View LT1913_3369067.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

LT1913 1 1913f 25v, 3.5a, 2.4mhz step-down switching regulator the lt ? 1913 is an adjustable frequency (200khz to 2.4mhz) monolithic buck switching regulator that accepts input voltages up to 25v. a high ef? ciency 95m switch is included on the die along with a boost schottky diode and the necessary oscillator, control, and logic circuitry. current mode topology is used for fast transient response and good loop stability. shutdown reduces input supply current to less than 1a while a resistor and capacitor on the run/ss pin provide a controlled output voltage ramp (soft-start). a power good ? ag signals when v out reaches 91% of the programmed output voltage. the LT1913 is available in 10-pin 3mm 3mm dfn packages with ex- posed pads for low thermal resistance. automotive battery regulation power for portable products distributed supply regulation industrial supplies wall transformer regulation wide input range: 3.6v to 25v 3.5a maximum output current adjustable switching frequency: 200khz to 2.4mhz low shutdown current: i q < 1a integrated boost diode synchronizable between 250khz to 2mhz power good flag saturating switch design: 95m on-resistance 0.790v feedback reference voltage output voltage: 0.79v to 25v thermal protection soft-start capability small 10-pin (3mm 3mm) dfn packages 5v step-down converter applicatio s u features descriptio u typical applicatio u sw fb v c pg rt v in bd v in 6.5v to 25v v out 5v 3.5a 10f 0.47f 680pf 47f 100k 15k 63.4k 4.7h 536k gnd off on LT1913 1913 ta01a run/ss boost sync ef? ciency , lt, ltc and ltm are registered trademarks of linear technology corporation. all other trademarks are the property of their respective owners. output current (a) 0 0.5 50 efficiency (%) 70 100 1 2 2.5 1913 g01 60 90 80 1.5 3 3.5 v in = 12v v out = 5v l = 4.7h f = 600khz v in = 24v
LT1913 2 1913f electrical characteristics v in , run/ss voltage .................................................25v boost pin voltage ...................................................50v boost pin above sw pin .........................................25v fb, rt, v c voltage .......................................................5v pg, bd voltage .........................................................25v (note 1) parameter conditions min typ max units minimum input voltage o 3 3.6 v quiescent current from v in v run/ss = 0.2v 0.01 0.5 a v bd = 3v, not switching o 0.45 1.2 ma v bd = 0, not switching 1.3 2.3 ma quiescent current from bd v run/ss = 0.2v 0.01 0.5 a v bd = 3v, not switching o 0.9 1.8 ma v bd = 0, not switching 110 a minimum bias v oltage (bd pin) 2.7 3 v the o denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v run/ss = 10v, v boost = 15v, v bd = 3.3v unless otherwise noted. (note 2) absolute axi u rati gs w ww u sync voltage ............................................................20v operating junction temperature range (note 2) LT1913e ............................................. ?40c to 125c LT1913i .............................................. ?40c to 125c storage temperature range ................... ?65c to 150c top view dd package 10-lead ( 3mm 3mm ) plastic dfn 10 9 6 7 8 4 5 3 11 2 1 rt v c fb pg sync bd boost sw v in run/ss pin configuration order information lead free finish tape and reel part marking* package description temperature range LT1913edd#pbf LT1913edd#trpbf ldjw 10-lead (3mm 3mm) plastic dfn ?40c to 125c LT1913idd#pbf LT1913idd#trpbf ldjw 10-lead (3mm 3mm) plastic dfn ?40c to 125c consult ltc marketing for parts speci? ed with wider operating temperature ranges. *the temperature grade is identi? ed by a label on the shipping container. consult ltc marketing for information on non-standard lead based ? nish parts. for more information on lead free part marking, go to: http://www.linear.com/leadfree/ for more information on tape and reel speci? cations, go to: http://www.linear.com/tapeandreel/
LT1913 3 1913f parameter conditions min typ max units feedback voltage 780 775 790 790 800 805 mv mv fb pin bias current (note 3) v fb = 0.8v, v c = 0.4v 10 40 na fb voltage line regulation 4v < v in < 25v 0.002 0.01 %/v error amp g m 525 mho error amp gain 2000 v c source current 60 a v c sink current 60 a v c pin to switch current gain 5.3 a/v v c clamp voltage 2.0 v switching frequency r t = 8.66k r t = 29.4k r t = 187k 2.2 1.0 200 2.45 1.1 230 2.7 1.25 260 mhz mhz khz minimum switch off-time 60 150 ns switch current limit duty cycle = 5% 4.6 5.4 6.0 a switch v cesat i sw = 3.5a 335 mv boost schottky reverse leakage v sw = 10v, v bd = 0v 0.02 2 a minimum boost voltage (note 4) 1.5 2.0 v boost pin current i sw = 1a 35 60 ma run/ss pin current v run/ss = 2.5v 5 8 a run/ss input voltage high 2.5 v run/ss input voltage low 0.2 v pg threshold offset from feedback voltage v fb rising 65 mv pg hysteresis 10 mv pg leakage v pg = 5v 0.1 1 a pg sink current v pg = 0.4v 200 800 a sync low threshold 0.5 v sync high threshold 0.8 v sync pin bias current v sync = 0v 0.1 a note 1: stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. exposure to any absolute maximum rating condition for extended periods may affect device reliability and lifetime. note 2: the LT1913e is guaranteed to meet performance speci? cations from 0c to 125c. speci? cations over the C40c to 125c operating temperature range are assured by design, characterization and correlation with statistical process controls. the LT1913i speci? cations are guaranteed over the C40c to 125c temperature range. note 3: bias current ? ows out of the fb pin. note 4: this is the minimum voltage across the boost capacitor needed to guarantee full saturation of the switch. the denotes the speci? cations which apply over the full operating temperature range, otherwise speci? cations are at t a = 25c. v in = 10v, v run/ss = 10v, v boost = 15v, v bd = 3.3v unless otherwise noted. (note 2) electrical characteristics
LT1913 4 1913f output current (a) 0 0.5 50 efficiency (%) 70 100 1 2 2.5 1913 g01 60 90 80 1.5 3 3.5 v in = 12v v out = 5v l = 4.7h f = 600khz v in = 24v output current (a) 0 0.5 50 efficiency (%) 70 100 1 2 2.5 1913 g02 60 90 80 1.5 3 3.5 v in = 12v v out = 3.3v l = 3.3h f = 600khz v in = 24v ef? ciency ef? ciency input voltage (v) 5 load current (a) 15 1913 g06 4.0 10 20 3.0 2.5 5.5 5.0 4.5 3.5 25 typical minimum v out = 3.3v t a = 25c l = 4.7h f = 600khz maximum load current duty cycle (%) 0 switch current limit(a) 40 1913 g08 4.5 20 60 3.5 3.0 6.0 5.5 5.0 4.0 80 100 switch current limit typical perfor a ce characteristics uw switch current (a) 0 boost pin current (ma) 15 45 60 75 120 1913 g11 30 90 105 03 12 45 boost pin current temperature (c) switch current limit (a) 4.0 4.5 5.5 5.0 1913 g09 3.5 3.0 2.0 2.5 6.5 6.0 duty cycle = 10 % duty cycle = 90 % C50 25 C25 0 50 75 100 150 125 switch current limit input voltage (v) 5 load current (a) 15 1913 g07 4.5 10 20 3.5 3.0 5.5 5.0 4.0 25 typical minimum v out = 5v t a = 25c l = 4.7h f = 600khz maximum load current switch current (a) 0 400 500 700 3 1913 g10 300 200 12 45 100 0 600 voltage drop (mv) switch voltage drop ef? ciency t a = 25c unless otherwise noted. output current (a) 0 0.5 50 efficiency (%) total power loss (w) 70 100 1 2 2.5 1913 g03 60 90 80 0.5 1.5 3.0 1.0 2.5 2.0 1.5 3 3.5 v in = 12v v out = 5v l = 4.7h f = 600khz
LT1913 5 1913f temperature (c) feedback voltage (mv) 800 1913 g12 760 840 780 820 C50 25 C25 0 50 75 100 150 125 feedback voltage temperature (c) frequency (mhz) 1.00 1.10 1913 g13 0.90 0.80 1.20 0.95 1.05 0.85 1.15 C50 25 C25 0 50 75 100 150 125 r t = 34.0k switching frequency fb pin voltage (mv) 0 switching frequency (khz) 800 1000 1200 600 1913 g14 600 400 200 400 800 500 100 300 700 900 200 0 r t = 34.0k frequency foldback temperature (c) minimum switch on time (ns) 80 100 120 1913 g 15 60 40 20 0 140 C50 25 C25 0 50 75 100 15 0 125 minimum switch on-time run/ss pin voltage (v) 0 switch current limit (a) 1.5 1913 g16 4 2 0.5 1 2 1 0 7 6 5 3 2.5 3 3.5 soft-start run/ss pin voltage (v) 0 run/ss pin current (a) 8 10 12 15 25 1913 g17 6 4 510 20 2 0 run/ss pin current boost diode current (a) 0 boost diode v f (v) 0.8 1.0 1.2 2.0 1913 g18 0.6 0.4 0 0.5 1.0 1.5 0.2 1.4 boost diode typical perfor a ce characteristics uw t a = 25c unless otherwise noted. fb pin error voltage (mv) C200 C50 v c pin current (a) C20 0 20 020 0 50 1913 g19 C40 C100 100 40 10 C10 30 C30 error amp output current load current (ma) 1 input voltage (v) 3.0 3.5 1000 0 1913 g20 2.5 2.0 10 100 1000 5.0 4.5 4.0 v out = 3.3v t a = 25c l = 4.7h f = 600khz minimum input voltage
LT1913 6 1913f 1 1000 0 10 100 1000 load current (ma) input voltage (v) 5.0 5.5 1913 g21 4.5 4.0 6.5 6.0 v out = 5v t a = 25 c l = 4.7h f = 600khz minimum input voltage temperature (c) v c voltage (v) 1.50 2.00 2.50 1913 g22 1.00 0.50 0 current limit clamp switching threshold C50 25 C25 0 50 75 100 150 125 temperature (c) threshold voltage (%) 85 90 95 1913 g23 80 75 C50 25 C25 0 50 75 100 150 125 1913 g25 i l 0.2a/div v sw 5v/div v out 10mv/div v in = 12v v out = 3.3v i load = 110ma 1s/div switching waveforms; discontinuous operation power good threshold 1913 g26 i l 0.5a/div v sw 5v/div v out 10mv/div v in = 12v v out = 3.3v i load = 1a 1s/div switching waveforms; continuous operation v c voltages typical perfor a ce characteristics uw t a = 25c unless otherwise noted.
LT1913 7 1913f pi fu ctio s uuu bd (pin 1): this pin connects to the anode of the boost schottky diode. bd also supplies current to the internal regulator. boost (pin 2): this pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar npn power switch. sw (pin 3): the sw pin is the output of the internal power switch. connect this pin to the inductor, catch diode and boost capacitor. v in (pin 4): the v in pin supplies current to the LT1913s internal regulator and to the internal power switch. this pin must be locally bypassed. run/ss (pin 5): the run/ss pin is used to put the LT1913 in shutdown mode. tie to ground to shut down the LT1913. tie to 2.5v or more for normal operation. if the shutdown feature is not used, tie this pin to the v in pin. run/ss also provides a soft-start function; see the applications information section. sync (pin 6): this is the external clock synchronization input. ground this pin when not used. tie to a clock source for synchronization. clock edges should have rise and fall times faster than 1s. do not leave pin ? oating. see synchronizing section in applications information. pg (pin 7): the pg pin is the open collector output of an internal comparator. pg remains low until the fb pin is within 9% of the ? nal regulation voltage. pg output is valid when v in is above 3.6v and run/ss is high. fb (pin 8): the LT1913 regulates the fb pin to 0.790v. connect the feedback resistor divider tap to this pin. v c (pin 9): the v c pin is the output of the internal error ampli? er. the voltage on this pin controls the peak switch current. tie an rc network from this pin to ground to compensate the control loop. rt (pin 10): oscillator resistor input. connecting a resistor to ground from this pin sets the switching frequency. exposed pad (pin 11): ground. the exposed pad must be soldered to pcb.
LT1913 8 1913f block diagram + C + C + C oscillator 200khz to 2.4mhz v c clamp soft-start slope comp r v in v in run/ss boost sw switch latch v c v out c2 c3 c f l1 d1 c c r c bd rt r2 gnd error amp r1 fb r t c1 pg 0.7v s q 1913 bd 4 5 10 7 1 2 3 9 11 8 6 internal 0.79v ref sync
LT1913 9 1913f the LT1913 is a constant frequency, current mode step- down regulator. an oscillator, with frequency set by rt, enables an rs ? ip-? op, turning on the internal power switch. 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 on the v c pin provides current limit. the v c pin is also clamped to the voltage on the run/ss pin; soft-start is implemented by generating a voltage ramp at the run/ss pin using an external resistor and capacitor. an internal regulator provides power to the control circuitry. the bias regulator normally draws power from the v in pin, but if the bd pin is connected to an external voltage higher than 3v bias power will be drawn from the external source (typically the regulated output voltage). this improves ef? ciency. the run/ss pin is used to place the LT1913 in shutdown, disconnecting the output and reducing the input current to less than 0.5a. 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 LT1913s operating frequency when the voltage at the fb pin is low. this frequency foldback helps to control the output current during startup and overload. the LT1913 contains a power good comparator which trips when the fb pin is at 91% of its regulated value. the pg output is an open-collector 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 LT1913 is enabled and v in is above 3.6v. operation
LT1913 10 1913f fb resistor network the output voltage is programmed with a resistor divider between the output and the fb pin. choose the 1% resis- tors according to: r1 = r2 v out 0.79 v C1 reference designators refer to the block diagram. setting the switching frequency the LT1913 uses a constant frequency pwm architecture that can be programmed to switch from 200khz to 2.4mhz by using a resistor tied from the rt pin to ground. a table showing the necessary rt value for a desired switching frequency is in figure 1. switching frequency (mhz) r t value (k ) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 215 140 100 78.7 63.4 53.6 45.3 39.2 34 26.7 22.1 18.2 15 12.7 10.7 9.09 figure 1. switching frequency vs. r t value operating frequency tradeoffs selection of the operating frequency is a tradeoff between ef? ciency, 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 ef? ciency, lower maximum input voltage, and higher dropout voltage. the highest acceptable switching frequency (f sw(max) ) for a given application can be calculated as follows: f sw max () = v d + v out t on min () v d + v in ?v sw () where v in is the typical input voltage, v out is the output voltage, v d is the catch diode drop (~0.5v) and v sw is the internal switch drop (~0.5v at max load). this equation shows that slower switching frequency is necessary to safely accommodate high v in /v out ratio. also, as shown in the next section, lower frequency allows a lower dropout voltage. the reason input voltage range depends on the switching frequency is because the LT1913 switch has ? nite minimum on and off times. the switch can turn on for a minimum of ~150ns and turn off for a minimum of ~150ns. typical minimum on time at 25c is 80ns. this means that the minimum and maximum duty cycles are: dc min = f sw t on min () dc max = 1? f sw t off min () where f sw is the switching frequency, the t on(min) is the minimum switch on time (~150ns), and the t off(min) is the minimum switch off time (~150ns). these equations show that duty cycle range increases when switching frequency is decreased. a good choice of switching frequency should allow ad- equate input voltage range (see next section) and keep the inductor and capacitor values small. input voltage range the maximum input voltage for LT1913 applications depends on switching frequency and absolute maxi- mum ratings of the v in and boost pins (25v and 50v respectively). while the output is in start-up, short-circuit, or other overload conditions, the switching frequency should be chosen according to the following equation: v in max () = v out + v d f sw t on min () ?v d + v sw where v in(max) is the maximum operating input voltage, v out is the output voltage, v d is the catch diode drop (~0.5v), v sw is the internal switch drop (~0.5v at max load), f sw is the switching frequency (set by r t ), and t on(min) is the minimum switch on time (~100ns). note that a higher switching frequency will depress the maximum applications information
LT1913 11 1913f operating input voltage. conversely, a lower switching frequency will be necessary to achieve safe operation at high input voltages. if the output is in regulation and no short-circuit, start- up, or overload events are expected, then input voltage transients of up to 25v are acceptable regardless of the switching frequency. in this mode, the LT1913 may enter pulse skipping operation where some switching pulses are skipped to maintain output regulation. in this mode the output voltage ripple and inductor current ripple will be higher than in normal operation. the minimum input voltage is determined by either the LT1913s minimum operating voltage of ~3.6v or by its maximum duty cycle (see equation in previous section). the minimum input voltage due to duty cycle is: v in min () = v out + v d 1? f sw t off min () ?v d + v sw where v in(min) is the minimum input voltage, and t off(min) is the minimum switch off time (150ns). note that higher switching frequency will increase the minimum input voltage. if a lower dropout voltage is desired, a lower switching frequency should be used. inductor selection for a given input and output voltage, the inductor value and switching frequency will determine the ripple current. the ripple current i l increases with higher v in or v out and decreases with higher inductance and faster switch- ing frequency. a reasonable starting point for selecting the ripple current is: 6 i l = 0.4(i out(max) ) where i out(max) is the maximum output load current. to guarantee suf? cient output current, peak inductor current must be lower than the LT1913s switch current limit (i lim ). the peak inductor current is: i l(peak) = i out(max) + 6 i l /2 where i l(peak) is the peak inductor current, i out(max) is the maximum output load current, and 6 i l is the inductor ripple current. the LT1913s switch current limit (i lim ) is 5.5a at low duty cycles and decreases linearly to 4.5a at dc = 0.8. the maximum output current is a function of the inductor ripple current: i out(max) = i lim C 6 i l /2 be sure to pick an inductor ripple current that provides suf? cient maximum output current (i out(max) ). the largest inductor ripple current occurs at the highest v in . to guarantee that the ripple current stays below the speci? ed maximum, the inductor value should be chosen according to the following equation: l = v out + v d f sw i l 1C v out + v d v in(max ) where v d is the voltage drop of the catch diode (~0.4v), v in(max) is the maximum input voltage, v out is the output voltage, f sw is the switching frequency (set by rt), and l is in the inductor value. the inductors rms current rating must be greater than the maximum load current and its saturation current should be about 30% higher. to keep the ef? ciency high, the series resistance (dcr) should be less than 0.05 , and the core material should be intended for high frequency applications. table 1 lists several vendors and suitable types. table 1. inductor vendors vendor url part series type murata www.murata.com lqh55d open tdk www.componenttdk.com slf10145 shielded toko www.toko.com d75c d75f shielded open sumida www.sumida.com cdrh74 cr75 cdrh8d43 shielded open shielded nec www.nec.com mplc073 mpbi0755 shielded shielded of course, such a simple design guide will not always re- sult in the optimum inductor for your application. a larger value inductor provides a slightly higher maximum load current and will reduce the output voltage ripple. if your applications information
LT1913 12 1913f load is lower than 3.5a, 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. there are several graphs in the typical performance characteristics section of this data sheet that show the maximum load current as a function of input voltage and inductor value for several popular output voltages. low inductance may result in discontinuous mode operation, which is okay but further reduces maximum load current. for details of maximum output current and discontinuous mode opera- tion, see linear technology application note 44. finally, for duty cycles greater than 50% (v out /v in > 0.5), there is a minimum inductance required to avoid subharmonic oscillations. see an19. input capacitor bypass the input of the LT1913 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 10f to 22f ceramic capacitor is adequate to bypass the LT1913 and will easily handle the ripple current. note that larger input capacitance is required when a lower switching frequency is used. if the input power source has high impedance, or there is signi? cant inductance due to long wires or cables, additional bulk capacitance may be necessary. this can be provided with a lower performance electrolytic capacitor. step-down regulators draw current from the input sup- ply in pulses with very fast rise and fall times. the input capacitor is required to reduce the resulting voltage ripple at the LT1913 and to force this very high frequency switching current into a tight local loop, minimizing emi. a 10f capacitor is capable of this task, but only if it is placed close to the LT1913 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 LT1913. a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. if the LT1913 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT1913s voltage rating. this situation is easily avoided (see the hot plugging safety section). for space sensitive applications, a 4.7f ceramic capaci- tor can be used for local bypassing of the LT1913 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 LT1913 to ~3.7v. output capacitor and output ripple the output capacitor has two essential functions. along with the inductor, it ? lters the square wave generated by the LT1913 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 LT1913s control loop. ceramic capacitors have very low equivalent series resistance (esr) and provide the best ripple performance. a good starting value is: c out = 100 v out f sw where f sw is in mhz, and c out is the recommended output capacitance in f. use x5r or x7r types. this choice will provide low output ripple and good transient response. transient performance can be improved with a higher value capacitor if the compensation network is also adjusted to maintain the loop bandwidth. a lower value of output capacitor can be used to save space and cost but transient performance will suffer. see the frequency compensation section to choose an appropriate compen- sation network. when choosing a capacitor, look carefully through the data sheet to ? nd out what the actual capacitance is under operating conditions (applied voltage and temperature). a physically larger capacitor, or one with a higher voltage applications information
LT1913 13 1913f rating, may be required. high performance tantalum or electrolytic capacitors can be used for the output capacitor. low esr is important, so choose one that is intended for use in switching regulators. the esr should be 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. catch diode the catch diode conducts current only during switch off time. average forward current in normal operation can be calculated from: i d(avg) = i out (v in C v out )/v in where i out is the output load current. the only reason to consider a diode with a larger current rating than necessary for nominal operation is for the worst-case condition of shorted output. the diode current will then increase to the typical peak switch current. peak reverse voltage is equal to the regulator input voltage. use a schottky diode with a reverse voltage rating greater than the input voltage. table 3 lists several schottky diodes and their manufacturers. table 3. diode vendors part number v r (v) i ave (a) v f at 3 a (mv) on semiconductor mbra340 40 3 500 diodes inc. b330 b320 30 20 3 3 500 450 frequency compensation the LT1913 uses current mode control to regulate the output. this simpli? es loop compensation. in particular, the LT1913 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 vendor phone url part series commands panasonic (714) 373-7366 www.panasonic.com ceramic, polymer, tantalum eef series kemet (864) 963-6300 www.kemet.com ceramic, tantalum t494, t495 sanyo (408) 749-9714 www.sanyovideo.com ceramic, polymer, tantalum poscap murata (408) 436-1300 www.murata.com ceramic avx www.avxcorp.com ceramic, tantalum tps series taiyo yuden (864) 963-6300 www.taiyo-yuden.com ceramic table 2. capacitor vendors applications information
LT1913 14 1913f v c pin, as shown in figure 2. generally a capacitor (c c ) and a resistor (r c ) in series to ground are used. in addi- tion, there may be lower value capacitor in parallel. this capacitor (c f ) is not part of the loop compensation but is used to ? 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. loop compensation determines the stability and transient performance. designing the compensation network is a bit 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 stabil- ity using a transient load. figure 2 shows an equivalent circuit for the LT1913 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 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 3 shows the transient response when the load cur- rent is stepped from 1a to 3a and back to 1a. boost and bias pin considerations capacitor c3 and the internal boost schottky diode (see the block diagram) are used to generate a boost volt- age that is higher than the input voltage. in most cases a 0.47f capacitor will work well. figure 2 shows three ways to arrange the boost circuit. the boost pin must be C + 0.8v sw v c g m = 525mho gnd 3m LT1913 1913 f02 r1 output esr c f c c r c error amplifier fb r2 c1 c1 current mode power stage g m = 5.3mho + polymer or tantalum ceramic c pl figure 3. transient load response of the LT1913 front page application as the load current is stepped from 1a to 3a. v out = 5v figure 2. model for loop response 1913 f03 i l 1a/div v out 100mv/div 10s/div applications information
LT1913 15 1913f more than 2.3v above the sw pin for best ef? ciency. for outputs of 3v and above, the standard circuit (figure 4a) is best. for outputs between 2.8v and 3v, use a 1f boost capacitor. a 2.5v output presents a special case because it is marginally adequate to support the boosted drive stage while using the internal boost diode. for reliable boost pin operation with 2.5v outputs use a good external schottky diode (such as the on semi mbr0540), and a 1f boost capacitor (see figure 4b). for lower output voltages the boost diode can be tied to the input (figure 4c), or to another supply greater than 2.8v. the circuit in figure 4a figure 5. the minimum input voltage depends on output voltage, load current and boost circuit 1913 f05 load current (ma) 1 input voltage (v) 4.0 4.5 5.0 10000 3.5 3.0 2.0 10 100 1000 1 10000 10 100 1000 2.5 6.0 5.5 to start (worst case) to run load current (ma) input voltage (v) 5.0 6.0 7.0 4.0 2.0 3.0 8.0 to run v out = 3.3v t a = 25c l = 8.2h f = 600khz v out = 5v t a = 25c l = 8.2h f = 600khz to start (worst case) applications information v in boost sw bd v in v out 4.7f c3 gnd LT1913 v in boost sw bd v in v out 4.7f c3 d2 gnd LT1913 v in boost sw bd v in v out 4.7f c3 gnd LT1913 1913 fo4 (4a) for v out > 2.8v (4b) for 2.5v < v out < 2.8v (4c) for v out < 2.5v figure 4. three circuits for generating the boost voltage is more ef? cient because the boost pin current and bd pin quiescent current comes from a lower voltage source. you must also be sure that the maximum voltage ratings of the boost and bd pins are not exceeded. the minimum operating voltage of an LT1913 application is limited by the minimum input voltage (3.6v) and by the maximum duty cycle as outlined in a previous section. for proper startup, the minimum input voltage is also limited by the boost circuit. if the input voltage is ramped slowly, or the LT1913 is turned on with its run/ss pin when the output is already in regulation, then the boost capacitor may not be fully charged. because the boost capacitor is
LT1913 16 1913f input. for example, if the synchronization signal will be 250khz and higher, the r t should be chosen for 200khz. to assure reliable and safe operation the LT1913 will only synchronize when the output voltage is near regulation as indicated by the pg ? ag. it is therefore necessary to choose a large enough inductor value to supply the required output current at the frequency set by the r t resistor. see inductor selection section. it is also important to note that slope compensation is set by the r t value: when the sync frequency is much higher than the one set by r t , the slope compensation will be signi? cantly reduced which may require a larger inductor value to prevent subharmonic oscillation. shorted and reversed input protection if the inductor is chosen so that it wont saturate exces- sively, an LT1913 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 LT1913 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 LT1913s output. if the v in pin is allowed to ? oat and the run/ss pin is held high (either by a logic signal or because it is tied to v in ), then the LT1913s 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 the run/ss pin, the sw pin current will drop to essentially zero. however, if the v in pin is grounded while 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 . 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 LT1913, requiring a higher input voltage to maintain regulation. soft-start the run/ss pin can be used to soft-start the LT1913, reducing the maximum input current during start-up. the run/ss pin is driven through an external rc ? lter to create a voltage ramp at this pin. figure 6 shows the start- up and shut-down waveforms with the soft-start circuit. by choosing a large rc time constant, the peak start-up current can be reduced to the current that is required to regulate the output, with no overshoot. choose the value of the resistor so that it can supply 20a when the run/ss pin reaches 2.5v. synchronization synchronizing the LT1913 oscillator to an external fre- quency can be done by connecting a square wave (with 20% to 80% duty cycle) to the sync pin. the square wave amplitude should have valleys that are below 0.3v and peaks that are above 0.8v (up to 6v). the LT1913 may be synchronized over a 250khz to 2mhz range. the r t resistor should be chosen to set the LT1913 switching frequency 20% below the lowest synchronization applications information figure 6. to soft-start the LT1913, add a resisitor and capacitor to the run/ss pin 1913 f06 i l 1a/div v run/ss 2v/div v out 2v/div run/ss gnd run 15k 0.22f 2ms/div
LT1913 17 1913f the output is held high, then parasitic diodes inside the LT1913 can pull large currents from the output through the sw pin and the v in pin. figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. pcb layout for proper operation and minimum emi, care must be taken during printed circuit board layout. figure 8 shows the recommended component placement with trace, ground plane and via locations. note that large, switched currents ? ow in the LT1913s v in and sw pins, the catch diode (d1) and the input capacitor (c1). the loop formed by these components should be as small as possible. these components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. place a local, unbroken ground plane below these components. 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. 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 LT1913 runs only when the input is present v in boost gnd fb run/ss v c sw d4 mbrs140 v in LT1913 1913 f07 v out backup vias to local ground plane vias to v out vias to run/ss vias to pg vias to v in outline of local ground plane 1913 f08 l1 c2 r rt r pg r c r2 r1 c c v out d1 c1 gnd vias to sync figure 8. a good pcb layout ensures proper, low emi operation applications information 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 LT1913 to additional ground planes within the circuit board and on the bottom side. hot plugging safely the small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT1913 circuits. however, these capaci- tors can cause problems if the LT1913 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 LT1913 can ring to twice the nominal input voltage, possibly exceeding the LT1913s rating and damaging the part. if the input supply is poorly controlled or the user will be plugging the LT1913 into an energized supply, the input network should be designed
LT1913 18 1913f to prevent this overshoot. figure 9 shows the waveforms that result when an LT1913 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. a good solution is shown in figure 9b. a 0.7 resistor is added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). a 0.1f capacitor improves high frequency ? ltering. for high input voltages its impact on ef? ciency is minor, reducing ef? ciency by 1.5 percent for a 5v output at full load operating from 24v. figure 9. a well chosen input network prevents input voltage overshoot and ensures reliable operation when the LT1913 is connected to a live supply + LT1913 4.7f v in 20v/div i in 10a/div 20s/div v in closing switch simulates hot plug i in (9a) (9b) low impedance energized 24v supply stray inductance due to 6 feet (2 meters) of twisted pair + LT1913 4.7f 0.1f 0.7 v in 20v/div i in 10a/div 20s/div danger ringing v in may exceed absolute maximum rating (9c) + LT1913 4.7f 22f 35v ai.ei. 1913 f09 v in 20v/div i in 10a/div 20s/div + applications information high temperature considerations the pcb must provide heat sinking to keep the LT1913 cool. the exposed pad on the bottom of the package must be soldered to a ground plane. this ground should be tied to large copper layers below with thermal vias; these lay- ers will spread the heat dissipated by the LT1913. place additional vias can reduce thermal resistance further. with these steps, the thermal resistance from die (or junction) to ambient can be reduced to ja = 35c/w or less. with 100 lfpm air? ow, this resistance can fall by another 25%. further increases in air? ow will lead to lower thermal re-
LT1913 19 1913f typical applications sistance. because of the large output current capability of the LT1913, it is possible to dissipate enough heat to raise the junction temperature beyond the absolute maximum of 125c. when operating at high ambient temperatures, the maximum load current should be derated as the ambient temperature approaches 125c. power dissipation within the LT1913 can be estimated by calculating the total power loss from an ef? ciency measure- ment and subtracting the catch diode loss and inductor loss. the die temperature is calculated by multiplying the LT1913 power dissipation by the thermal resistance from junction to ambient. other linear technology publications application notes 19, 35 and 44 contain more detailed descriptions and design information for buck regulators and other switching regulators. the lt1376 data sheet has a more extensive discussion of output ripple, loop compensation and stability testing. design note 100 shows how to generate a bipolar output supply using a buck regulator. applications information 5v step-down converter sw fb v c pg rt v in bd v in 6.5v to 25v v out 5v 3.5a 10 f 0.47 f 47 f 100k f = 600khz d: on semi mbra340 l: nec mplc0730l4r7 d 15k 63.4k l 4.7 h 536k gnd 680pf on off LT1913 1913 ta02 run/ss boost sync
LT1913 20 1913f typical applications 3.3v step-down converter sw fb v c pg rt v in bd v in 4.8v to 25v v out 3.3v 3.5a 4.7f 0.47f 47f 100k f = 600khz d: on semi mbra340 l: nec mplc0730l3r3 d 19k 63.4k l 3.3h gnd 680pf on off LT1913 1913 ta03 run/ss boost sync 316k 2.5v step-down converter sw fb v c pg rt v in bd v in 4v to 25v v out 2.5v 3.5a 4.7f 1f 47f 100k f = 600khz d1: on semi mbra340 d2: mbr0540 l: nec mplc0730l3r3 d1 15.4k 63.4k l 3.3h 215k gnd 680pf on off LT1913 d2 1913 ta04 run/ss boost sync
LT1913 21 1913f typical applications 12v step-down converter sw fb v c pg rt v in bd v in 15v to 25v v out 12v 3.5a 10f 0.47f 47f 50k f = 600khz d: on semi mbra340 l: nec mbp107558r2p d 17.4k 63.4k l 8.2h gnd 680pf on off LT1913 1913 ta06 run/ss boost sync 715k 5v, 2mhz step-down converter sw fb v c pg rt v in bd v in 8.6v to 22v v out 5v 2.5a 4.7f 0.47f 22f 100k f = 2mhz d: on semi mbra340 l: nec mplc0730l2r2 d 15k 12.7k l 2.2h gnd 680pf on off LT1913 1913 ta05 run/ss boost sync 536k
LT1913 22 1913f typical applications 1.8v step-down converter sw fb v c pg rt v in bd v in 3.6v to 25v v out 1.8v 3.5a 4.7f 0.47f 47f 100k f = 500khz d: on semi mbra340 l: nec mplc0730l3r3 d 16.9k 78.7k l 3.3h 127k gnd 680pf on off LT1913 1913 ta08 run/ss boost sync
LT1913 23 1913f dd package 10-lead plastic dfn (3mm 3mm) (reference ltc dwg # 05-08-1699) package description 3.00 0.10 (4 sides) note: 1. drawing to be made a jedec package outline m0-229 variation of (weed-2). check the ltc website data sheet for current status of variation assignment 2. drawing not to scale 3. all dimensions are in millimeters 4. dimensions of exposed pad on bottom of package do not include mold flash. mold flash, if present, shall not exceed 0.15mm on any side 5. exposed pad shall be solder plated 6. shaded area is only a reference for pin 1 location on the top and bottom of package 0.38 0.10 bottom view?exposed pad 1.65 0.10 (2 sides) 0.75 0.05 r = 0.115 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 1103 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 0.675 0.05 3.50 0.05 package outline 0.25 0.05 0.50 bsc
LT1913 24 1913f linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com ? linear technology corporation 2007 lt 1207 ? printed in usa part number description comments lt1766 60v, 1.2a (i out ), 200khz, high ef? ciency step-down dc/dc converter v in : 5.5v to 60v, v out(min) = 1.2v, i q = 2.5ma, i sd = 25a, tssop16/e package lt1933 500ma (i out ), 500khz step-down switching regulator in sot-23 v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.6ma, i sd < 1a, thinsot tm package lt1936 36v, 1.4a (i out ), 500khz, high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 1.2v, i q = 1.9ma, i sd < 1a, ms8e package lt1940 dual 25v, 1.4a (i out ), 1.1mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 25v, v out(min) = 1.2v, i q = 3.8ma, i sd < 30a, tssop16e package lt1976/lt1967 60v, 1.2a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converters with burst mode operation v in : 3.3v to 60v, v out(min) = 1.2v, i q = 100a, i sd < 1a, tssop16e package lt3434/lt3435 60v, 2.4a (i out ), 200khz/500khz, high ef? ciency step-down dc/dc converters with burst mode operation v in : 3.3v to 60v, v out(min) = 1.2v, i q = 100a, i sd < 1a, tssop16 package lt3437 60v, 400ma (i out ), micropower step-down dc/dc converter with burst mode operation v in : 3.3v to 60v, v out(min) = 1.25v, i q = 100a, i sd < 1a, 3mm 3mm dfn10 and tssop16e packages lt3480 36v with transient protection to 60v, 2a (i out ), 2.4mhz, high ef? ciency 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 dfn10 and msop10e packages lt3481 34v with transient protection to 36v, 2a (i out ), 2.8mhz, high ef? ciency 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 dfn10 and msop10e packages lt3493 36v, 1.4a (i out ), 750khz high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 0.8v, i q = 1.9ma, i sd < 1a, 2mm 3mm dfn6 package lt3505 36v with transient protection to 40v, 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 34v, v out(min) = 0.78v, i q = 2ma, i sd = 2a, 3mm 3mm dfn8 and msop8e packages lt3508 36v with transient protection to 40v, dual 1.4a (i out ), 3mhz, high ef? ciency step-down dc/dc converter v in : 3.7v to 37v, v out(min) = 0.8v, i q = 4.6ma, i sd = 1a, 4mm 4mm qfn24 and tssop16e packages lt3680 36v, 3.5a(i out ), 2.4mhz high ef? ciency step-down dc/dc converter v in : 3.6v to 36v, v out(min) = 0.79v, i q = 75a, i sd < 1a, 3mm 3mm dfn, msop10e lt3684 34v with transient protection to 36v, 2a (i out ), 2.8mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 34v, v out(min) = 1.26v, i q = 850a, i sd < 1a, 3mm 3mm dfn10 and msop10e packages lt3685 36v with transient protection to 60v, dual 2a (i out ), 2.4mhz, high ef? ciency step-down dc/dc converter v in : 3.6v to 38v, v out(min) = 0.78v, i q = 70a, i sd < 1a, 3mm 3mm dfn10 and msop10e packages typical applicatio u related parts sw fb v c pg rt v in bd v in 3.6v to 25v v out 1.2v 3.5a 4.7f 0.47f 100f f = 500khz d: on semi mbra340 l: nec mplc0730l3r3 d 17k 78.7k l 3.3h gnd 470pf on off LT1913 1913 ta09 run/ss boost sync 100k 52.3k 1.2v step-down converter

▲Up To Search▲

Price & Availability of LT1913

	To Download LT1913 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .