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1 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb applicatio s u features descriptio u typical applicatio u 500ma high efficiency, low noise, inductorless step-down dc/dc converter spread spectrum step-down converter 1.5v efficiency vs input voltage (burst mode operation) the ltc ? 3251/ltc3251-1.2/ltc3251-1.5 are 2-phase charge pump step-down dc/dc converters that produce a regulated output from a 2.7v to 5.5v input. the parts use switched capacitor fractional conversion to achieve twice the typical efficiency of a linear regulator. no inductors are required. v out is resistor programmable from 0.9v to 1.6v or fixed at 1.2v or 1.5v, with up to 500ma of load current available. a unique 2-phase spread spectrum architecture provides a very low noise regulated output as well as low noise at the input.* the parts have four operating modes: continuous spread spectrum, spread spectrum with burst mode operation, super burst tm mode operation and shutdown. low operating current (35 a in burst mode operation, 10 a in super burst mode operation) and low external parts count make the ltc3251/ltc3251-1.2/ltc3251-1.5 ideally suited for space-constrained battery-powered applications. the parts are short-circuit and overtempera- ture protected, and are available in a thermally enhanced 10-pin msop package. up to 500ma output current no inductors 2.7v to 5.5v input voltage range 2x efficiency improvement over ldos 2-phase, spread spectrum operation for low input and output noise shutdown disconnects load from v in adjustable output voltage range: 0.9v to 1.6v fixed output voltages: 1.2v, 1.5v super burst, burst and burst defeat operating modes low operating current: i in = 35 a (burst mode ? operation) super burst operating current: i in = 10 a low shutdown current: i in = 0.01 a typ soft-start limits inrush current at turn-on short-circuit and overtemperature protected available in a thermally enhanced 10-pin msop package handheld devices cellular phones portable electronic equipment dsp power supplies md0 md1 ltc3251-1.5 v in c1 + 1 f 1 f 1-cell li-ion or 3-cell nimh off on c1 C gnd v out c2 + c2 C mode 1 f 10 f v out = 1.5v 500ma 3251 ta01 19 2 3 4 5, 11 7 8 6 10 input voltage (v) 3 efficiency (%) 60 80 100 5 3251 ta02 40 20 50 70 90 30 10 0 3.5 4 4.5 5.5 ltc3251-1.5 ldo i out = 200ma , lt, ltc and ltm are registered trademarks of linear technology corporation. burst mode is a registered trademark of linear technology corporation. super burst is a trademark of linear technology corporation. all other trademarks are the property of their respective owners. protected by us patents including 6411531.
2 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb absolute axi u rati gs w ww u package/order i for atio uu w consult ltc marketing for parts specified with wider operating temperature ranges. electrical characteristics parameter conditions min typ max units v in minimum operating voltage (notes 4,5) 2.7 v v in maximum operating voltage (note 5) 5.5 v v in continuous mode operating current i out = 0ma, v md0 = 0, v md1 = v in 35 ma spread spectrum disabled mode = v in 3.75 6 ma v in burst mode operating current i out = 0ma, v md0 = v in , v md1 = 0 35 60 a spread spectrum disabled mode = v in 35 60 a v in super burst mode operating current i out = 0ma, v md0 = v in , v md1 = v in 10 15 a spread spectrum disabled mode = v in 10 15 a v in shutdown current v md0 = 0v, v md1 = 0v (note 5) 0.01 1 a v fb regulation voltage (ltc3251) i out = 0ma, 2.7v v in 5.5v 0.78 0.8 0.82 v v out regulation voltage (ltc3251-1.2) i out 200ma, 2.7v v in 5.5v (note 5) 1.15 1.2 1.25 v continuous mode or burst mode operation i out 300ma, 2.8v v in 5.5v (note 5) 1.15 1.2 1.25 v i out 500ma, 3v v in 5.5v (note 5) 1.15 1.2 1.25 v v out regulation voltage (ltc3251-1.2) i out 40ma 1.15 1.2 1.25 v super burst operation v out regulation voltage (ltc3251-1.5) i out 100ma, 3.1v v in 5.5v (note 5) 1.44 1.5 1.56 v continuous mode or burst mode operation i out 200ma, 3.2v v in 5.5v (note 5) 1.44 1.5 1.56 v i out 300ma, 3.3v v in 5.5v (note 5) 1.44 1.5 1.56 v i out 500ma, 3.5v v in 5.5v (note 5) 1.44 1.5 1.56 v v out regulation voltage (ltc3251-1.5) i out 40ma 1.44 1.5 1.56 v super burst operation i out continuous output current (ltc3251) v md0 = 0, v md1 = v in or v md0 = v in , v md1 = 0 500 ma i out super burst output current (ltc3251) v md0 = v in , v md1 = v in 40 ma load regulation (ltc3251) 0ma i out 500ma, referred to fb pin 0.045 mv/ma order part number mse part marking exposed pad is gnd (pin 11), must be soldered to pcb t jmax = 125 c, ja = 40 c/w, jc = 10 c/w v in to gnd ................................................... C0.3v to 6v md0, md1, mode and fb to gnd . C 0.3v to (v in + 0.3v) i out (note 2) ...................................................... 650ma ltc3251emse the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = 3.6v, c1 = c2 = 1 f, c in = 1 f, c out = 10 f, v mode = 0v for ltc3251-1.2v or ltc3251-1.5, v out = 1.5v for ltc3251, all capacitors ceramic, unless otherwise noted. ltb4 operating temperature range (note 3) ... C40 c to 85 c storage temperature range .................. C 65 c to 150 c lead temperature (soldering, 10 sec)................... 300 c order part number mse part marking exposed pad is gnd (pin 11), must be soldered to pcb t jmax = 125 c, ja = 40 c/w, jc = 10 c/w ltc3251emse-1.2 ltc3251emse-1.5 ltagm ltabe 1 2 3 4 5 md0 v in c1 + c1 C gnd 10 9 8 7 6 mode md1 c2 + v out c2 C top view mse package 10-lead plastic msop 11 1 2 3 4 5 md0 v in c1 + c1 C gnd 10 9 8 7 6 mode md1 c2 + v out c2 C top view mse package 10-lead plastic msop 11 (notes 1, 7) order options tape and reel: add #tr lead free: add #pbf lead free tape and reel: add #trpbf lead free part marking: http://www.linear.com/leadfree/
3 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb v in (v) 2.7 10 9 8 7 6 5 4 3 2 1 0 4.2 5.2 3251 g17 3.2 3.7 4.7 i cc (ma) C40 c 25 c 85 c v in (v) 2.7 50 45 40 85 c 25 c C40 c 35 30 25 20 4.2 5.2 3251 g02 3.2 3.7 4.7 i in ( a) v in (v) 2.7 7 6 5 4 3 2 1 0 4.2 5.2 3251 g01 3.2 3.7 4.7 i in (ma) C40 c 25 c 85 c typical perfor a ce characteristics uw no load supply current vs supply voltage (continuous mode spread spectrum enabled) no load supply current vs supply voltage (burst mode operation) parameter conditions min typ max units line regulation (ltc3251) i out = 500ma, 2.7v v in 5.5v 0.2 %/v spread spectrum frequency range f min switching frequency 0.7 1.0 mhz f max switching frequency 1.6 2 mhz spread spectrum disabled frequency mode = v in 1.3 1.6 2 mhz md0, md1 input high voltage 2.7v v in 5.5v 0.8 1.2 v md0, md1 input low voltage 2.7v v in 5.5v 0.4 0.8 v md0, md1 input high current md0 = v in , md1 = v in C1 1 a md0, md1 input low current md0 = 0v, md1 = 0v C1 1 a fb input current (ltc3251) v fb = 0.85v C50 50 na mode input high voltage (ltc3251-1.2/ltc3251-1.5) 2.7v v in 5.5v 50 70 %/v in mode input low voltage (ltc3251-1.2/ltc3251-1.5) 2.7v v in 5.5v 30 50 %/v in mode input high current (ltc3251-1.2/ltc3251-1.5) mode = v in C1 1 a mode input low current (ltc3251-1.2/ltc3251-1.5) mode = 0v C1 1 a turn-on time (burst or continuous mode operation) r ol = 3 ? , (note 5) 1 ms open-loop output impedance (ltc3251) v in = 3v, i out = 200ma (note 6) 0.45 0.7 ? 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: based on long term current density limitations. note 3: the ltc3251e is guaranteed to meet specified performance from 0 c to 70 c. specifications over the C 40 c to 85 c operating temperature range are assured by design, characterization and correlation with statistical process controls. note 4: minimum operating voltage required for regulation is: v in 2 ? (v out + r ol ? i out ) note 5: v mode = 0v or v mode = v in for ltc3251-1.2/ltc3251-1.5. note 6: output not in regulation; r ol = (v in /2 C v out )/i out . (v fb = 0.76v). burst or continuous mode operation. note 7: this ic includes overtemperature protection that is intended to protect the device during momentary overload conditions. junction temperature will exceed 125 c when overtemperature protection is active. continuous operation above the specified maximum operating junction temperature may impair device reliability. the denotes the specifications which apply over the full operating temperature range, otherwise specifications are at t a = 25 c. v in = 3.6v, c1 = c2 = 1 f, c in = 1 f, c out = 10 f, v mode = 0v for ltc3251-1.2v or ltc3251-1.5, v out = 1.5v for ltc3251, all capacitors ceramic, unless otherwise noted. electrical characteristics no load supply current vs supply voltage (continuous mode, spread spectrum disabled)
4 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb 1.5v output voltage vs supply voltage (burst mode operation/ continuous mode) v in (v) 3 v out (v) 1.52 1.56 1.60 5 3251 g04 1.48 1.44 1.50 1.54 1.58 1.46 1.42 1.40 3.5 4 4.5 5.5 i out = 0ma i out = 250ma i out = 500ma t a = 25 c 1.2v output voltage vs supply voltage (burst mode operation/ continuous mode) v in (v) 2.7 1.300 1.260 1.220 1.180 1.140 1.280 1.240 1.200 1.160 1.120 1.100 4.2 5.2 3251 g05 3.2 3.7 4.7 v out (v) i out = 0ma i out = 500ma i out = 250ma t a = 25 c 1.5v output voltage vs supply voltage (super burst mode operation) v in (v) 3 v out (v) 1.52 1.56 1.60 5 3251 g06 1.48 1.44 1.50 1.54 1.58 1.46 1.42 1.40 3.5 4 4.5 5.5 t a = 25 c 0ma 10ma 40ma fb voltage vs output current (burst mode operation/ continuous mode) 1.5v output efficiency vs output current (burst mode operation) i out (ma) 0 0.780 v fb (v) 0.785 0.790 0.795 0.800 0.805 100 200 300 400 3251 g07 500 600 t a = 25 c v out = 1.5v i out (ma) 0.1 0 efficiency (%) 10 30 40 50 100 70 1 10 3251 g08 20 80 90 60 100 1000 v in = 3.3v md0 = v in , md1 = 0v v in = 4v v in = 5v v in = 3.6v 1.5v output efficiency vs output current (super burst mode operation) i out (ma) 0.1 0 efficiency (%) 10 30 40 50 100 70 1 10 3251 g09 20 80 90 60 100 v in = 3.3v v in = 4v v in = 5v v in = 3.6v md0 = md1 = v in typical perfor a ce characteristics uw no load supply current vs supply voltage (super burst mode operation) v in (v) 2.7 20 16 12 8 4 18 14 10 6 2 0 4.2 5.2 3251 g02 3.2 3.7 4.7 i cc ( a) 85 c 25 c C40 c 1.2v output voltage vs supply voltage (super burst mode operation) 1.2v output efficiency vs output current (burst mode operation) v in (v) 2.7 1.30 1.28 1.26 1.24 1.22 1.20 1.18 1.16 1.14 1.12 1.10 4.2 5.2 3251 g18 3.2 3.7 4.7 v out (v) 0ma 10ma 40ma t a = 25 c i out (ma) 0.1 100 80 60 40 20 90 70 50 30 10 0 10 1000 3251 g19 1 100 efficiency (%) v in = 2.7v v in = 3.5v v in = 4.5v v in = 3v md0 = v in , md1 = 0v
5 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb typical perfor a ce characteristics uw md0/md1 input threshold voltage vs supply voltage max/min oscillator frequency vs supply voltage v in (v) 2.7 1.2 1.0 0.8 85 c C40 c 0.6 1.1 0.9 0.7 0.5 0.4 4.2 5.2 3251 g10 3.2 3.7 4.7 md0/md1 threshold (v) 25 c v in (v) 2.7 0.8 frequency (mhz) 1.0 1.2 1.4 1.6 2.0 3.2 3.7 4.2 4.7 3251 g11 5.2 1.8 0.9 1.1 1.3 1.5 1.9 1.7 C40 c max C40 c min 25 c max 25 c min 85 c max 85 c min output transient response (continuous mode) 450ma 50ma i out v out 20mv/div (ac) t a = 25 c10 s/div 3251 g13 c out = 10 f x5r 6.3v v out = 1.5v output transient response (burst mode operation) 450ma 50ma i out v out 20mv/div (ac) t a = 25 c10 s/div 3251 g14 c out = 10 f x5r 6.3v v out = 1.5v supply transient response (continuous mode) 4.5v 3.5v v in v out 20mv/div (ac) t a = 25 c20 s/div 3251 g15 c out = 10 f x5r 6.3v i out = 250ma v out = 1.5v ltc3251-1.5 output voltage ripple spread spect disabled 10mv/div (ac) spread spect enabled 10mv/div (ac) t a = 25 c 200ns/div 3251 g16 c out = 10 f x5r 6.3v i out = 500ma v out = 1.5v v out 20mv/div (ac) v in
6 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb uu u pi fu ctio s md0 (pin 1)/md1 (pin 9): switching mode input pins. the mode input pins are used to set the operating mode of the ltc3251. the modes of operation are: md1 md0 operating mode 0 0 shutdown 0 1 spread spectrum with burst 1 0 continuous spread spectrum 1 1 super burst md0 and md1 are high impedance cmos inputs and must not be allowed to float. v in (pin 2): input supply voltage. operating v in may be between 2.7v and 5.5v. bypass v in with a 1 f low esr ceramic capacitor to gnd (c out ). c1 + (pin 3): flying capacitor 1 positive terminal (c1). c1 (pin 4): flying capacitor 1 negative terminal (c1). gnd (pin 5, 11): ground. connect to a ground plane for best performance. c2 (pin 6): flying capacitor 2 negative terminal (c2). v out (pin 7): regulated output voltage. v out is discon- nected from v in during shutdown. bypass v out with a low esr ceramic capacitor to gnd (c in ). see v out capacitor selection for capacitor size requirements. c2 + (pin 8): flying capacitor 2 positive terminal (c2). fb (pin 10) (ltc3251): feedback input pin. an output divider should be connected from v out to fb to program the output voltage. mode (pin 10) (ltc3251-1.2/ltc3251-1.5): spread spectrum operation mode pin. a low voltage on mode enables spread spectrum operation. when mode is high spread spectrum operation is disabled and switching occurs at the maximum operating frequency.
7 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb si plified w block diagra w 3 2 1 9 4 charge pump 1 spread spectrum oscillator c1 + c1 C c2 + c2 C 7 v out 10 5 11 fb mode charge pump 2 8 6 switch control and soft-start v in md0 md1 gnd 3251 bd C + burst detect circuit internal on ltc3251-1.2/ ltc3251-1.5 10 ltc3251-1.2/ ltc3251-1.5 only overtemp
8 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb operatio u (refer to block diagram) the ltc3251 family of parts use a dual phase switched capacitor charge pump to step down v in to a regulated output voltage. regulation is achieved by sensing the output voltage through an external resistor divider and modulating the charge pump output current based on the error signal. a 2-phase nonoverlapping clock activates the two charge pumps. the two charge pumps work in paral- lel, but out of phase from each other. on the first phase of the clock, current is transferred from v in , through the external flying capacitor 1, to v out via the switches of charge pump 1. not only is current being delivered to v out on the first phase, but the flying capacitor is also being charged. on the second phase of the clock, flying capaci- tor 1 is connected from v out to ground, transferring the charge stored during the first phase of the clock to v out via the switches of charge pump 1. charge pump 2 operates in the same manner, but with the phases of the clock reversed. this dual phase architecture achieves extremely low output and input noise by providing constant charge transfer from v in to v out . using this method of switching, only half of the output current is delivered from v in , thus achieving twice the efficiency over a conventional ldo. a spread spectrum oscillator, which utilizes random switching frequencies between 1mhz and 1.6mhz, sets the rate of charging and discharging of the flying capacitors. the ltc3251-1.2/ ltc3251-1.5 mode pin can be used to disable spread spectrum operation which causes switching to occur at 1.6mhz. the part also has two types of low current burst mode operation to improve efficiency even at light loads. in shutdown mode, all circuitry is turned off and the ltc3251 family draws only leakage current from the v in supply. furthermore, v out is disconnected from v in . the md0 and md1 pins are cmos inputs with threshold voltages of approximately 0.8v to allow regulator control with low voltage logic levels. the mode pin is also cmos, but has a threshold of about 1/2 ? v in . the ltc3251 family is in shutdown when a logic low is applied to both mode pins. since md0, md1 and mode pins are high impedance cmos inputs, they should never be allowed to float. always drive md0, md1 and mode with valid logic levels. short-circuit/thermal protection the ltc3251 family has built-in short-circuit current limiting as well as overtemperature protection. during short-circuit conditions, internal circuitry automatically limits the output current to approximately 800ma. at higher temperatures, or in cases where internal power dissipation causes excessive self heating on chip (i.e., output short circuit), the thermal shutdown circuitry will shut down the charge pumps when the junction tempera- ture exceeds approximately 160 c. it will re-enable the charge pumps once the junction temperature drops back to approximately 150 c. the ltc3251 will cycle in and out of thermal shutdown without latch-up or damage until the overstress condition is removed. long term overstress (i out > 650ma and/or t j > 125 c) should be avoided as it can degrade the performance or shorten the life of the part. soft-start to prevent excessive current flow at v in during start-up, the ltc3251 family has built-in soft-start circuitry. soft- start is achieved by increasing the amount of current available to the output charge storage capacitor linearly over a period of approximately 500 s. soft-start is en- abled whenever the device is brought out of shutdown, and is disabled shortly after regulation is achieved. spread spectrum operation switching regulators can be particularly troublesome where electromagnetic interference (emi) is concerned. switch- ing regulators operate on a cycle-by-cycle basis to transfer power to an output. in most cases the frequency of operation is either fixed or is a constant based on the output load. this method of conversion creates large components of noise at the frequency of operation (funda- mental) and multiples of the operating frequency (har- monics). figure 1a shows a conventional buck switching converter. figures 1b and 1c are the input and output noise spectrums for the buck converter of figure 1 with v in = 3.6v, v out = 1.5v and i out = 500ma.
9 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb figure 1a. conventional buck switching converter figure 1b. conventional buck converter output noise spectrum with 22 f output capacitor (i o = 500ma) figure 1c. conventional buck converter input noise spectrum with 10 f input capacitor (i o = 500ma) figure 2a. ltc3251 buck converter figure 2b. ltc3251 output noise spectrum with 10 f output capacitor (i o = 500ma) figure 2c. ltc3251 input noise spectrum with 1 f input capacitor (i o = 500ma) operatio u (refer to block diagram) unlike conventional buck converters, the ltc3251s inter- nal oscillator is designed to produce a clock pulse whose period is random on a cycle-by-cycle basis, but fixed between 1mhz and 1.6mhz. this has the benefit of spread- ing the switching noise over a range of frequencies, thus significantly reducing the peak noise. figures 2b and 2c are the input and output noise spectrums for the ltc3251 of figure 2a with v in = 3.6v, v out = 1.5v and i out = 500ma. note the significant reduction in peak output noise (>20dbm) with only 1/2 the output capacitance and the virtual elimination of input harmonics with only 1/10 the input capacitance. spread spectrum operation is used exclusively in continuous mode and for output currents greater than about 50ma in burst mode operation. low current burst mode operation to improve efficiency at low output currents, a burst mode function is included in the ltc3251 family of parts. an output current sense is used to detect when the required output current drops below an internally set threshold (50ma typ). when this occurs, the part shuts down the internal oscillator and goes into a low current operating state. the part will remain in the low current operating state until the output voltage has dropped enough to require another burst of current. when the output current exceeds 50ma, the part will operate in continuous mode. unlike traditional charge pumps, where the burst current is dependant on many factors (i.e., supply, switch strength, sw in comp 22 f 3251 f01a v out 4.7 h fb v in gnd 10 f 1 f 10nh* *10nh = 1cm of pcb trace out in c1 + c1 C c2 + c2 C 10 f 3251 f02a v out ltc3251 fb v in gnd 1 f 1 f1 f 1 f 10nh* *10nh = 1cm of pcb trace C40 C50 noise (dbm) C60 C70 C80 C90 start freq: 100khz rbw: 10khz stop freq: 30mhz 3251 f01b start freq: 100khz rbw: 10khz stop freq: 30mhz noise (dbm) C40 C50 C60 C70 C80 C90 3251 f02b noise (dbm) start freq: 100khz rbw: 10khz stop freq: 30mhz C40 C50 C60 C70 C80 C90 3251 f01c start freq: 100khz rbw: 10khz stop freq: 30mhz noise (dbm) C40 C50 C60 C70 C80 C90 3251 f02c
10 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb operatio u (refer to block diagram) figure 3 capacitor selection, etc.), the parts burst current is set by the burst threshold and hysteresis. this means that the v out ripple voltage in burst mode operation will be fixed and is typically 15mv with a 10 f output capacitor. ultralow current super burst mode operation to further optimize the supply current for low output current requirements, a super burst mode operaton is included in the ltc3251 family of parts. this mode is very similar to burst mode operation, but much of the internal circuitry and switch is shut down to further reduce supply current. in super burst mode operation an internal hyster- etic comparator is used to enable/disable charge transfer. the hysteresis of the comparator and the amount of current deliverable to the output are limited to keep output ripple low. the v out ripple voltage in super burst mode operation is typically 35mv with a 10 f output capacitor. the ltc3251 family can deliver 40ma of current in super burst mode operation but does not switch to continuous mode. the mode pin of the ltc3251-1.2 and ltc3251- 1.5 has no effect on operation in super-burst mode. v out capacitor selection the style and value of capacitors used with the ltc3251 family determine several important parameters such as regulator control loop stability, output ripple and charge pump strength. the dual phase nature of the ltc3251 family minimizes output noise significantly but not completely. what small ripple that does exist is controlled by the value of c out directly. increasing the size of c out will proportionately reduce the output ripple. the esr (equivalent series resistance) of c out plays the dominant role in output noise. when a part switches between clock phases there is a period where all switches are turned off. this blanking period shows up as a spike at the output and is a direct function of the output current times the esr value. to reduce output noise and ripple, it is suggested that a low esr (<0.08 ? ) ceramic capacitor be used for c out . tanta- lum and aluminum capacitors are not recommended be- cause of their high esr. both the style and value of c out can significantly affect the stability of the ltc3251 family. as shown in the block diagram, the ltc3251 family uses a control loop to adjust the strength of the charge pump to match the current required at the output. the error signal of this loop is stored directly on the output charge storage capacitor. thus the charge storage capacitor also serves to form the dominant pole for the control loop. the desired output voltage also affects stability. as the divider ratio (r a /r b ) drops, the effective closed-loop gain increases, thus re- quiring a larger output capacitor for stability. figure 3 shows the suggested output capacitor for optimal tran- sient response. the value of the output capacitance should not drop below the minimum capacitance line to prevent excessive ringing or instability. (see ceramic capacitor selection guidelines section). likewise excessive esr on the output capacitor will tend to degrade the loop stability. the closed loop output impedance of the ltc3251 is approximately: r v v o out ?? 0 045 08 .? . for example, with the output programmed to 1.5v, the r o is 0.085 ? , which produces a 40mv output change for a 500ma load current step. for stability and good load transient response, it is important for the output capacitor to have 0.08 ? or less of esr. ceramic capacitors typically have exceptional esr, and combined with a tight board layout, should yield excellent stability and load transient performance. v out (v) 0.9 c out ( f) 8 14 15 16 1.1 1.3 1.4 3251 f03 6 12 10 7 13 4 5 11 9 1.0 1.2 1.5 1.6 optimum capacitance minimum capacitance
11 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb figure 5. 10nh inductor used for additional input noise reduction operatio u (refer to block diagram) figure 4. 10nh inductor used for additional output noise reduction v out 10nh (trace inductance) 1 f 10 f 3251 f04 v out ltc3251 gnd v in 10nh (trace inductance) 1 f 3251 f05 v in supply ltc3251 gnd further output noise reduction can be achieved by filtering the ltc3251 output through a very small series inductor as shown in figure 4. a 10nh inductor will reject the fast output transients caused by the blanking period. the 10nh inductor can be fabricated on the pc board with about 1cm (0.4") of 1mm wide pc board trace. flying capacitor selection warning: a polarized capacitor such as tantalum or alumi- num should never be used for the flying capacitors since their voltages can reverse upon start-up of the ltc3251. ceramic capacitors should always be used for the flying capacitors. the flying capacitors control the strength of the charge pump. in order to achieve the rated output current, it is necessary for the flying capacitor to have at least 0.4 f of capacitance over operating temperature with a 2v bias (see ceramic capacitor selection guidelines). if only 200ma or less of output current is required for the application, the flying capacitor minimum can be reduced to 0.15 f. ceramic capacitor selection guidelines capacitors of different materials lose their capacitance with higher temperature and voltage at different rates. for example, a ceramic capacitor made of x5r or x7r material will retain most of its capacitance from C 40 c to 85 c, whereas a z5u or y5v style capacitor will lose consider- able capacitance over that range (60% to 80% loss typ). z5u and y5v capacitors may also have a very strong voltage coefficient, causing them to lose an additional 60% or more of their capacitance when the rated voltage is applied. therefore, when comparing different capaci- tors, it is often more appropriate to compare the amount of achievable capacitance for a given case size rather than discussing the specified capacitance value. for example, over rated voltage and temperature conditions, a 4.7 f, 10v, y5v ceramic capacitor in an 0805 case may not provide any more capacitance than a 1 f, 10v, x5r or x7r available in the same 0805 case. in fact, over bias and v in capacitor selection the dual phase architecture used by the ltc3251 family makes input noise filtering much less demanding than conventional charge pump regulators. the input current should be continuous at about i out /2. the blanking period described in the v out section also effects the input. for this reason it is recommended that a low esr, 1 f (0.4 f min) or greater ceramic capacitor be used for c in (see ceramic capacitor selection guidelines section). in cases where the supply impedance is high, heavy output transients can cause significant input transients. these input transients feed back to the output which slows the output transient recovery and increases overshoot and output impedance. this effect can generally be avoided by using low impedance supplies and short supply connec- tions. if this is not possible, a 4.7 f capacitor is recom- mended for the input capacitor. aluminum and tantalum capacitors are not recommended because of their high esr. further input noise reduction can be achieved by filtering the input through a very small series inductor as shown in figure 5. a 10nh inductor will reject the fast input tran- sients caused by the blanking period, thereby presenting a nearly constant load to the input supply. for economy, the 10nh inductor can be fabricated on the pc board with about 1cm (0.4") of 1mm wide pc board trace.
12 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb operatio u (refer to block diagram) figure 6. recommended layout temperature range, the 1 f, 10v, x5r or x7r will provide more capacitance than the 4.7 f, 10v, y5v. the capacitor manufacturers data sheet should be consulted to deter- mine what value of capacitor is needed to ensure mini- mum capacitance values are met over operating tempera- ture and bias voltage. below is a list of ceramic capacitor manufacturers and how to contact them: avx www.avxcorp.com kemet www.kemet.com murata www.murata.com taiyo yuden www.t-yuden.com tdk www.tdk.com layout considerations due to the high switching frequency and transient currents produced by the ltc3251, careful board layout is neces- sary for optimal performance. a true ground plane and short connections to all capacitors will improve perfor- mance and ensure proper regulation under all conditions. figure 6 shows the recommended layout configuration. gnd v in v out 3251 f06 c2 1 f c o 10 f r b r a c1 1 f c i 1 f c a 5pf ltc3251 components not used on the ltc3251-1.2 or ltc3251-1.5 the flying capacitor pins c1 + , c1 C , c2 + , c2 C will have very high edge rate wave forms. the large dv/dt on these pins can couple energy capacitively to adjacent printed circuit board runs. magnetic fields can also be generated if the flying capacitors are not close to the part (i.e., the loop area is large). to decouple capacitive energy transfer, a faraday shield may be used. this is a grounded pc trace between the sensitive node and the ics pins. for a high quality ac ground, it should be returned to a solid ground plane that extends all the way to the part. keep the fb trace of the ltc3251 away from or shielded from the flying capacitor traces or degraded performance could result. thermal management if the junction temperature increases above approximately 160 c, the thermal shutdown circuitry will automatically deactivate the output. to reduce the maximum junction temperature, a good thermal connection to the pc board is recommended. connecting the 10-pin mse paddle directly to a ground plane, and maintaining a solid ground plane under the device on one or more layers of the pc board, can reduce the thermal resistance of the package and pc board considerably. using this method a ja of 40 c/w should be achieved. the actual power dissipated by the ltc3251 (pd) can be calculated by the following equation: pd v vi in out out = ? ? ? ? ? ? 2 C power efficiency the power efficiency ( ) of the ltc3251 family is approxi- mately double that of a conventional linear regulator. this occurs because the input current for a 2-to-1 step-down charge pump is approximately half the output current. for an ideal 2-to-1 step-down charge pump the power effi- ciency is given by: ? == p p vi vi v v out in out out in out out in ? ? 1 2 2
13 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb operatio u (refer to block diagram) at moderate to high output power the switching losses and quiescent current of the ltc3251 family is negligible and the expression above is valid. for example with v in = 3.6v, i out = 200ma and v out regulating to 1.5v the measured efficiency is 81% which is in close agreement with the theoretical 83.3% calculation. programming the ltc3251 output voltage (fb pin) the ltc3251 is programmed to an arbitrary output volt- age via an external resistive divider. figure 7 shows the required voltage divider connection. the voltage divider ratio is given by the expression: r r v v a b out = 08 1 . C for a 1.5v output, r o is 0.085 ? , which produces a 40mv output change for a 500ma load current step. thus, the user may want to target an unloaded output voltage slightly higher than desired to compensate for the output load conditions. the output may be programmed for regulation voltages of 0.9v to 1.6v. since the ltc3251 employs a 2-to-1 charge pump archi- tecture, it is not possible to achieve output voltages greater than half the available input voltage. the minimum v in supply required for regulation can be determined by the following equation: v in(min) 2 ? (v out(min) + i out ? r ol ) the compensation capacitor (c a ) is necessary to counter- act the pole caused by the large valued resistors r a and r b , and the input capacitance of the fb pin. for best results, c a should be 5pf for all r a or r b greater than 10k and can be omitted if both r a and r b are less than 10k. disabling spread spectrum operation on the ltc3251-1.2/ltc3251-1.5 (mode pin) spread spectrum operation can be disabled by driving mode high. when mode is high, switching takes place at the maximum operating frequency (typ 1.6mhz). the advantage of spread spectrum operation is that it reduces the peak noise at and above the operating frequency at the expense of a slightly increased noise floor and slightly increased low frequency ripple caused by the converter compensating for the changing operating frequency. us- ers who do not need the peak noise reduction gained by using spread spectrum may wish to disable spread spec- trum, thus improving the low frequency input/output ripple. typical values for total voltage divider resistance can range from several k ? s up to 1m ? . the user may want to consider load regulation when setting the desired output voltage. the closed loop output impedance of the ltc3251 is approximately: r v v o out ?? 0 045 08 .? . figure 7. programming the ltc3251 v out c out c a r a r b 3251 f07 v out ltc3251 fb gnd 0.8v r a r b 1 + ()
14 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb typical applicatio s u md0 md1 ltc3251 v in 19 c1 + 1 f 1 f c1 C gnd 7 8 6 10 2 off on v in 3.3v 3 4 5,11 v out c2 + c2 C fb 1 f 10 f v out = 1.4v i out 350ma 4.12k 5.36k 3251 ta03 md0 md1 ltc3251 v in 19 c1 + 1 f 1 f 1-cell li-ion or 3-cell nimh off on c1 C gnd 7 8 6 10 2 3 4 5,11 v out c2 + c2 C fb 1 f 10 f v out = 0.9v 500ma 73.2k 5pf 536k 3251 ta05 4.7 f 0.9v output continuous/burst mode operation with shutdown 3.3v to 1.4v conversion, continuous spread spectrum operation with shutdown
15 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb 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. u package descriptio mse package 10-lead plastic msop (reference ltc dwg # 05-08-1663) msop (mse) 0603 0.53 0.152 (.021 .006) seating plane 0.18 (.007) 1.10 (.043) max 0.17 C 0.27 (.007 C .011) typ 0.127 0.076 (.005 .003) 0.86 (.034) ref 0.50 (.0197) bsc 12 3 45 4.90 0.152 (.193 .006) 0.497 0.076 (.0196 .003) ref 8 9 10 10 1 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 C 6 typ detail a detail a gauge plane 5.23 (.206) min 3.20 C 3.45 (.126 C .136) 0.889 0.127 (.035 .005) recommended solder pad layout 0.305 0.038 (.0120 .0015) typ 2.083 0.102 (.082 .004) 2.794 0.102 (.110 .004) 0.50 (.0197) bsc bottom view of exposed pad option 1.83 0.102 (.072 .004) 2.06 0.102 (.081 .004)
16 ltc3251/ ltc3251-1.2/ltc3251-1.5 32511215fb part number description comments ltc1514 50ma, 650khz, step-up/down charge pump v in : 2.7v to 10v, v out : 3v or 5v, regulated output, i q : 60 a, with low battery comparator i sd : 10 a, s8 package ltc1515 50ma, 650khz, step-up/down charge pump v in : 2.7v to 10v, v out : 3.3v or 5v, regulated output, i q : 60 a, with power-on reset i sd : <1 a, s8 package lt1776 500ma (i out ), 200khz, high efficiency 90% efficiency, v in : 7.4v to 40v, v out(min) : 1.24v, step-down dc/dc converter i q : 3.2ma, i sd : 30 a, n8, s8 packages ltc1911-1.5/ 250ma, 1.5mhz, high efficiency up to 90% efficiency, v in : 2.7v to 5.5v, v out : 1.5v/1.8v, regulated output, ltc1911-1.8 step-down charge pump i q : 180 a, i sd : 10 a, ms8 package ltc3250-1.5 250ma, 1.5mhz, high efficiency up to 90% efficiency, v in : 3.1v to 5.5v, v out : 1.5v, regulated output, step-down charge pump i q : 35 a, i sd : <1 a, thinsot package ltc3252 250ma, dual, low noise, inductorless up to 90% efficiency, v in : 2.7v to 5.5v, v out : 0.9v to 1.6v, step-down dc/dc converter i q : 60 a, dfn package ltc3404 600ma (i out ), 1.4mhz, synchronous 95% efficiency, v in : 2.7v to 6v, v out(min) : 0.8v, step-down dc/dc converter i q : 10 a, i sd : <1 a, ms8 package ltc3405/ltc3405a 300ma (i out ), 1.5mhz, synchronous 95% efficiency, v in : 2.7v to 6v, v out(min) : 0.8v, step-down dc/dc converter i q : 20 a, i sd : <1 a, thinsot package ltc3406/ltc3406b 600ma (i out ), 1.5mhz, synchronous 95% efficiency, v in : 2.5v to 5.5v, v out(min) : 0.6v, step-down dc/dc converter i q : 20 a, i sd : <1 a, thinsot package ltc3411 1.25a (i out ), 4mhz, synchronous step-down 95% efficiency, v in : 2.5v to 5.5v, v out(min) : 0.8v, dc/dc converter i q : 60 a, i sd : <1 a, ms package ltc3412 2.5a (i out ), 4mhz, synchronous step-down 95% efficiency, v in : 2.5v to 5.5v, v out(min) : 0.8v, dc/dc converter i q : 60 a, i sd : <1 a, tssop-16e package ltc3440 600ma (i out ), 2mhz, synchronous 95% efficiency, v in : 2.5v to 5.5v, v out : 2.5v to 5.5v, buck-boost dc/dc converter i q : <25 a, i sd : 1 a, ms package ltc3441 1.2a (i out ), 1mhz, synchronous 95% efficiency, v in : 2.4v to 5.5v, v out : 2.4v to 5.25v, buck-boost dc/dc converter i q : <25 a, i sd : 1 a, dfn package lt 0306 rev b ? printed in usa ? linear technology corporation 2003 related parts linear technology corporation 1630 mccarthy blvd., milpitas, ca 95035-7417 (408) 432-1900 fax: (408) 434-0507 www.linear.com u typical applicatio md0 md1 ltc3251-1.2 v in 19 c1 + 1 f 1 f 1-cell li-ion or 3-cell nimh c1 C gnd 7 8 6 10 2 3 4 5,11 v out c2 + c2 C mode 1 f 10 f x5r 6.3v v out = 1.2v i out up to 300ma, v in 2.8v i out up to 500ma, v in 3.0v 3251 ta04 p 1.2v output with mprocessor control of operating modes (spread spectrum disabled)

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