View LTC3700EMSTRPBF_4822416.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

1 ltc3700 3700f load current (ma) 1 efficiency (%) 90 86 82 78 74 70 66 62 58 54 50 10 100 1000 3700 f01a v out = 1.8v r sense = 0.068 v in = 3.3v v in = 5v v in = 4.2v constant frequency step-down dc/dc controller with ldo regulator the ltc ? 3700 is a constant frequency current mode step- down (buck) dc/dc controller with excellent ac and dc load and line regulation. the on-chip 150ma low dropout (ldo) linear regulator can be powered from the buck controllers input supply, its own independent input supply or the buck regulators output. the buck controller incor- porates an undervoltage lockout feature that shuts down the controller when the input voltage falls below 2.1v. the buck regulator provides a 2.5% output voltage accu- racy. it consumes only 210 m a of quiescent current in nor- mal operation with the ldo consuming an additional 50 m a. in shutdown, a mere 10 m a (combined) is consumed. for applications where efficiency is a prime consideration, the buck controller is configured for burst mode operation which enhances efficiency at low output current. to fur- ther maximize the life of a battery source, the external p-channel mosfet is turned on continuously in dropout (100% duty cycle). high constant operating frequency of 550khz allows the use of a small external inductor. the ldo is protected by both current limit and thermal shutdown circuits. the ltc3700 is available in a tiny 10-pin msop. n notebook computers n portable instruments n one or two li-ion battery-powered applications n dual output regulator in tiny 10-pin msop n high efficiency: up to 94% n wide v in range: 2.65v to 9.8v n constant frequency 550khz operation n 150ma ldo regulator with current limit and thermal shutdown protection n high output currents easily achieved n burst mode ? operation at light load n low dropout: 100% duty cycle n current mode operation for excellent line and load transient response n 0.8v reference allows low output voltages n low quiescent current: 260 m a total n shutdown mode draws only 10 m a supply current n common power good output for both supplies , ltc and lt are registered trademarks of linear technology corporation. burst mode is a registered trademark of linear technology corporation. + sense ltc3700 pgate v fb ldo v fb2 pgood v in v in2 i th /run gnd m1 l1 10 h c2 47 f 6v 100k 80.6k d1 10k 169k c4 2.2 f 16v 78.7k v out2 2.5v at 150ma 3700 f01 v in2 3.3v v out1 1.8v at 1a c1, c3: taiyo yuden emk325bj106mnt c2: sanyo poscap 6tpa47m c4: murata grm42-6x7r225k016al d1: motorola mbrm120t3 l1: coiltronics up1b-100 m1: si3443dv r1: dale 0.25w v in1 5v c3 10 f 10v c1 10 f 10v r1 0.068 220pf figure 1. high efficiency 5v to 1.8v/1a buck with 3.3v to 2.5v/150ma ldo buck efficiency vs load current descriptio u features applicatio s u typical applicatio u
2 ltc3700 3700f absolute m axi m u m ratings w ww u (note 1) buck input supply voltage (v in ) ................C 0.3v to 10v sense C , pgate voltages ............. C 0.3v to (v in + 0.3v) v fb , i th /run voltages ..............................C 0.3v to 2.4v pgate peak output current (< 10 m s) ....................... 1a ldo input supply voltage (v in2 ) .................C 0.3v to 6v ldo, v fb2 voltages ..................... C 0.3v to (v in2 + 0.3v) pgood voltage .........................................C 0.3v to 10v ldo peak output current (< 10 m s) ..................... 500ma storage ambient temperature range ... C 65 c to 150 c operating temperature range (note 2) ... C40 c to 85 c junction temperature (note 3) ............................. 150 c lead temperature (soldering, 10 sec).................. 300 c package/order i n for m atio n w u u t jmax = 150 c, q ja = 230 c/ w ms part marking order part number ltc3700ems ltxn consult ltc marketing for parts specified with wider operating temperature ranges. electrical characteristics the l denotes specifications that apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = v in2 = 4.2v unless otherwise specified. (note 2) 1 2 3 4 5 v in2 ldo v fb2 pgood gnd 10 9 8 7 6 i th /run v fb sense v in pgate top view ms package 10-lead plastic msop parameter conditions min typ max units buck dc/dc controller input dc supply current typicals at v in = 4.2v (note 4) normal operation 2.65v v in 9.8v 210 340 m a sleep mode 2.65v v in 9.8v 200 330 m a shutdown 2.65v v in 9.8v, v ith /run = 0v 10 30 m a uvlo v in < uvlo threshold 10 30 m a undervoltage lockout threshold v in falling l 1.90 2.10 2.60 v v in rising l 2.00 2.20 2.65 v shutdown threshold (at i th /run) l 0.15 0.30 0.45 v start-up current source v ith /run = 0v 0.25 0.5 0.85 m a regulated feedback voltage (note 5), 0 c to 70 c l 0.780 0.800 0.820 v (note 5), C40 c to 85 c l 0.770 0.800 0.830 v output voltage line regulation 2.65v v in 9.8v (note 5) 0.1 mv/v output voltage load regulation i th /run sinking 5 m a (note 5) 4 mv/ m a i th /run sourcing 5 m a (note 5) 4 mv/ m a v fb input current (note 5) 10 50 na overvoltage protect threshold measured at v fb 0.820 0.860 0.910 v overvoltage protect hysteresis 20 mv oscillator frequency v fb = 0.8v 500 550 650 khz v fb = 0v 110 khz gate drive rise time c load = 3000pf 40 ns gate drive fall time c load = 3000pf 40 ns peak current sense voltage (note 6) 120 mv peak current sense voltage in burst mode 30 mv
3 ltc3700 3700f note 1: absolute maximum ratings are those values beyond which the life of a device may be impaired. note 2: the ltc3700 is guaranteed to meet specifications from 0 c to 70 c. specifications over the C40 c to 85 c operating temperature range are assured by design, characterization and correlation with statistical process controls. note 3: t j is calculated from the ambient temperature t a and power dissipation p d according to the following formula: t j = t a + (p d ? q ja c/w) note 4: dynamic supply current is higher due to the gate charge being delivered at the switching frequency. note 5: the ltc3700 is tested in a feedback loop that servos v fb to the output of the error amplifier. note 6: peak current sense voltage is reduced dependent on duty cycle to a percentage of value as given in figure 2. note 7: guaranteed by design; not tested in production. note 8: pgood values are expressed as a percentage difference from the respective regulated feedback voltage as given in the table. parameter conditions min typ max units electrical characteristics the l denotes specifications that apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = v in2 = 4.2v unless otherwise specified. (note 2) ldo regulator v in2 input voltage 2.4 6 v input dc supply current typicals at v in2 = 4.2v normal operation with buck enabled 2.4v v in2 6v 50 100 m a normal operation with buck undervoltage 2.4v v in2 6v 100 150 m a shutdown with buck enabled 2.4v v in2 6v, v ith/run = 0v 0 1 m a shutdown with buck undervoltage 2.4v v in2 6v, v ith/run = 0v 8 24 m a regulated feedback voltage 0 c t a 70 c, i ldo = 1ma l 0.780 0.800 0.830 v C40 c t a 85 c, i ldo = 1ma l 0.765 0.800 0.835 v output voltage line regulation (unity-gain feedback) with buck enabled 2.65v v in 9.8v 0.05 mv/v with buck enabled 2.4v v in2 6v, i ldo = 1ma 4 mv/v with buck undervoltage 2.4v v in2 6v, i ldo = 1ma 4 mv/v output voltage load regulation 1ma i load 150ma 0.06 0.12 mv/ma v fb2 input current 010 na ldo short-circuit current v ldo = 0v 150 200 ma ldo dropout v in2 = 3.3v, i ldo = 150ma 270 mv v in2 = 6v, i ldo = 150ma 170 mv overtemperature trip point (note 7) 150 c overtemperature hysteresis (note 7) 5 c pgood feedback voltage pgood threshold (note 8) pgood high-to-low v fb or v fb2 falling C 12 C 7.5 % v fb or v fb2 rising 7.5 12 % pgood low-to-high v fb or v fb2 rising C 10 C 5.0 % v fb or v fb2 falling 5.0 10 % pgood on-resistance v ith/run = 0v, v in = v in2 = 4.2v, v pgood = 100mv 135 180 w
4 ltc3700 3700f typical perfor a ce characteristics uw buck dc/dc controller temperature ( c) 55 35 15 5 25 45 65 85 105 125 v fb voltage (mv) 3700 g01 805 804 803 802 801 800 799 798 797 796 795 v in = 4.2v i th /run = v fb no load temperature ( c) 55 35 15 5 25 45 65 85 105 125 normalized frequency shift (%) 3700 g02 10 8 6 4 2 0 ? ? ? ? ?0 v in = 4.2v temperature ( c) 55 35 15 5 25 45 65 85 105 125 trip voltage (v) 3700 g03 2.30 2.28 2.26 2.24 2.20 2.00 2.18 2.16 2.14 2.12 2.10 v in rising v in falling temperature ( c) 55 35 15 5 25 45 65 85 105 125 i th /run voltage (mv) 3700 g04 400 380 360 340 320 300 280 260 240 220 200 v in = 4.2v duty cycle (%) 20 30 40 50 60 70 80 90 100 trip voltage (mv) 3700 g05 130 120 110 100 90 80 70 60 50 v in = 4.2v t a = 25 c v fb voltage vs temperature normalized oscillator frequency vs temperature undervoltage lockout trip voltage vs temperature shutdown threshold vs temperature maximum (v in C sense C ) voltage vs duty cycle buck supply current vs input voltage v in input voltage (v) 2 150 v in supply current ( a) 160 180 190 200 250 220 4 6 7 3700 g10 170 230 240 210 35 8 9 10 i th /run = v fb v in2 = 0v t a = 25 c
5 ltc3700 3700f typical perfor a ce characteristics uw v in input voltage (v) 2 3 4 5 6 7 8 910 pgood r on ( ) 3700 g09 300 270 240 210 180 150 120 90 60 30 0 v in2 = 0v v pgood = 100mv t a = 25 c pgood r on vs input voltage ldo pass fet r on vs input voltage v in2 input voltage (v) 2 2.5 3 3.5 4 4.5 5 5.5 6 r on ( ) 3700 g08 4.0 3.7 3.4 3.1 2.8 2.5 2.2 1.9 1.6 1.3 1.0 v in = 0 ildo = 100ma t a = 25 c temperature ( c) 55 35 15 5 25 45 65 85 105 125 v fb2 voltage (mv) 3700 g06 850 840 830 820 810 800 790 780 770 760 750 v in2 = 4.2v ldo = v fb2 i load = 100ma i load = 10ma i load = 10 a i load = 1ma v in2 input voltage (v) 2.4 2.85 3.3 3.75 4.2 4.65 5.1 5.55 6 v fb2 voltage (mv) 3700 g07 850 840 830 820 810 800 790 780 770 760 750 t a = 25 c ldo = v fb2 i load = 100ma i load = 1ma i load = 10 a i load = 10ma v fb2 voltage vs temperature ldo line regulation (v fb2 voltage vs supply) ldo regulator load transient response ldo supply current vs input voltage v in2 input voltage (v) 2 20 v in2 supply current ( a) 30 50 60 70 120 90 3 4 4.5 3700 g11 40 100 110 80 2.5 3.5 5 5.5 6 v in = 0v v in = 9.8v ldo = v fb2 i ldo = 10 a t a = 25 c 150 100 50 0 0 i ldo (ma) 50ma/div d v ldo 20mv/div ac coupled t a = 25 c20 m s/div 3700 g12 v in2 = 3.3v v ldo = 2.5v c ldo = 10 m f
6 ltc3700 3700f v in2 (pin 1): ldo input supply pin. must be closely decoupled to gnd (pin 5). ldo (pin 2): ldo output pin. must be closely decoupled to gnd (pin 5) with a low esr ceramic capacitor 3 2.2 m f. v fb2 (pin 3): ldo feedback voltage. receives the feed- back voltage from an external resistor divider between ldo (pin 2) and gnd (pin 5). pgood (pin 4): open-drain power good output. this pin will pull to ground if either voltage output of the buck or the ldo [sensed at v fb (pin 9) and v fb2 (pin 3), respectively] is out of range. when both voltage outputs are valid, this pin will go to a high impedance state. gnd (pin 5): common ground pin for both buck and ldo. pgate (pin 6): gate drive for bucks external p-channel mosfet. this pin swings from 0v to v in . v in (pin 7): buck input supply pin. must be closely decoupled to gnd (pin 5). sense C (pin 8): the negative input to the current com- parator of the buck. monitors switch current of external p-channel mosfet. v fb (pin 9): buck feedback voltage. receives the feed- back voltage from an external resistor divider between buck output and gnd (pin 5). i th /run (pin 10): this pin performs two functions. it serves as the error amplifier compensation point for the buck, as well as a common run control input for both the buck and the ldo. the current comparator threshold of the buck increases with this voltage. nominal voltage range for this pin is 0.7v to 1.9v. forcing this pin below 0.3v causes both the buck and the ldo to be shut down. in shutdown all functions are disabled, the pgate pin is held high and the ldo output will go to a high impedance state. pi n fu n ctio n s uuu
7 ltc3700 3700f fu n ctio n al diagra uu w v in2 switching logic and blanking circuit pgood + + 0.5 a 0.3v 0.3v 0.15v ovp short-circuit detect shdn 1.2v uv 3700 fd v ref + 60mv v ref 0.8v 0.8v 0.74v v fb2 v fb 0.86v v in rs1 voltage reference slope comp icmp r s q freq foldback osc sense v in 7 5 4 + 8 + ldo v fb2 4 3 1 eamp v fb + 9 pgate v in 6 ldo 2 i th /run v in 0.3v v ref 0.8v 10 + shdn cmp + burst cmp sleep gnd + undervoltage lockout overtemperature detect v in2 4 pgood shdn v in
8 ltc3700 3700f operatio u (refer to functional diagram) main control loop (buck controller) the ltc3700 is a constant frequency current mode switch- ing regulator. during normal operation, the external p-channel power mosfet is turned on each cycle when the oscillator sets the rs latch (rs1) and turned off when the current comparator (icmp) resets the latch. the peak inductor current at which icmp resets the rs latch is controlled by the voltage on the i th /run pin, which is the output of the error amplifier eamp. an external resistive divider connected between v out and ground allows the eamp to receive an output feedback voltage v fb . when the load current increases, it causes a slight decrease in v fb relative to the 0.8v reference, which in turn causes the i th /run voltage to increase until the average inductor current matches the new load current. the main control loop is shut down by pulling the i th /run pin low. releasing i th /run allows an internal 0.5 m a current source to charge up the external compensation network. when the i th /run pin reaches 0.3v, the main control loop is enabled with the i th /run voltage then pulled up to its zero current level of approximately 0.7v. as the external compensation network continues to charge up, the corre sponding output current trip level follows, allowing normal operation. comparator ovp guards against transient overshoots > 7.5% by turning off the external p-channel power mosfet and keeping it off until the fault is removed. burst mode operation the buck enters burst mode operation at low load cur- rents. in this mode, the peak current of the inductor is set as if v ith /run = 1v (at low duty cycles) even though the voltage at the i th /run pin is at a lower value. if the inductors average current is greater than the load require- ment, the voltage at the i th /run pin will drop. when the i th /run voltage goes below 0.85v, the sleep signal goes high, turning off the external mosfet. the sleep signal goes low when the i th /run voltage goes above 0.925v and the buck resumes normal operation. the next oscilla- tor cycle will turn the external mosfet on and the switch- ing cycle repeats. dropout operation when the input supply voltage decreases towards the output voltage, the rate of change of inductor current during the on cycle decreases. this reduction means that the external p-channel mosfet will remain on for more than one oscillator cycle since the inductor current has not ramped up to the threshold set by eamp. further reduc- tion in input supply voltage will eventually cause the p-channel mosfet to be turned on 100%, i.e., dc. the output voltage will then be determined by the input voltage minus the voltage drop across the mosfet, the sense resistor and the inductor. undervoltage lockout to prevent operation of the p-channel mosfet below safe input voltage levels, an undervoltage lockout is incorpo- rated into the buck input supply. when the input supply voltage drops below approximately 2.1v, the p-channel mosfet and all circuitry is turned off except the under- voltage block, which draws only several microamperes. short-circuit protection when the output is shorted to ground, the frequency of the oscillator will be reduced to about 110khz. this lower frequency allows the inductor current to safely discharge, thereby preventing current runaway. the oscillators fre- quency will gradually increase to its designed rate when the feedback voltage again approaches 0.8v. overvoltage protection as a further protection, the overvoltage comparator in the buck will turn the external mosfet off when the feedback voltage has risen 7.5% above the reference voltage of 0.8v. this comparator has a typical hysteresis of 20mv. slope compensation and inductors peak current the inductors peak current is determined by: i v r pk ith sense = () . 07 10
9 ltc3700 3700f the ldo is protected by both current limit and thermal shutdown circuits. current limit is set such that the output voltage will start dropping out when the load current reaches approximately 200ma. with a short-circuited ldo output, the device will limit the sourced current to approximately 225ma. the thermal shutdown circuit has a typical trip point of 150 c with a typical hysteresis of 5 c. in thermal shutdown, the ldo pass device is turned off. frequency compensation of the ldo is accomplished by forcing the dominant pole at the output. for stability, a low esr ceramic capacitor 3 2.2 m f is required from ldo to gnd. for improved transient response, particularly at heavy loads, it is recommended to use the largest value of capacitor available in the same size considered. both the buck and the ldo share the same internally generated bandgap reference voltage for their feedback reference. when both input supplies are present, the internal reference is powered by the buck input supply (v in ). for this reason, line regulation for the ldo output is specified both with respect to v in and v in2 if the buck is present and with respect only to v in2 if the buck is disabled. the same is true for v in2 supply current, which will be higher when the buck is disabled by the current draw of the internal reference. when the buck is operating below 40% duty cycle. how- ever, once the duty cycle exceeds 40%, slope com- pensation begins and effectively reduces the peak induc- tor current. the amount of reduction is given by the curves in figure 2. soft-start an internal default soft-start circuit is employed at power up and/or when coming out of shutdown. the soft-start circuit works by internally clamping the voltage at the i th / run pin to the corresponding zero-current level and gradually raising the clamp voltage such that the minimum time required for the programmed switch current to reach its maximum is approximately 0.5msec. after the soft- start circuit has timed out, it is disabled until the part is put in shutdown again or the input supply is cycled. ldo regulator the 150ma low dropout (ldo) regulator on the ltc3700 employs an internal p-channel mosfet pass device be- tween its input supply (v in2 ) and the ldo output pin. the pass fet has an on-resistance of approximately 1.5 w (with v in2 = 4.2v) with a strong dependence on input supply voltage. the dropout voltage is simply the fet on- resistance multiplied by the load current when in dropout. operatio u (refer to functional diagram) duty cycle (%) 0 10 20 30 40 50 60 70 80 90 100 sf = i out /i out(max) (%) 3700 f02 110 100 90 80 70 60 50 40 30 20 10 i ripple = 0.4i pk at 5% duty cycle i ripple = 0.2i pk at 5% duty cycle v in = 4.2v figure 2. maximum output current vs duty cycle
10 ltc3700 3700f the basic ltc3700 application circuit is shown in figure 1. external component selection for the buck is driven by the load requirement and begins with the selection of l1 and r sense (= r1). next, the power mosfet, m1 and the output diode d1 are selected followed by c in (= c1) and c out (= c2). r sense selection for output current r sense is chosen based on the required output current. with the current comparator monitoring the voltage devel- oped across r sense , the threshold of the comparator determines the inductors peak current. the output cur- rent the buck can provide is given by: i r i out sense ripple =- 012 2 . where i ripple is the inductor peak-to-peak ripple current (see inductor value calculation section). a reasonable starting point for setting ripple current is i ripple = (0.4)(i out ). rearranging the above equation, it becomes: r i sense out =< 1 10 ()( ) for duty cycle 40% however, for operation that is above 40% duty cycle, slope compensation effect has to be taken into consideration to select the appropriate value to provide the required amount of current. using figure 2, the value of r sense is: applicatio n s i n for m atio n wu u u r sf i sense out = ()( )( ) 10 100 inductor value calculation the operating frequency and inductor selection are inter- related in that higher operating frequencies permit the use of a smaller inductor for the same amount of inductor ripple current. however, this is at the expense of efficiency due to an increase in mosfet gate charge losses. the inductance value also has a direct effect on ripple current. the ripple current, i ripple , decreases with higher inductance or frequency and increases with higher v in or v out . the inductors peak-to-peak ripple current is given by: i vv fl vv vv ripple in out out d in d = -+ + ? ? ? ? () where f is the operating frequency. accepting larger values of i ripple allows the use of low inductances, but results in higher output voltage ripple and greater core losses. a reasonable starting point for setting ripple current is i ripple = 0.4(i out(max) ). remember, the maximum i ripple occurs at the maximum input voltage.
11 ltc3700 3700f applicatio n s i n for m atio n wu u u molypermalloy (from magnetics, inc.) is a very good, low loss core material for toroids, but it is more expensive than ferrite. a reasonable compromise from the same manu- facturer is kool m m . toroids are very space efficient, especially when you can use several layers of wire. be- cause they generally lack a bobbin, mounting is more difficult. however, new designs for surface mount that do not increase the height significantly are available. power mosfet selection an external p-channel power mosfet must be selected for use with the ltc3700. the main selection criteria for the power mosfet are the threshold voltage v gs(th) and the on resistance r ds(on) , reverse transfer capacitance c rss and total gate charge. since the ltc3700 is designed for operation down to low input voltages, a sublogic level threshold mosfet (r ds(on) guaranteed at v gs = 2.5v) is required for applications that work close to this voltage. when these mosfets are used, make sure that the input supply to the buck is less than the absolute maximum v gs rating, typically 8v. the required minimum r ds(on) of the mosfet is gov- erned by its allowable power dissipation. for applications that may operate the ltc3700 in dropout, i.e., 100% duty cycle, at its worst case the required r ds(on) is given by: r p ip ds on p out max dc () () % = = () + () 100 2 1 d where p p is the allowable power dissipation and d p is the temperature dependency of r ds(on) . (1 + d p) is generally given for a mosfet in the form of a normalized r ds(on) vs temperature curve, but d p = 0.005/ c can be used as an approximation for low voltage mosfets. kool m m is a registered trademark of magnetics, inc. in burst mode operation on the ltc3700, the ripple current is normally set such that the inductor current is continuous during the burst periods. therefore, the peak- to-peak ripple current must not exceed: i r ripple sense 003 . this implies a minimum inductance of: l vv f r vv vv min in out sense out d in d = - ? ? ? ? + + ? ? ? ? 003 . (use v in(max) = v in ) a smaller value than l min could be used in the circuit; however, the inductor current will not be continuous during burst periods. inductor core selection once the value for l is known, the type of inductor must be selected. high efficiency converters generally cannot af- ford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite, molypermalloy or kool m m ? cores. actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. as inductance increases, core losses go down. unfortunately, increased inductance requires more turns of wire and therefore copper losses will in- crease. ferrite designs have very low core losses and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. ferrite core material saturates hard, which means that inductance collapses abruptly when the peak design cur- rent is exceeded. this results in an abrupt increase in inductor ripple current and consequent output voltage ripple. do not allow the core to saturate!
12 ltc3700 3700f applicatio n s i n for m atio n wu u u in applications where the maximum duty cycle is less than 100% and the buck is in continuous mode, the r ds(on) is governed by: r p dc i p ds on p out () @ () + () 2 1 d where dc is the maximum operating duty cycle of the buck. output diode selection the catch diode carries load current during the off-time. the average diode current is therefore dependent on the p-channel switch duty cycle. at high input voltages the diode conducts most of the time. as v in approaches v out the diode conducts only a small fraction of the time. the most stressful condition for the diode is when the output is short-circuited. under this condition the diode must safely handle i peak at close to 100% duty cycle. therefore, it is important to adequately specify the diode peak current and average power dissipation so as not to exceed the diode ratings. under normal load conditions, the average current con- ducted by the diode is: i vv vv i d in out in d out = - + ? ? ? ? the allowable forward voltage drop in the diode is calcu- lated from the maximum short-circuit current as: v p i f d sc max ? () where p d is the allowable power dissipation and will be determined by efficiency and/or thermal requirements. a fast switching diode must also be used to optimize efficiency. schottky diodes are a good choice for low forward drop and fast switching times. remember to keep lead length short and observe proper grounding (see board layout checklist) to avoid ringing and increased dissipation. c in and c out selection in continuous mode, the source current of the p-channel mosfet is a square wave of duty cycle (v out + v d )/ (v in + v d ). to prevent large voltage transients, a low esr input capacitor sized for the maximum rms current must be used. the maximum rms capacitor current is given by: cii vvv v in rms max out in out in required ? - () [] 12 / this formula has a maximum value at v in = 2v out , where i rms = i out /2. this simple worst-case condition is com- monly used for design because even significant deviations do not offer much relief. note that capacitor manufacturers ripple current ratings are often based on 2000 hours of life. this makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. several capacitors may be paralleled to meet the size or height requirements in the design. due to the high operating frequency of the ltc3700, ceramic capacitors can also be used for c in . always consult the manufacturer if there is any question.
13 ltc3700 3700f applicatio n s i n for m atio n wu u u the selection of c out is driven by the required effective series resistance (esr). typically, once the esr require- ment is satisfied, the capacitance is adequate for filtering. the output ripple ( d v out ) is approximated by: d v i esr fc out ripple out ?+ ? ? ? ? 1 8 where f is the operating frequency, c out is the output capacitance and i ripple is the ripple current in the induc- tor. the output ripple is highest at maximum input voltage since d i l increases with input voltage. manufacturers such as nichicon, united chemicon and sanyo should be considered for high performance through- hole capacitors. the os-con semiconductor dielectric capacitor available from sanyo has the lowest esr (size) product of any aluminum electrolytic at a somewhat higher price. once the esr requirement for c out has been met, the rms current rating generally far exceeds the i ripple(p-p) requirement. figure 3. line regulation of v ref and v ith input voltage (v) 2.0 normalized voltage (%) 105 100 95 90 85 80 75 2.2 2.4 2.6 2.8 3700 f03 3.0 v ref v ith in surface mount applications, multiple capacitors may have to be paralleled to meet the esr or rms current handling requirements of the application. aluminum elec- trolytic and dry tantalum capacitors are both available in surface mount configurations. in the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. an excellent choice is the avx tps, avx tpsv and kemet t510 series of surface mount tantalum, available in case heights ranging from 2mm to 4mm. other capacitor types include sanyo os-con, nichicon pl series and panasonic sp. low supply voltage operation although the ltc3700 can function down to 2.1v (typ), the maximum allowable output current is reduced when v in decreases below 3v. figure 3 shows the amount of change as the supply is reduced down to 2.2v. also shown in figure 3 is the effect of v in on v ref as v in goes below 2.3v.
14 ltc3700 3700f figure 4. setting output voltage (buck controller) 9 v fb v out1 ltc3700 r1 3700 f04 r2 figure 5. foldback current limiting v fb i th /run v out1 ltc3700 r1 3700 f05 r2 9 10 d fb1 d fb2 + setting output voltage (buck controller) the buck develops a 0.8v reference voltage between the feedback (pin 9) terminal and ground (see figure 4). by selecting resistor r1, a constant current is caused to flow through r1 and r2 to set the overall output voltage. the regulated output voltage is determined by: v r r out1 08 1 2 1 =+ ? ? ? ? . for most applications, an 80k resistor is suggested for r1. to prevent stray pickup, locate resistors r1 and r2 close to ltc3700. foldback current limiting as described in the output diode selection, the worst- case dissipation occurs with a short-circuited output when the diode conducts the current limit value almost continuously. to prevent excessive heating in the diode, 3 v out2 r3 3700 f06 r4 v fb2 2 ldo ltc3700 figure 6. setting output voltage (ldo regulator) applicatio n s i n for m atio n wu u u foldback current limiting can be added to reduce the current in proportion to the severity of the fault. foldback current limiting is implemented by adding di- odes d fb1 and d fb2 between the output and the i th /run pin as shown in figure 5. in a hard short (v out = 0v), the current will be reduced to approximately 50% of the maximum output current. setting output voltage (ldo regulator) the ldo develops a 0.8v reference voltage between v fb2 (pin 3) and ground (see figure 6), similar to the buck controller. the regulated output voltage v out2 is given by: v r r out2 08 1 4 3 =+ ? ? ? ? . for most applications, an 80k resistor is suggested for r3. to prevent stray pickup, locate resistors r3 and r4 close to ltc3700.
15 ltc3700 3700f package descriptio n u ms package 10-lead plastic msop (reference ltc dwg # 05-08-1661) msop (ms) 0802 0.53 0.01 (.021 .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 0.27 (.007 ?.011) typ 0.13 0.076 (.005 .003) 0.86 (.034) ref 0.50 (.0197) bsc 12 3 45 4.90 0.15 (1.93 .006) 0.497 0.076 (.0196 .003) ref 8 9 10 7 6 3.00 0.102 (.118 .004) (note 3) 3.00 0.102 (.118 .004) note 4 note: 1. dimensions in millimeter/(inch) 2. drawing not to scale 3. dimension does not include mold flash, protrusions or gate burrs. mold flash, protrusions or gate burrs shall not exceed 0.152mm (.006") per side 4. dimension does not include interlead flash or protrusions. interlead flash or protrusions shall not exceed 0.152mm (.006") per side 5. lead coplanarity (bottom of leads after forming) shall be 0.102mm (.004") max 0.254 (.010) 0 ?6 typ detail ? detail ? gauge plane 5.23 (.206) min 3.2 ?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 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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
16 ltc3700 3700f part number description comments lt1375/lt1376 1.5a, 500khz step-down switching regulators high frequency, small inductor, high efficiency ltc1622 low input voltage current mode step-down dc/dc controller v in 2v to 10v, i out up to 4.5a, synchronizable to 750khz optional burst mode operation, 8-lead msop ltc1624 high efficiency so-8 n-channel switching regulator controller n-channel drive, 3.5v v in 36v ltc1625 no r sense tm synchronous step-down regulator 97% efficiency, no sense resistor, current mode ltc1649 3.3v input synchronous controller no need for 5v supply, uses standard logic gate mosfets, i out up to 15a ltc1702a 550khz, 2 phase, dual synchronous controller two channels, minimum c in and c out , i out up to 15a ltc1704 synchronous step-down controller plus linear regulator controller no current sense required, n-channel mosfet drivers, adjustable soft-start ltc1735 single, high efficiency, low noise synchronous switching controller high efficiency 5v to 3.3v conversion at up to 15a lt1761 100ma, low noise, ldo micropower regulators in thinsot tm 1.8v v in 20v, 300mv dropout at 100ma ltc1771 ultralow supply current step-down dc/dc controller 10 m a supply current, 93% efficiency, 1.23v v out 18v, 2.8v v in 20v ltc1772 constant frequency current mode step-down with burst mode operation for higher efficiency dc/dc controller in thinsot at light load current ltc1773 95% efficient synchronous step-down controller 2.65v v in 8.5v, 0.8v v out v in , current mode, 550khz ltc1778 no r sense current mode synchronous step-down controller up to 97% efficiency, 4v v in 36v, 0.8v v out (0.9)(v in ), i out up to 20a ltc1872 thinsot step-up controller 2.5v v in 9.8v, 550khz, 90% efficiency ltc3404 1.4mhz monolithic synchronous step-down controller 2.65v v in 6v, 700ma output current, 8-lead msop ltc3406/ltc3406b 600ma (i out ), 1.5mhz synchronous step-down converter v in = 2.5v to 5.5v, 95% efficiency, thinsot, b version: burst mode defeat no r sense and thinsot are trademarks of linear technology corporation. lt/tp 0203 2k ? printed in usa related parts ? linear technology corporation 2001 linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 l fax: (408) 434-0507 l www.linear.com + 8 71 6 sense ltc3700 pgate 9 10 v fb 2 3 ldo v fb2 4 pgood v in v in2 i th /run 5 gnd m1 l1 15 h c2 47 f 6v 249k 80.6k 10k 220pf d1 169k c3 2.2 f 16v 78.7k v out2 2.5v at 150ma 3700 ta01 v out1 3.3v at 1a v in1 5v c1 10 f 16v r1 0.05 c1: taiyo yuden emk325bj106mnt c2: sanyo poscap 6tpa47m c3: murata grm42-6x7r225k016al d1: motorola mbrs130lt3 l1: coiltronics up1b150 m1: si3443dv r1: dale 0.25w 5v input supply to 3.3v/1a high efficiency output and 2.5v/150ma low noise output typical applicatio u

▲Up To Search▲

Price & Availability of LTC3700EMSTRPBF

	To Download LTC3700EMSTRPBF Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .