) 0 65 efficiency (%) 70 75 80 85 95 0.5 1 1936 ta01b 1.5 90 v out = 5v v in = 12v v out = 3.3v l , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. downloaded from: http:///
lt1936 2 1936fd pin configuration electrical characteristics absolute maximum ratings v in voltage ................................................. C0.4v to 36v boost voltage .........................................................43v boost above sw voltage ........................................20v shdn voltage ............................................ C0.4v to 36v fb, v c , comp voltage .................................................6v operating temperature range (note 2) lt1936e ............................................... C40c to 85c lt1936i .............................................. C40c to 125c lt1936h ............................................ C40c to 150c maximum junction temperature lt1936e, lt1936i ............................................. 125c lt1936h ........................................................... 150c storage temperature range ................... C65c to 150c lead temperature (soldering, 10 sec) .................. 300c (note 1) 12 3 4 boost v in sw gnd 87 6 5 compv c fbshdn top view 9 ms8e package 8-lead plastic msop ja = 40c/w, jc = 10c/w exposed pad (pin 9) is gnd, must be soldered to pcb the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 12v, v boost = 17v, unless otherwise noted. (note 2) parameter conditions min typ max units undervoltage lockout 3.45 3.6 v quiescent current v fb = 1.5v 1.8 2.5 ma quiescent current in shutdown v shdn = 0v 0.1 2 a fb voltage 1.175 1.200 1.215 v fb pin bias current (note 4) v fb = 1.20v, e and i grades h grade 5050 200300 nana fb voltage line regulation v in = 5v to 36v 0.01 %/v error amp gm v c = 0.5v, i vc = 5a 250 s error amp voltage gain v c = 0.8v, 1.2v 150 order information lead free finish tape and reel part marking package description temperature range lt1936ems8e#pbf lt1936ems8e#trpbf ltbmt 8-lead plastic msop C40c to 85c lt1936ims8e#pbf lt1936ims8e#trpbf ltbrv 8-lead plastic msop C40c to 125c lt1936hms8e#pbf lt1936hms8e#trpbf ltbwb 8-lead plastic msop C40c to 150c lead based finish tape and reel part marking package description temperature range lt1936ems8e lt1936ems8e#tr ltbmt 8-lead plastic msop C40c to 85c lt1936ims8e lt1936ims8e#tr ltbrv 8-lead plastic msop C40c to 125c lt1936hms8e lt1936hms8e#tr ltbwb 8-lead plastic msop C40c to 150c consult ltc marketing for parts speci? ed with wider operating temperature ranges. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/ downloaded from: http:///
lt1936 3 1936fd load current (a) 0 60 efficiency (%) 70 80 90 100 0.5 1.0 1936 g02 1.5 v out = 3.3v t a = 25c d1 = dfls140ll1 = 10h, toko d63cb v in = 5v v in = 12v v in = 24v load current (a) 0 60 efficiency (%) 70 80 90 100 0.5 1.0 1936 g01 1.5 v out = 5v t a = 25c d1 = dfls140ll1 = 15h, toko d63cb v in = 12v v in = 24v duty cycle (%) 0 0 current limit (a) 0.5 1.0 1.5 2.0 2.5 3.0 20 40 typ min 60 80 1936 g03 100 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 lt1936e is guaranteed to meet performance speci? cations from 0c to 70c. speci? cations over the C40c to 85c operating temperature range are assured by design, characterization and correlation the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 12v, v boost = 17v, unless otherwise noted. (note 2) v c clamp 1.8 v v c switch threshold 0.7 v internal compensation r 50 k internal compensation c v comp = 1v 150 pf comp pin leakage v comp = 1.8v, e and i grades h grade 12 aa switching frequency v fb = 1.1v v fb = 0v 400 500 40 600 khz khz maximum duty cycle 87 92 % switch current limit (note 3) 1.9 2.2 2.6 a switch v cesat i sw = 1.2a 410 520 mv switch leakage current 2 a minimum boost voltage above sw i sw = 1.2a 2 2.2 v boost pin current i sw = 1.2a 28 50 ma boost pin leakage v sw = 0v 0.1 1 a shdn input voltage high 2.3 v shdn input voltage low 0.3 v shdn pin current v shdn = 2.3v (note 5) v shdn = 12v (note 5) v shdn = 0v 34 140 0.01 50 240 0.1 aa a with statistical process controls. the lt1936i speci? cations are guaranteed over the C40c to 125c temperature range. the lt1936h speci? cations are guaranteed over the C40c to 150c temperature range. note 3: current limit guaranteed by design and/or correlation to static test. slope compensation reduces current limit at higher duty cycle.note 4: current ? ows out of pin. note 5: current ? ows into pin. typical performance characteristics ef? ciency, v out = 5v ef? ciency, v out = 3.3v switch current limit downloaded from: http:///
lt1936 4 1936fd input voltage (v) 0 1.0 load current (a) 1.2 1.4 1.6 1.8 10 20 51 52 5 1936 g04 30 v out = 5v l = 15h l = 10h input voltage (v) 0 1.0 load current (a) 1.2 1.4 1.6 1.8 10 20 51 52 5 1936 g05 30 v out = 3.3v l = 6.8h l = 10h switch current (a) 0 0 switch voltage drop (mv) 100 200 300 400 600 0.5 1.0 1936 g06 1.5 500 t a = C45c t a = 85c t a = 25c temperature (c) 1.185 feedback voltage (v) 1.195 1.210 1936 g07 1.190 1.205 1.200 C50 C25 0 25 50 75 100 150 125 temperature (c) C50 3.0 uvlo (v) 3.2 3.4 3.6 3.8 C25 0 25 50 1936 g08 75 100 150 125 temperature (c) C50 400 switching frequency (khz) 450 500 550 600 C25 0 25 50 1936 g09 75 100 150 125 fb pin voltage (v) 0 700 t a = 25c 600500 400 300 200 100 0 1936 g10 0.5 1.0 1.5 switching frequency (khz) shdn pin voltage (v) 0 0 switch current limit (a) 0.5 1.0 1.5 2.0 2.5 3.0 t a = 25c dc = 30% 12 34 1936 g11 shdn pin voltage (v) 0 0 shdn pin current (a) 50 100 150 200 48 1936 g12 12 16 t a = 25c typical performance characteristics maximum load current maximum load current switch voltage drop feedback voltage undervoltage lockout switching frequency frequency foldback soft-start shdn pin current downloaded from: http:///
lt1936 5 1936fd load current (ma) 1 4.0 input voltage (v) 4.5 5.0 5.5 6.0 6.5 7.0 10 100 1000 1936 g13 v out = 5v t a = 25c l = 15h load current (ma) 1 3.0 input voltage (v) 3.5 4.0 4.5 5.0 10 100 1000 1936 g14 v out = 3.3v t a = 25c l = 10h temperature (c) C50 switch current limit (a) 2.0 2.5 3.0 25 75 1936 g15 1.5 1.0 C25 0 50 100 150 125 0.5 0 v sw 10v/div i l 500ma/div v out 20mv/div 1s/div v in = 12v v out = 3.3v i out = 1a l = 10hc out = 22f 1936 g16 v sw 10v/div i l 500ma/div v out 20mv/div 1s/div v in = 12v v out = 3.3v i out = 50ma l = 10hc out = 22f 1936 g17 temperature (c) C50 C25 0 0.5 v c voltage (v) 1.5 05 0 7 5 1936 g18 1.0 2.5 2.0 25 100 150 125 current limit clamp switching threshold fb pin voltage (v) 0 C60 v c pin current (a) C40 C20 0 20 60 1 1936 g19 2 40 t a = 25c v c = 0.5v typical performance characteristics minimum input voltage minimum input voltage switch current limit switching waveforms switching waveforms, discontinuous mode v c voltages error amp output current downloaded from: http:///
lt1936 6 1936fd block diagram pin functions boost (pin 1): the boost pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. v in (pin 2): the v in pin supplies current to the lt1936s internal regulator and to the internal power switch. this pin must be locally bypassed. sw (pin 3): the sw pin is the output of the internal power switch. connect this pin to the inductor, catch diode and boost capacitor. gnd (pin 4): tie the gnd pin to a local ground plane below the lt1936 and the circuit components. return the feedback divider to this pin. shdn (pin 5): the shdn pin is used to put the lt1936 in shutdown mode. tie to ground to shut down the lt1936. tie to 2.3v or more for normal operation. if the shutdown feature is not used, tie this pin to the v in pin. shdn also provides a soft-start function; see the applications infor- mation. do not drive shdn more than 5v above v in . fb (pin 6): the lt1936 regulates its feedback pin to 1.200v. connect the feedback resistor divider tap to this pin. set the output voltage according to v out = 1.200v (1 + r1/r2). a good value for r2 is 10k.v c (pin 7): the v c pin is used to compensate the lt1936 control loop by tying an external rc network from this pin to ground. the comp pin provides access to an internal rc network that can be used instead of the external components. comp (pin 8): to use the internal compensation network, tie the comp pin to the v c pin. otherwise, tie comp to ground or leave it ? oating. exposed pad (pin 9): the exposed pad must be soldered to the pcb and electrically connected to ground. use a large ground plane and thermal vias to optimize thermal performance. 4 r driver q1 s osc slope comp frequencyfoldback int reg and uvlo v c g m 1.200v 1936 bd 8 2 5 qq 1 3 boost sw 6 fb gnd r c 50k c c 150pf v out r1r2 l1 d2 c3 c1 d1 v in c2 v in on off comp 7 v c r4 c5 c4 r3 shdn downloaded from: http:///
lt1936 7 1936fd operation (refer to block diagram) the lt1936 is a constant frequency, current mode step- down regulator. a 500khz oscillator enables an rs ? ip-? op, turning on the internal 1.9a power switch q1. an ampli- ? er and comparator monitor the current ? owing between the v in and sw pins, turning the switch off when this current reaches a level determined by the voltage at v c . an error ampli? er measures the output voltage through an external resistor divider tied to the fb pin and servos the v c pin. if the error ampli? ers output increases, more current is delivered to the output; if it decreases, less current is delivered. an active clamp (not shown) on the v c pin provides current limit. the v c pin is also clamped to the voltage on the shdn pin; soft-start is implemented by generating a voltage ramp at the shdn pin using an external resistor and capacitor. an internal regulator provides power to the control circuitry. this regulator includes an undervoltage lockout to prevent switching when v in is less than ~3.45v. the shdn pin is used to place the lt1936 in shutdown, disconnecting the output and reducing the input current to less than 2a. the switch driver operates from either the input or from the boost pin. an external capacitor and diode are used to generate a voltage at the boost pin that is higher than the input supply. this allows the driver to fully saturate the internal bipolar npn power switch for ef? cient opera- tion. the oscillator reduces the lt1936s operating frequency when the voltage at the fb pin is low. this frequency foldback helps to control the output current during startup and overload. downloaded from: http:///
lt1936 8 1936fd applications information fb resistor networkthe 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 1.200 ?1 r2 should be 20k or less to avoid bias current errors. reference designators refer to the block diagram. input voltage range the input voltage range for lt1936 applications depends on the output voltage and the absolute maximum ratings of the v in and boost pins. the minimum input voltage is determined by either the lt1936s minimum operating voltage of ~3.45v or by its maximum duty cycle. the duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: dc vv vv v out d in sw d = + + ? where v d is the forward voltage drop of the catch diode (~0.5v) and v sw is the voltage drop of the internal switch (~0.5v at maximum load). this leads to a minimum input voltage of: v vv dc vv in min out d max ds w () ? = + + with dc max = 0.87. the maximum input voltage is determined by the absolute maximum ratings of the v in and boost pins and by the minimum duty cycle dc min = 0.08: v vv dc vv in max out d min ds w () ? = + + note that this is a restriction on the operating input voltage; the circuit will tolerate transient inputs up to the absolute maximum ratings of the v in and boost pins. inductor selection and maximum output current a good ? rst choice for the inductor value is l = 2.2 (v out + v d ) where v d is the voltage drop of the catch diode (~0.4v) and l is in h. with this value the maximum output cur- rent will be above 1.2a at all duty cycles and greater than 1.4a for duty cycles less than 50% (v in > 2 v out ). 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 2.6a. to keep the ef? ciency high, the series resistance (dcr) should be less than 0.1, and the core material should be intended for high frequency applications. table 1 lists several vendors and suitable types. table 1. inductor vendors vendor url part series type murata www.murata.com lqh55d open tdk www.component.tdk.com slf7045 slf10145 shieldedshielded toko www.toko.com d62cb d63cbd75c d75f shieldedshielded shielded open sumida www.sumida.com cr54 cdrh74cdrh6d38 cr75 openshielded shielded open of course, such a simple design guide will not always result in the optimum inductor for your application. a larger value provides a slightly higher maximum load current and will reduce the output voltage ripple. if your load is lower than 1.2a, then you can decrease the value of the inductor and operate with higher ripple current. this allows you to use a physically smaller inductor, or one with a lower dcr resulting in higher ef? ciency. be aware that if the inductance differs from the simple rule above, then the maximum load current will depend on input volt- age. there are several graphs in the typical performance characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. low downloaded from: http:///
lt1936 9 1936fd inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. for details of maximum output current and discontinuous mode 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. choosing l greater than 1.6 (v out + v d ) h prevents subharmonic oscillations at all duty cycles.catch diode a 1a schottky diode is recommended for the catch diode, d1. the diode must have a reverse voltage rating equal to or greater than the maximum input voltage. the on semiconductor mbrm140 is a good choice. it is rated for 1a dc at a case temperature of 110c and 1.5a at a case temperature of 95c. diode incorporateds dfls140l is rated for 1.1a average current; the dfls240l is rated for 2a average current. the average diode current in an lt1936 application is approximately i out (1 C dc). input capacitor bypass the input of the lt1936 circuit with a 4.7f or higher value ceramic capacitor of x7r or x5r type. y5v types have poor performance over temperature and ap- plied voltage, and should not be used. a 4.7f ceramic is adequate to bypass the lt1936 and will easily handle the ripple current. however, if the input power source has high impedance, or there is signi? cant 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 lt1936 and to force this very high frequency switching current into a tight local loop, minimizing emi. a 4.7f capacitor is capable of this task, but only if it is placed close to the lt1936 and the catch diode; see the pcb layout section. a second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the lt1936. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the lt1936 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the lt1936s voltage rating. this situation is easily avoided; see the hot plugging safety section. for space sensitive applications, a 2.2f ceramic capaci- tor can be used for local bypassing of the lt1936 input. however, the lower input capacitance will result in in- creased input current ripple and input voltage ripple, and may couple noise into other circuitry. also, the increased voltage ripple will raise the minimum operating voltage of the lt1936 to ~3.7v. output capacitor the output capacitor has two essential functions. along with the inductor, it ? lters the square wave generated by the lt1936 to produce the dc output. in this role it determines the output ripple, and low impedance at the switching frequency is important. the second function is to store energy in order to satisfy transient loads and stabilize the lt1936s control loop. ceramic capacitors have very low equivalent series re- sistance (esr) and provide the best ripple performance. a good value is: c v out out = 150 where c out is 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 high value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. a lower value of output capacitor can be used, but transient performance will suffer. with an external compensation network, the loop gain can be lowered to compensate for the lower capacitor value. when using the internal compensa- tion network, the lowest value for stable operation is: c v out out > 66 applications information downloaded from: http:///
lt1936 10 1936fd applications information this is the minimum output capacitance required, not the nominal capacitor value. for example, a 3.3v output requires 20f of output capacitance. if a small 22f, 6.3v ceramic capacitor is used, the circuit may be unstable be- cause the effective capacitance is lower than the nominal capacitance when biased at 3.3v. look carefully at the capacitors data sheet to ? nd out what the actual capaci- tance is under operating conditions (applied voltage and temperature). a physically larger capacitor, or one with a higher voltage rating, may be required. high performance electrolytic capacitors can be used for the output capacitor. low esr is important, so choose one that is intended for use in switching regulators. the esr should be speci? ed by the supplier, and should be 0.05 or less. such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low esr. table 2 lists several capacitor vendors. frequency compensation the lt1936 uses current mode control to regulate the output. this simpli? es loop compensation. in particular, the lt1936 does not require the esr of the output capacitor for stability, so you are free to use ceramic capacitors to achieve low output ripple and small circuit size. frequency compensation is provided by the components tied to the v c pin, as shown in figure 1. generally a capaci- tor (c c ) and a resistor (r c ) in series to ground are used. in addition, there may be lower value capacitor in parallel. this capacitor (c f ) is not part of the loop compensation but is used to ? lter noise at the switching frequency, and is required only if a phase-lead capacitor is used or if the output capacitor has high esr. an alternative to using external compensation components is to use the internal rc network by tying the comp pin to the v c pin. this re- duces component count but does not provide the optimum transient response when the output capacitor value is high, and the circuit may not be stable when the output capacitor value is low. if the internal compensation network is not used, tie comp to ground or leave it ? oating. loop compensation determines the stability and transient performance. designing the compensation network is a bit figure 1. model for loop response table 2. capacitor vendors vendor phone url part series comments panasonic (714) 373-7366 www.panasonic.com ceramic, polymer, tantalum eef series kemet (864) 963-6300 www.kemet.com ceramic, tantalum t494, t495 sanyo (408) 749-9714 www.sanyovideo.com ceramic, polymer, tantalum poscap murata (404) 436-1300 www.murata.com ceramic avx www.avxcorp.com ceramic, tantalum tps series taiyo yuden (864) 963-6300 www.taiyo-yuden.com ceramic C + 1.2v sw v c comp gnd 50k 600k 150pf lt1936 1936 f01 r1 output esr c f c c r c error amplifier fb r2 c1 c1 current mode power stage g m = 2mho g m = 250mho + polymer or tantalum ceramic c pl downloaded from: http:///
lt1936 11 1936fd applications information complicated and the best values depend on the application and in particular the type of output capacitor. a practical approach is to start with one of the circuits in this data sheet that is similar to your application and tune the com- pensation network to optimize the performance. stability should then be checked across all operating conditions, including load current, input voltage and temperature. the lt1375 data sheet contains a more thorough discussion of loop compensation and describes how to test the stability using a transient load. figure 1 shows an equivalent circuit for the lt1936 control loop. the error ampli? er is a transconductance ampli? er with ? nite output impedance. the power section, consisting of the modulator, power switch and inductor, is modeled as a transconductance ampli? er generating an output figure 2. transient load response of the lt1936 with different output capacitors as the load current is stepped from 200ma to 800ma. v out = 3.3v comp c out = 22f (avx 1210zd226mat) (2a)(2b) (2c) (2d) v out 100mv/div v out 100mv/div v out 100mv/div v out 100mv/div i out 500ma/div 800ma200ma 50s/div 1936 f02 v c comp c out = 22f 2 v c comp c out = 150f (4tpc150m) v c comp 220k 100pf c out = 150f (4tpc150m) v c current proportional to the voltage at the v c pin. note that the output capacitor integrates this current, and that the capacitor on the v c pin (c c ) integrates the error ampli? er output current, resulting in two poles in the loop. in most cases a zero is required and comes from either the output capacitor esr or from a resistor r c in series with c c . this simple model works well as long as the value of the inductor is not too high and the loop crossover frequency is much lower than the switching frequency. a phase lead capacitor (c pl ) across the feedback divider may improve the transient response.figure 2 compares the transient response across several output capacitor choices and compensation schemes. in each case the load current is stepped from 200ma to 800ma and back to 200ma. downloaded from: http:///
lt1936 12 1936fd applications information boost pin considerationscapacitor c3 and diode d2 are used to generate a boost voltage that is higher than the input voltage. in most cases a 0.22f capacitor and fast switching diode (such as the 1n4148 or 1n914) will work well. figure 3 shows two ways to arrange the boost circuit. the boost pin must be at least 2.3v above the sw pin for best ef? ciency. for outputs of 3v and above, the standard circuit (figure 3a) is best. for outputs between 2.8v and 3v, use a 0.47f capacitor and a schottky diode. for lower output voltages the boost diode can be tied to the input (figure 3b), or to another supply greater than 2.8v. the circuit in figure 3a is more ef? cient because the boost pin current comes from a lower voltage. you must also be sure that the maximum voltage rating of the boost pin is not exceeded. a 2.5v output presents a special case. this is a popular output voltage, and the advantage of connecting the boost circuit to the output is that the circuit will accept a 36v maximum input voltage rather than 20v (due to the boost pin rating). however, 2.5v is marginally adequate to support the boosted drive stage at low ambient tem- peratures. therefore, special care and some restrictions on operation are necessary when powering the boost pin from a 2.5v output. minimize the voltage loss in the boost circuit by using a 1f boost capacitor and a good, low drop schottky diode (such as the on semi mbr0540). because the required boost voltage increases at low temperatures, the circuit will supply only 1a of output current when the ambient temperature is C45c, increasing to 1.2a at 0c. also, the minimum input voltage to start the boost circuit is higher at low temperature. see the typical applications section for a 2.5v schematic and performance curves. the minimum operating voltage of an lt1936 application is limited by the undervoltage lockout (~3.45v) and by the maximum duty cycle as outlined above. for proper start-up, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, or the lt1936 is turned on with its shdn pin when the output is already in regulation, then 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 4 shows a plot of minimum load to start and to run as a function of input voltage. in many cases the discharged output capacitor will present a load to the switcher, which will allow it to start. the plots show the worst-case situation where v in is ramping very slowly. for lower start-up voltage, the boost diode can be tied to v in ; however, this restricts the input range to one-half of the absolute maximum rating of the boost pin.at light loads, the inductor current becomes discontinu- ous and the effective duty cycle can be very high. this reduces the minimum input voltage to approximately 300mv above v out . at higher load currents, the inductor current is continuous and the duty cycle is limited by the maximum duty cycle of the lt1936, requiring a higher input voltage to maintain regulation. soft-start the shdn pin can be used to soft-start the lt1936, reducing the maximum input current during start-up. the shdn pin is driven through an external rc ? lter to create a voltage ramp at this pin. figure 5 shows the start-up waveforms with and without the soft-start circuit. by choosing a large figure 3. two circuits for generating the boost voltage v in boost gnd sw v in lt1936 (3a) d2 v out c3 v boost C v sw ? v out max v boost ? v in + v out v in boost gnd sw v in lt1936 ( 3b) d2 1933 f03 v out c3 v boost C v sw ? v in max v boost ? 2v in downloaded from: http:///
lt1936 13 1936fd applications information rc time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. choose the value of the resistor so that it can supply 60a when the shdn pin reaches 2.3v. shorted and reversed input protectionif the inductor is chosen so that it wont saturate exces- sively, an lt1936 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 lt1936 is absent. this may occur in battery charging ap- plications or in battery backup systems where a battery or some other supply is diode or-ed with the lt1936s output. if the v in pin is allowed to ? oat and the shdn pin is held high (either by a logic signal or because it is tied to v in ), then the lt1936s internal circuitry will pull its quiescent current through its sw pin. this is ? ne if your system can tolerate a few ma in this state. if you ground figure 5. to soft-start the lt1936, add a resistor and capacitor to the shdn pin. v in = 12v, v out = 3.3v, c out = 2 22f, r load = 3.3 figure 4. the minimum input voltage depends on output voltage, load current and boost circuit minimum input voltage v out = 3.3v minimum input voltage v out = 5v load current (ma) 0 3.0 input voltage (v) 3.5 4.0 4.5 5.0 5.5 6.0 10 to start to run 100 1000 1936 f04a v out = 3.3v t a = 25c l = 10h load current (ma) 1 input voltage (v) 6 7 1936 f04b 54 10 100 1000 8 to start v out = 5v t a = 25c l = 15h to run run run 5v/div i in 500ma/div 50s/div v out 5v/div run 5v/div i in 500ma/div v out 5v/div shdn gnd 1936 f05a 0.5ms/div 1936 f05b run 15k 0.22f shdn gnd downloaded from: http:///
lt1936 14 1936fd applications information the shdn pin, the sw pin current will drop to essentially zero. however, if the v in pin is grounded while the output is held high, then parasitic diodes inside the lt1936 can pull large currents from the output through the sw pin and the v in pin. figure 6 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. figure 6. 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 lt1936 runs only when the input is present figure 7. a good pcb layout ensures low emi operation pcb layoutfor proper operation and minimum emi, care must be taken during printed circuit board layout. figure 7 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents ? ow in the lt1936s v in and sw pins, the catch diode (d1) and the input capacitor (c2). 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 and v c nodes small so that the ground traces will shield them from the sw and boost nodes. the exposed pad on the bottom of the package must be soldered to ground so that the pad acts as a heat sink. to keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the lt1936 to additional ground planes within the circuit board and on the bottom side. high temperature considerations the die temperature of the lt1936 must be lower than the maximum rating of 125c (150c for the h grade). this is generally not a concern unless the ambient temperature is above 85c. for higher temperatures, care should be taken in the layout of the circuit to ensure good heat sink- ing of the lt1936. the maximum load current should be derated as the ambient temperature approaches 125c (150c for the h grade). the die temperature is calculated by multiplying the lt1936 power dissipation by the thermal resistance from junction to ambient. power dissipation within the lt1936 can be estimated by calculating the total power loss from an ef? ciency measurement and subtracting the catch diode loss. the resulting temperature rise at full load is nearly independent of input voltage. thermal resistance depends on the layout of the circuit board, but values from 40c/w to 60c/w are typical. die temperature rise was measured on a 4-layer, 5cm 6.5cm circuit board in still air at a load current of 1.4a. for 12v input to 3.3v output the die temperature elevation above ambient was 26c; for 24v in to 3.3v out the rise was 31c; for 12v in to 5v the rise was 31c and for 24v in to 5v the rise was 34c. v in boost comp gnd fb shdnv c sw d4 mbrs140 v in lt1936 1936 f06 v out backup d1 r2 r4 c2 c3 d2 minimize lt1936 c2, d1 loop in gnd r1 c1 l1 gnd vias out 1936 f07 downloaded from: http:///
lt1936 15 1936fd applications information hot plugging safelythe small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of lt1936 circuits. however, these capaci- tors can cause problems if the lt1936 is plugged into a live supply (see linear technology application note 88 for a complete discussion). the low loss ceramic capacitor combined with stray inductance in series with the power source forms an under damped tank circuit, and the voltage at the v in pin of the lt1936 can ring to twice the nominal input voltage, possibly exceeding the lt1936s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the lt1936 into an energized supply, the input network should be designed to prevent this overshoot. figure 8 shows the waveforms that result when an lt1936 circuit is connected to a 24v supply through six feet of 24-gauge twisted pair. the ? rst plot is the response with a 4.7f ceramic capacitor at the input. the input voltage rings as high as 50v and the input current peaks at 26a. figure 8. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the lt1936 is connected to a live supply + + lt1936 4.7f v in 20v/div i in 10a/div 20s/div v in closing switch simulates hot plug i in (8a)(8b) (8c) low impedance energized 24v supply stray inductance due to 6 feet (2 meters) of twisted pair + + lt1936 4.7f 22f 35v ai.ei. lt1936 4.7f 0.1f 0.7 1936 f08 v in 20v/div i in 10a/div 20s/div v in 20v/div i in 10a/div 20s/div danger ringing v in may exceed absolute maximum rating of the lt1936 downloaded from: http:///
lt1936 16 1936fd applications information one method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. in figure 8b an aluminum electrolytic capacitor has been added. this capacitors high equivalent series resistance damps the circuit and eliminates the voltage overshoot. the extra capacitor improves low frequency ripple ? ltering and can slightly improve the ef? ciency of the circuit, though it is likely to be the largest component in the circuit. an alternative solution is shown in figure 8c. a 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). a 0.1f capacitor improves high frequency ? ltering. this solution is smaller and less expensive than the electrolytic capacitor. for high input voltages its impact on ef? ciency is minor, reducing ef? ciency by one percent for a 5v output at full load operating from 24v. 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. outputs greater than 6v for outputs greater than 6v, add a resistor of 1k to 2.5k across the inductor to damp the discontinuous ringing of the sw node, preventing unintended sw current. the 12v step-down converter circuit in the typical applica- tions section shows the location of this resistor. also note that for outputs above 6v, the input voltage range will be limited by the maximum rating of the boost pin. the 12v circuit shows how to overcome this limitation using an additional zener diode. 3.3v step-down converter typical applications v in 4.5v to 36v on off d1 d2 l1 10h r1 17.4k c247f 1936 ta03 c14.7f c3 0.22f v out 3.3v1.2a v in boost v c gnd comp fb lt1936 shdn sw r210k downloaded from: http:///
lt1936 17 1936fd typical applications 5v step-down converter 1.8v step-down converter 1.2v step-down converter ef? ciency, 1.8v output ef? ciency, 1.2v output v in 3.6v to 20v on off d1 d2 l1 4.7h r1 10k d1: dfls140ld2: 1n4148 l1: toko d63cb c247f 2 1936 ta05a c14.7f c3 0.22f v out 1.8v1.3a v in boost v c gnd comp fb lt1936 shdn sw r220k load current (a) 0 50 efficiency (%) power loss (w) 60 70 80 90 0 0.5 1.0 1.5 2.0 0.5 1 1936 ta05b 1.5 power loss v in = 5v v in = 12v v out = 1.8v t a = 25c v in 3.6v to 20v on off d1 d2 l1 3.3h d1: dfls140ld2: 1n4148 l1: toko d63cb c247f 2 1936 ta06a c14.7f c3 0.22f v out 1.2v1.3a v in boost v c gnd comp fb lt1936 shdn sw 100k load current (a) 0 50 55 efficiency (%) power loss (w) 60 65 70 75 80 0 0.5 1.0 1.5 2.0 0.5 1 1936 ta06b 1.5 power loss v in = 5v v in = 12v v out = 1.2v t a = 25c v in 6.3v to 36v on off d1 d2 l1 15h r1 31.6k c222f 1936 ta04 c14.7f c3 0.22f v out 5v1.2a v in boost v c gnd comp fb lt1936 shdn sw r210k downloaded from: http:///
lt1936 18 1936fd typical applications 2.5v step-down converter ef? ciency, 2.5v output minimum input voltage 12v step-down converter load current (a) 0 60 efficiency (%) 70 80 90 100 0.5 1.0 1936 ta07b 1.5 v out = 2.5v t a = 25c v in = 12v v in = 5v load current (ma) 1 input voltage (v) 4.5 5.0 1936 ta07c 4.03.5 3.0 10 100 1000 5.5 v out = 2.5v to start t a = C45c to run t a = C45c to start t a = 25c to run t a = 25c v in 3.6v to 36v on off d1 d2 l1 6.2h r1 11k d1: dfls140ld2: mbro540 l1: toko d63cb c247f 1936 ta07a c14.7f c3 1f v out 2.5v1.2a t a > 0c v in boost v c gnd comp fb lt1936 shdn sw r210k v in 14.5v to 36v on off d1 d2 l1 22h r1 182k c222f 1936 ta08 c12.2f d1: mbrm140d2: 1n4148 d3: cmdz5235b c3 0.22f 1.8k v out 12v1.2a d3 6.8v v in boost v c gnd comp fb lt1936 shdn sw r220k downloaded from: http:///
lt1936 19 1936fd 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. package description ms8e package 8-lead plastic msop, exposed die pad (reference ltc dwg # 05-08-1662 rev e) msop (ms8e) 0908 rev e 0.53 0.152 (.021 .006) seating plane 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 0.18 (.007) 0.254 (.010) 1.10 (.043) max 0.22 C 0.38 (.009 C .015) typ 0.86 (.034) ref 0.65 (.0256) bsc 0 C 6 typ detail a detail a gauge plane 12 3 4 4.90 0.152 (.193 .006) 8 8 1 bottom view of exposed pad option 7 6 5 3.00 0.102 (.118 .004) (note 3) 3.00 0.102 (.118 .004) (note 4) 0.52 (.0205) ref 1.83 0.102 (.072 .004) 2.06 0.102 (.081 .004) 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 2.083 0.102 (.082 .004) 2.794 0.102 (.110 .004) 0.889 0.127 (.035 .005) recommended solder pad layout 0.42 0.038 (.0165 .0015) typ 0.65 (.0256) bsc 0.1016 0.0508 (.004 .002) detail b detail b corner tail is part of the leadframe feature. for reference only no measurement purpose 0.05 ref 0.29ref downloaded from: http:///
lt1936 20 1936fd linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2006 lt 1108 rev d printed in usa related parts typical application 2.5v step-down converter minimum input voltage part number description comments lt1676 60v, 440ma (i out ), 100khz, high ef? ciency step-down dc/dc converter v in : 7.4v to 60v, v out(min) = 1.24v, i q = 3.2ma, i sd = 2.5a, so-8 package lt1765 25v, 2.75a (i out ), 1.25mhz, high ef? ciency step-down dc/dc converter v in : 3v to 25v, v out(min) = 1.20v, i q = 1ma, i sd = 15a, so-8 and 16-lead tssope packages lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(min) = 1.20v, i q = 2.5ma, i sd = 25a, 16-lead tssop/tssope packages lt1767 25v, 1.2a (i out ), 1.25mhz, high ef? ciency step-down dc/dc converter v in : 3v to 25v, v out(min) = 1.20v, i q = 1ma, i sd = 6a, ms8/ms8e packages lt1776 40v, 550ma (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 7.4v to 40v, v out(min) = 1.24v, i q = 3.2ma, i sd = 30a, n8/so-8 packages lt1933 600ma, 500khz, step-down switching regulator in sot-23 v in : 3.6v to 36v, v out(min) = 1.25v, i q = 1.6ma, i sd < 1a, thinsot? package lt1940 25v, dual 1.4a (i out ), 1.1mhz, high ef? ciency step-down dc/dc converter v in : 3v to 25v, v out(min) = 1.2v, i q = 3.8ma, i sd < 1a, 16-lead tssope package lt1956 60v, 1.2a (i out ), 500khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(min) = 1.20v, i q = 2.5ma, i sd = 25a, 16-lead tssop/tssope packages lt1976 60v, 1.2a (i out ), 200khz, high ef? ciency 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, 16-lead tssope package lt3010 80v, 50ma, low noise linear regulator v in : 1.5v to 80v, v out(min) = 1.28v, i q = 30a, i sd < 1a, ms8e package lt c ? 3407 dual 600ma (i out ), 1.5mhz, synchronous step-down dc/dc converter v in : 2.5v to 5.5v, v out(min) = 0.6v, i q = 40a, i sd < 1a, 10-lead mse package ltc3412 2.5a (i out ), 4mhz, synchronous step-down dc/dc converter v in : 2.5v to 5.5v, v out(min) = 0.8v, i q = 60a, i sd < 1a, 16-lead tssope package ltc3414 4a (i out ), 4mhz, synchronous step-down dc/dc converter v in : 2.3v to 5.5v, v out(min) = 0.8v, i q = 64a, i sd < 1a, 20-lead tssope package lt3430/lt3431 60v, 2.75a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converters v in : 5.5v to 60v, v out(min) = 1.20v, i q = 2.5ma, i sd = 30a, 16-lead tssope package burst mode is a registered trademark of linear technology corporation. thinsot is a trademark of linear technology corporation. v in 3.6v to 20v on off d1 d2 l1 8.2h d1: dfls140ld2: 1n4148 l1: toko d63cb c247f 1936 ta09a c14.7f c3 0.22f v out 2.5v1.3a v in boost v c gnd comp fb lt1936 shdn sw r210k r1 11k load current (ma) 1 input voltage (v) 4.5 5.0 1936 ta09b 4.03.5 3.0 10 100 1000 5.5 v out = 2.5v connecting the boost circuit to theinput lowers the minimum input voltage to run and to start to less than 3.7v at all loads downloaded from: http:///

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