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_______________________________________________________________ maxim integrated products 1 for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxims website at www.maxim-ic.com. high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 general description the MAX15108 high-efficiency, current-mode, synchro - nous step-down switching regulator with integrated power switches delivers up to 8a of output current. the regulator operates from 2.7v to 5.5v and provides an output voltage from 0.6v up to 95% of the input voltage, making the device ideal for distributed power systems, portable devices, and preregulation applications. the ic utilizes a current-mode control architecture with a high gain transconductance error amplifier. the current-mode control architecture facilitates easy compensation design and ensures cycle-by-cycle current limit with fast response to line and load transients. the regulator offers a selectable skip-mode functionality to reduce current consumption and achieve a higher effi - ciency at light output load. the low r ds(on) integrated switches ensure high efficiency at heavy loads while minimizing critical inductance, making the layout design a much simpler task with respect to discrete solutions. the ics simple layout and footprint assures first-pass success in new designs. the regulator features a 1mhz, factory-trimmed fixed- frequency pwm mode operation. the high switching frequency, along with the pwm current-mode architecture allows for a compact, all ceramic capacitor design. the ic features a capacitor-programmable soft-start to reduce input inrush current. internal control circuitry ensures safe-startup into a prebiased output. power sequencing is controlled with the enable input and power-good output. the ic is available in a 20-bump (4 x 5 array), 2.5mm x 2mm, wlp package and is fully specified over the -40 n c to +85 n c temperature range. applications distributed power systems ddr memory base stations portable devices notebook power server power features s continuous 8a outpu t current s efficiency up to 96% s 1% accuracy over load, line, and temperature s operates from a 2.7v to 5.5v supply s adjustable output from 0.6v to 0.95 x v in s programmable soft-start s safe startup into prebiased output s external reference input s 1mhz switching frequency s stable with low-esr ceramic output capacitors s skip mode or forced pwm mode s enable input and power-good output for power- supply sequencing s cycle-by-cycle overcurrent protection s fully protected features against overcurrent and overtemperature s input undervoltage lockout s 20-bump (4 x 5 array), 2.5mm x 2mm, wlp package 19-5917; rev 0; 6/11 + denotes a lead(pb)-free/rohs-compliant package. ordering information typical operating circuit evaluation kit available part temp range pin-package MAX15108ewp+ -40 n c to +85 n c 20 wlp skip 2.7v to 5.5v MAX15108 en in lx pgnd fb comp output inx pgood ss
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 2 ______________________________________________________________________________________ in, pgood to pgnd .............................................. -0.3v to +6v lx to pgnd ................................................ -0.3v to (v in + 0.3v) lx to pgnd ..................................... -1v to (v in + 0.3v) for 50ns en, comp, fb, ss, skip to pgnd ............ -0.3v to (v in + 0.3v) lx current (note 1) ............................................... -12a to +12a output short-circuit duration .................................... continuous continuous power dissipation (t a = +70 n c) wlp (derate 21.3mw/ n c above t a = +70 n c) .......... 745.5mw operating temperature range .......................... -40 n c to +85 n c operating junction temperature (note 2) ...................... +105 n c storage temperature range ............................ -65 n c to +150 n c soldering temperature (reflow) (note 3) ........................ +260 n c electrical characteristics (v in = 5v, c ss = 4.7nf, t a = t j = -40 n c to +85 n c. typical values are at t a = +25 n c, unless otherwise noted.) (note 4) absolute maximum ratings note 1: lx has internal clamp diodes to pgnd and in. do not exceed the power dissipation limits of the device when forward biasing these diodes. note 2: limit the junction temperature to +105 n c for continuous operation at full current. note 3: the wlp package is constructed using a unique set of package techniques that impose a limit on the thermal profile the device can be exposed to during board-level solder attach and rework. this limit permits only the use of the solder pro - files recommended in the industry-standard specification jedec 020a, paragraph 7.6, table 3 for ir/vpr and convection reflow. preheating is required. hand or wave soldering is not allowed. stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. these are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. parameter symbol conditions min typ max units in voltage range v in 2.7 5.5 v in shutdown supply current v en = 0v 0.3 3 f a in supply current i in v en = 5v, v fb = 0.75v, not switching 3.4 6 ma v in undervoltage lockout threshold lx starts switching, v in rising 2.6 2.7 v v in undervoltage lockout hysteresis lx stops switching, v in falling 200 mv error amplifier transconductance g mv 1.4 ms voltage gain a vea 90 db fb set-point accuracy v fb over line, load, and temperature 594 600 606 mv fb input bias current i fb -100 +100 na comp to current-sense transconductance g mod 25 a/v comp clamp low v fb = 0.68v 0.93 v compensation ramp valley 1 v power switches high-side switch current-limit threshold i hscl 14 a low-side switch sink current-limit threshold 14 a low-side switch source current-limit threshold 14 a
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 _______________________________________________________________________________________ 3 electrical characteristics (continued) (v in = 5v, c ss = 4.7nf, t a = t j = -40 n c to +85 n c. typical values are at t a = +25 n c, unless otherwise noted.) (note 4) note 4: specifications are 100% production tested at t a = +25 n c. limits over the operating temperature range are guaranteed by design and characterization. parameter symbol conditions min typ max units lx leakage current v en = 0v 10 f a rms lx output current 8 a oscillator switching frequency f sw 850 1000 1150 khz maximum duty cycle d max 94 % minimum controllable on-time 100 ns enable en input high threshold voltage v en rising 1.3 v en input low threshold voltage v en falling 0.4 v en input leakage current v en = 5v 1 f a skip skip input high threshold voltage v skip rising 1.3 v skip input low threshold voltage v skip falling 0.4 v skip input leakage current v skip = 5v 30 f a zero-crossing current threshold i lx falling 0.7 a on-time in skip mode 335 ns soft-start, prebias soft-start current i ss v ss = 0.45v, sourcing 10 f a ss discharge resistance r ss i ss = 10ma, sinking 8.5 i ss prebias mode stop voltage ss rising 0.58 v hiccup number of consecutive current-limit events to hiccup 8 events timeout 1024 clock cycles power-good output pgood threshold fb rising 0.54 0.56 0.58 v pgood threshold hysteresis fb falling 25 mv pgood v ol i pgood = 5ma, v fb = 0.5v 22 100 mv pgood leakage v pgood = 5v, v fb = 0.68v 1 f a thermal shutdown thermal shutdown threshold +160 n c thermal shutdown hysteresis temperature falling 25 n c
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 4 ______________________________________________________________________________________ typical operating characteristics (circuit of typical application circuit , t a = +25 n c, unless otherwise noted.) efficiency vs. output current (v in = 5v, pwm mode) MAX15108 toc01a output current (a) efficiency (%) 7 6 4 5 2 3 1 10 20 30 40 50 60 70 80 90 100 0 0 8 v out = 3.3v v out = 2.5v v out = 1.2v v out = 0.9v v out = 1.5v v out = 1.8v output voltage vs. supply voltage (pwm mode, v out = 1.5v) MAX15108 toc04a supply voltage (v) output voltage (v) 1.485 1.490 1.495 1.500 1.505 1.510 1.515 1.520 1.480 5.1 4.7 3.9 4.3 3.5 3.1 2.7 5.5 i load = 2a i load = 8a efficiency vs. output current (v in = 3.3v, skip mode) MAX15108 toc02b output current (a) efficiency (%) 7 6 4 5 2 3 1 10 20 30 40 50 60 70 80 90 100 0 0 8 v out = 2.5v v out = 1.8v v out = 1.2v v out = 0.9v v out = 1.5v output voltage vs. supply voltage (skip mode, v out = 1.5v) MAX15108 toc04b supply voltage (v) output voltage (v) 1.48 1.49 1.50 1.51 1.52 1.53 1.54 1.55 1.45 1.46 1.47 5.1 4.7 3.9 4.3 3.5 3.1 2.7 5.5 i load = 8a i load = 2a switching frequency vs. input voltage MAX15108 toc03 input voltage (v) switching frequency (khz) 5.1 4.7 3.9 4.3 3.5 3.1 990 1000 1010 1020 1030 1040 1050 1060 1070 1080 980 2.7 5.5 efficiency vs. output current (v in = 3.3v, pwm mode) MAX15108 toc01b output current (a) efficiency (%) 7 6 4 5 2 3 1 10 20 30 40 50 60 70 80 90 100 0 0 8 v out = 2.5v v out = 1.8v v out = 1.2v v out = 0.9v v out = 1.5v efficiency vs. output current (v in = 5v, skip mode) MAX15108 toc02a output current (a) efficiency (%) 7 6 4 5 2 3 1 10 20 30 40 50 60 70 80 90 100 0 0 8 v out = 3.3v v out = 2.5v v out = 1.2v v out = 0.9v v out = 1.5v v out = 1.8v
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 _______________________________________________________________________________________ 5 typical operating characteristics (continued) (circuit of typical application circuit , t a = +25 n c, unless otherwise noted.) output voltage vs. output current (pwm mode, v out = 1.5v) MAX15108 toc05a output current (a) output voltage (v) 7 6 5 4 3 2 1 1.48 1.49 1.50 1.51 1.52 1.53 1.47 0 8 v in = 5v v in = 3.3v output voltage error % vs. supply voltage MAX15108 toc06 supply voltage (v) output voltage error (%) 5.1 4.7 3.9 4.3 3.5 3.1 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 -0.5 2.7 5.5 normalized at v in = 3.3v v out = 1.2v v out = 2.5v v out = 0.9v v out = 1.5v v out = 1.8v i load = 8a switching waveforms (i out = 8a, v in = 5v) MAX15108 toc08 v out 10mv/div ac-coupled i lx 5a/div v lx 2v/div 400ns/div output voltage vs. output current (skip mode, v out = 1.5v) MAX15108 toc05b output current (a) output voltage (v) 7 6 5 4 3 2 1 1.48 1.49 1.50 1.51 1.52 1.53 1.47 0 8 v in = 5v v in = 3.3v load-transient response (v in = 5v, v out = 1.5v) MAX15108 toc07 v out 50mv/div ac-coupled 8a 4a i load 2a/div 40s/div switching waveform in skip mode (i out = 10ma) MAX15108 toc09 v out 10mv/div ac-coupled i lx 2a/div v lx 2v/div 20s/div soft-start waveforms (i load = 8a) MAX15108 toc11 v en 2v/div v out 1v/div v pgood 2v/div v lx 2v/div i lx 5a/div 1ms/div
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 6 ______________________________________________________________________________________ typical operating characteristics (continued) (circuit of typical application circuit , t a = +25 n c, unless otherwise noted.) input shutdown current vs. supply voltage MAX15108 toc12 supply voltage (v) input shutdown current (a) 0.4 0.8 1.2 1.6 2.0 0 5.1 4.7 3.9 4.3 3.5 3.1 2.7 5.5 overload hiccup mode MAX15108 toc14 i in 2a/div i out 10a/div v out 1v/div 400s/div v out = 0v only in a short shutdown waveform (i load = 8a) MAX15108 toc10 v en 2v/div v out v pgood 1v/div v lx 5v/div i lx 5a/div 10s/div input current vs. input voltage MAX15108 toc13 input voltage (v) input current (ma) 1 2 3 4 5 0 5.1 4.7 3.9 4.3 3.5 3.1 2.7 5.5 no-load, skip mode rms input current vs. supply voltage MAX15108 toc15 supply voltage (v) rms input current (a) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 5.1 4.7 3.9 4.3 3.5 3.1 2.7 5.5 short-circuit on output soft-start waveforms (i load = 8a) MAX15108 toc11 v en 2v/div v out 1v/div v pgood 2v/div v lx 2v/div i lx 5a/div 1ms/div
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 _______________________________________________________________________________________ 7 typical operating characteristics (continued) (circuit of typical application circuit , t a = +25 n c, unless otherwise noted.) soft-start (skip mode) MAX15108 toc17b i lx 2a/div v pgood 2v/div v out 1v/div v ss 500mv/div 400s/div enable into prebiased 0.5v output (no load, pwm mode) MAX15108 toc19a i lx 2a/div v pgood 2v/div v out 1v/div v en 2v/div 400s/div fb voltage vs. temperature (v out = 1.5v) MAX15108 toc16 temperature (c) fb voltage (v) 60 35 10 -15 0.590 0.595 0.600 0.605 0.610 0.615 0.585 -40 65 no load v in = 5v, skip mode v in = 5v, pwm mode v in = 3.3v, skip mode v in = 3.3v, pwm mode enable into prebiased 0.5v output (8a load, pwm mode) MAX15108 toc18 i lx 5a/div v pgood 2v/div v out 1v/div v en 2v/div 400s/div enable into prebiased 0.5v output (no load, skip mode) MAX15108 toc19b i lx 2a/div v pgood 2v/div v out 1v/div v en 2v/div 400s/div soft-start (pwm mode) MAX15108 toc17a i lx 2a/div v pgood 2v/div v out 1v/div v ss 500mv/div 400s/div
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 8 ______________________________________________________________________________________ pin description pin configuration bump name function a1, a5, b1, c1, d1 pgnd power ground. low-side switch source terminal. connect pgnd and the return terminals of input and output capacitors to the power ground plane. a2, a3, b2, c2 lx inductor connection. connect lx to the switching side of the inductor. lx is high impedance when the device is in shutdown mode. a4 pgood open-drain power-good output. pgood goes low when v fb is below 530mv. b3, c3, d3 in input power supply. input supply range is 2.7v to 5.5v. bypass in with a minimum 10 f f ceramic capacitor to pgnd. see the typical application circuit. b4 i.c. internally connected. leave unconnected. b5 fb feedback input. connect fb to the center tap of an external resistive voltage-divider from the output to pgnd to set the output voltage from 0.6v to 95% of v in . c4 skip skip mode input. connect skip to en to select skip mode or leave unconnected for fixed- frequency pwm operation. c5 ss soft-start. connect a capacitor from ss to pgnd to set the startup time. see the soft-start section for details on setting the soft-start time. ss is also an external reference input. apply an external voltage reference from 0v to v in - 1.5v to drive soft-start externally. d2 inx internally unconnected. inx is not internally connected to in. however, do externally connect inx to in to increase the area of the power plane for optimal heat dissipation. d4 en enable input. en is a digital input that turns the regulator on and off. drive en high to turn on the regulator. connect to in for always-on operation. d5 comp error amplifier output. connect compensation network from comp to signal ground (sgnd). see the compensation design guidelines section. wlp bump view 5 4 3 2 1 b c d a MAX15108 pgnd pgood lx lx pgnd fb i.c. in lx pgnd ss skip in lx pgnd comp en in inx pgnd
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 _______________________________________________________________________________________ 9 functional diagram 10a MAX15108 ss en fb ground sense buffer error amplifier comp 0.58v skip strong prebias forced_start skpm ck ss buffer 0.6v c ramp ck oscillator control logic in in lx lx ramp gen in current-sense amplifier high-side current limit low-side source-sink current limit and zero-crossing comparator sink source skpm pgood zx 0.555v rising, 0.53v falling power-good comparator voltage reference bias generator en logic, in uvlo thermal shdn shdn skip-mode logic skpm in inx lx pgnd
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 10 _____________________________________________________________________________________ detailed description the MAX15108 high-efficiency, current-mode switching regulator delivers up to 8a of output current. the regu - lator provides output voltages from 0.6v to (0.95 x v in ) with 2.7v to 5.5v input supplies, making the device ideal for on-board point-of-load applications. the ic delivers current-mode control architecture using a high gain transconductance error amplifier. the current- mode control architecture facilitates easy compensation design and ensures cycle-by-cycle current limit with fast response to line and load transients. the regulator features a 1mhz fixed switching fre - quency, allowing for all-ceramic capacitor designs with fast transient responses. the high operating frequency minimizes the size of external components. the ic is available in a 2.5mm x 2mm (4 x 5 array), 0.5mm pitch wlp package. the regulator offers a selectable skip-mode function to reduce current consumption and achieve a high effi - ciency at light output loads. the low r ds(on) integrated switches ensure high efficiency at heavy loads while minimizing critical inductance, making the layout design a much simpler task than that of discrete solutions. the ics simple layout and footprint assure first-pass success in new designs. the ic features pwm current-mode control, allowing for an all-ceramic capacitor solution. the regulator offers capacitor-programmable soft-start to reduce input inrush current. the device safely starts up into a prebiased output. the ic includes an enable input and open-drain pgood output for sequencing with other devices. controller function pwm logic the controller logic block determines the duty cycle of the high-side mosfet under different line, load, and temperature conditions. under normal operation, where the current-limit and temperature protection are not trig - gered, the controller logic block takes the output from the pwm comparator to generate the driver signals for both high-side and low-side mosfets. the control logic block controls the break-before-make logic and all the necessary timing. the high-side mosfet turns on at the beginning of the oscillator cycle and turns off when the comp voltage crosses the internal current-mode ramp waveform. the internal ramp is the sum of the compensation ramp and the current-mode ramp derived from the inductor current (current sense block). the high-side mosfet also turns off if the maximum duty cycle exceeds 95%, or when the current limit is reached. the low-side mosfet turns on for the remainder of the switching cycle. starting into a prebiased output the ic can soft-start into a prebiased output without dis - charging the output capacitor. in safe prebiased startup, both low-side and high-side mosfets remain off to avoid discharging the prebiased output. pwm operation starts when the voltage on ss crosses the voltage on fb. the ic can start into a prebiased voltage higher than the nominal set point without abruptly discharging the output. forced pwm operation starts when the ss volt - age reaches 0.58v, forcing the converter to start. when the low-side sink current-limit threshold of 1a is reached, the low-side switch turns off before the end of the clock period. the low-side sink current limit is 1a. the high- side switch turns on until one of the following conditions is satisfied: ? high-side source current hits the reduced high-side current limit (14a). the high-side switch turns off for the remaining time of clock period. ? the clock period ends. reduced high-side current limit is activated in order to recirculate the current into the high-side power switch rather than into the internal high-side body diode, which can cause damage to the device. the high-side current limit is set to 14a. low-side sink current limit protects the low-side switch from excessive reverse current during prebiased operation. enable input the ic features independent device enable control and power-good signal that allow for flexible power sequencing. drive the enable input (en) high to enable the regulator, or connect en to in for always-on opera - tion. power-good (pgood) is an open-drain output that deasserts when v fb is above 555mv, and asserts low if v fb is below 530mv.
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 ______________________________________________________________________________________ 11 programmable soft-start (ss) the ic utilizes a soft-start feature to slowly ramp up the regulated output voltage to reduce input inrush current during startup. connect a capacitor from ss to sgnd to set the startup time. see the setting the soft-start startup time section for capacitor selection details. error amplifier a high-gain error amplifier provides accuracy for the voltage feedback loop regulation. connect a compen - sation network between comp and sgnd. see the compensation design guidelines section. the error amplifier transconductance is 1.4ms. comp clamp low is set to 0.93v, just below the pwm ramp compensation valley, helping comp to rapidly return to the correct set point during load and line transients. pwm comparator the pwm comparator compares comp voltage to the current-derived ramp waveform (lx current to comp voltage transconductance value is 25a/v). to avoid instability due to subharmonic oscillations when the duty cycle is around 50% or higher, a compensation ramp is added to the current-derived ramp waveform. the compensation ramp slope (0.3v x 1mhz = 0.3v/ f s) is equivalent to half of the inductor current down-slope in the worst case (load 2a, current ripple 30% and maxi - mum duty-cycle operation of 95%). the compensation ramp valley is set to 1v. overcurrent protection and hiccup when the converter output is connected to ground or the device is overloaded, each high-side mosfet current- limit event (14a) turns off the high-side mosfet and turns on the low-side mosfet. a 3-bit counter incre - ments on each current-limit event. the counter is reset after three consecutive events of high-side mosfet turn-on without reaching the current limit. if the current- limit condition persists, the counter fills up reaching eight events. the control logic then discharges ss, stops both high-side and low-side mosfets and waits for a hiccup period (1024 clock cycles) before attempting a new soft- start sequence. the hiccup-mode also operates during soft-start. thermal shutdown protection the ic contains an internal thermal sensor that limits the total power dissipation to protect it in the event of an extended thermal fault condition. when the die tempera - ture exceeds +160 n c, the thermal sensor shuts down the device, turning off the dc-dc converter to allow the die to cool. after the die temperature falls by 25 n c, the device restarts, following the soft-start sequence. skip mode operation the ic operates in skip mode when skip is connected to en. when in skip mode, lx output becomes high impedance when the inductor current falls below 0.7a. the inductor current does not become negative. during a clock cycle, if the inductor current falls below the 0.7a threshold (during off-time), the low side turns off. at the next clock cycle, if the output voltage is above the set point the pwm logic keeps both high-side and low-side mosfets off. if instead the output voltage is below the set point, the pwm logic drives the high-side on for a minimum fixed on-time (330ns). in this way, the system skips cycles, reducing the frequency of operations, and switches only as needed to service load at the cost of an increase in output voltage ripple. see the skip mode frequency and output ripple section for details. in skip mode, power dissipation is reduced and efficiency improved at light loads because the internal power mosfets do not switch at every clock cycle. skip mode must be decided before or at the same time that the part is enabled. changing of skip mode operation with the part operating is not allowed.
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 12 _____________________________________________________________________________________ applications information setting the output voltage connect a voltage-divider (r1 and r2, see figure 1) from out to fb to pgnd to set the dc-dc converter output voltage. choose r1 and r2 so that the dc errors due to the fb input bias current do not affect the output- voltage precision. with lower value resistors, the dc error is reduced, but the amount of power consumed in the resistive divider increases. a typical tradeoff value for r2 is 5k i , but values between 1k i and 20k i are acceptable. once r2 is chosen, calculate r1 using: out 1 2 fb v r r - 1 v ? ? = ? ? ? ? where the feedback threshold voltage v fb = 0.6v. inductor selection a large inductor value results in reduced inductor ripple current, leading to a reduced output ripple voltage. a high-value inductor is of a larger physical size with a higher series resistance (dcr) and a lower saturation current rating. choose inductor values to produce a ripple current equal to 30% of the load current. choose the inductor with the following formula: out out sw l in v v l 1- f i v ? ? = ? ? ? ? ? where f sw is the internally fixed 1mhz switching fre - quency, and d i l is the estimated inductor ripple current (typically set to 0.3 x i load ). in addition, the peak induc - tor current, i l_pk , must always be below the high-side current-limit value, i hscl , and the inductor saturation current rating, i l_sat . ensure that the following relationship is satisfied: l_pk load l hscl l_sat 1 i i i min(i ,i ) 2 = + ? < input capacitor selection for a step-down converter, the input capacitor c in helps to keep the dc input voltage steady, in spite of discon - tinuous input ac current. use low-esr capacitors to minimize the voltage ripple due to esr. size c in using the following formula: load out in sw in_ripple in i v c f v v = ? figure 1. peak current-mode regulator transfer model l o q hs control logic v comp v out pwm comparator comp r c r out g mv v in power modulator output filter and load note: the g mod stage shown above models the average current o f the inductor injected into the output load. this represents a simplification for the power modulator stage drawn above . error amplifier feedback divider compensation ramp g mc dcr i l q ls v out v out i l esr c out r load c c ref *c cc is optional. r out = a vea /g mv *c cc fb r 1 r 2 g mod c
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 ______________________________________________________________________________________ 13 make sure that the selected capacitance can accom - modate the input ripple current given by: out in out rms o in v (v - v ) i i v = if necessary, use multiple capacitors in parallel to meet the rms current rating requirement. output capacitor selection use low-esr ceramic capacitors to minimize the voltage ripple due to esr. use the following formula to estimate the total output voltage peak-to-peak ripple: out out out esr_cout sw in sw out v v 1 v 1- r f l v 8 f c ? ? ? ? ? = + ? ? ? ? ? ? ? ? select the output capacitors to produce an output ripple voltage that is less than 2% of the set output voltage. setting the soft-start startup time the soft-start feature ramps up the output voltage slowly, reducing input inrush current during startup. size the c ss capacitor to achieve the desired soft-start time , t ss, using: ss ss ss fb i x t c v = i ss , the soft-start current, is 10 f a, and v fb , the output feedback voltage threshold, is 0.6v. when using large c out capacitance values, the high-side current limit can trigger during the soft-start period. to ensure the correct soft-start time, t ss , choose c ss large enough to satisfy: out ss ss out hscl_min out fb v i c c (i - i ) v >> i hscl_min is the minimum high-side switch current-limit value. an external tracking reference with steady-state value between 0v and v in - 1.5v can be applied to ss. in this case, connect an rc network from external tracking ref - erence and ss as in figure 2. set r ss to approximately 1k i . in this application, r ss is needed to ensure that, during hiccup period, ss can be internally pulled down. when an external reference is connected to ss, the soft- start must be provided externally. skip mode frequency and output ripple in skip mode, the switching frequency (f skip ) and output ripple voltage (v out-ripple ) shown in figure 3 are cal - culated as follows: t on is a fixed time by design (330ns, typ); the peak inductor current reached is: in out skip limit on v v i t 2 l ? ? = t off1 is the time needed for the inductor current to reach the zero-crossing (~0a): skip-limit off1 out l i t v = figure 2. setting soft-start time figure 3. skip-mode waveforms MAX15108 c ss r ss v ref_ext ss i l v out i skip-limit t on t off1 t off2 = n x t ck i load v out-ripple
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 14 _____________________________________________________________________________________ during t on and t off1 , the output capacitor stores a charge equal to: ( ) 2 skip-limit load in out out out 1 1 l i - i v - v v q 2 ? ? + ? ? ? ? ? = during t off2 (= n x t ck , number of clock cycles skipped), the output capacitor loses this charge: out off2 load q t i ? = ( ) 2 skip-limit load in out out off2 load 1 1 l i - i v - v v t 2 i ? ? + ? ? ? ? = finally, frequency in skip mode is: skip on off1 off2 1 f t t t = + + output ripple in skip mode is: ( ) ( ) out-ripple cout-ripple esr-ripple skip-limit load on esr,cout skip-limit load out v v v i - i t r i - i c = + = + ( ) ( ) out-ripple skip-limit esr,cout skip-limit load out in out v l i r i - i c v - v = ? ? + ? ? ? ? ? ? size c out based on the above formula to limit output ripple in skip mode. compensation design guidelines the ic uses a fixed-frequency, peak-current-mode con - trol scheme to provide easy compensation and fast tran - sient response. the inductor peak current is monitored on a cycle-by-cycle basis and compared to the comp voltage (output of the voltage error amplifier). the regu - lators duty cycle is modulated based on the inductors peak current value. this cycle-by-cycle control of the inductor current emulates a controlled current source. as a result, the inductors pole frequency is shifted beyond the gain bandwidth of the regulator. system stability is provided with the addition of a simple series capacitor-resistor from comp to pgnd. this pole-zero combination serves to tailor the desired response of the closed-loop system. the basic regulator loop consists of a power modulator (comprising the regulators pulse- width modulator, compensation ramp, control circuitry, mosfets, and inductor), the capacitive output filter and load, an output feedback divider, and a voltage- loop error amplifier with its associated compensation circuitry. see figure 1. the average current through the inductor is expressed as: l mod comp i g v = where i l is the average inductor current and g mod is the power modulators transconductance. for a buck converter: out load l v r i = where r load is the equivalent load resistor value. combining the above two relationships, the power mod - ulators transfer function in terms of v out with respect to v comp is: out load l load mod comp l mod v r i r g v i g = = having defined the power modulators transfer function gain, the total system loop gain can be written as follows (see figure 1): ( ) ( ) ( ) ( ) ( ) out c c c cc c out c cc c out r sc r 1 s c c r r 1 s c || c r || r 1 + = ? ? + + + ? ? ? ? + ? ? ( ) ( ) out mod load out load sc esr 1 g r sc esr r 1 + = ? ? + + ? ? 2 vea 1 2 out r a gain r r r = + where r out is the quotient of the error amplifiers dc gain, a vea , divided by the error amplifiers transconduc - tance, g mv ; r out is much larger than r c . 2 fb 1 2 out r v r r v = +
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 ______________________________________________________________________________________ 15 also, c c is much larger than c cc , therefore: c cc c c c c + and c cc cc c || c c rewriting: ( ) ( ) ( ) ( ) c c fb vea out vea c cc c mv out mod load out load sc r 1 v gain a v a sc 1 sc r 1 g sc esr 1 g r sc esr r 1 + = ? ? ? ? + + ? ? ? ? ? ? ? ? ? ? + ? ? + + ? ? the dominant poles and zeros of the transfer loop gain are shown below: mv p1 avea_db/20 c g f 2 10 c = ( ) p2 out load 1 f 2 c esr r = + p3 cc c 1 f 2 c r = z1 c c 1 f 2 c r = z2 out 1 f 2 c esr = the order of pole-zero occurrence is: p1 p2 z1 z2 p3 f f f f f < < < under heavy load, f p2 , approaches f z1 . a graphical representation of the asymptotic system closed-loop response, including dominant pole and zero locations is shown in figure 3. figure 4. asymptotic loop response of peak current-mode regulator 1st asymptote v fb x v out -1 x 10 a vea [db]/20 x g mod x r load 2nd asymptote v fb x v out -1 x g mv x (c c ) -1 x g mod x r load 3rd asymptote v fb x v out -1 x g mv x (c c ) -1 x g mod x r load x (c out (esr + r load )) -1 4th asymptote v fb x v out -1 x g mv x r c x g mod x r load x (c out (esr + r load )) -1 5th asymptote v fb x v out -1 x g mv x r c x g mod x (esr || r load ) 6th asymptote v fb x v out -1 x g mv x (c cc ) -1 x g mod x (esr || r load ) unity gain rad/s 3rd pole (c cc r c ) -1 2nd zero (c out esr) -1 1st zero (c c r c ) -1 2nd pole (c out (esr + r load )) -1 1st pole g mv x (10 a vea [db]/20 c c ) -1 co
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 16 _____________________________________________________________________________________ if c out is large, or exhibits a lossy equivalent series resistance (large esr), the circuits second zero might come into play around the crossover frequency (f co = /2 g ). in this case, a third pole can be induced by a second (optional) small compensation capaci - tor (c cc ), connected from comp to pgnd. the loop responses fourth asymptote (in bold, figure 4) is the one of interest in establishing the desired crossover fre - quency (and determining the compensation component values). a lower crossover frequency provides for stable closed-loop operation at the expense of a slower load and line transient response. increasing the crossover frequency improves the transient response at the (poten - tial) cost of system instability. a standard rule of thumb sets the crossover frequency p 1/10th of the switching frequency. first, select the passive and active power components that meet the applications requirements. then, choose the small-signal compensation compo - nents to achieve the desired closed-loop frequency response and phase margin as outlined in the closing the loop: designing the compensation circuitry section . closing the loop: designing the compensation circuitry select the desired crossover frequency. choose f co approximately 1/10th of the switching frequency f sw , or f co 100khz. select r c using the transfer-loops fourth asymptote gain (assuming f co > f p1 , f p2 , and f z1 and setting the overall loop gain to unity) as follows: ( ) fb mv c mod load out co out load v 1 g r g r v 1 2 f c esr r = + therefore: ( ) co out load out c fb mv mod load 2 f c esr r v r v g g r + = for r load much greater than esr, the equation can be further simplified as follows: out co out c fb mv mod v 2 f c r v g g = where v fb is equal to 0.6v. determine c c by selecting the desired first system zero, f z1 , based on the desired phase margin. typically, set - ting f z1 below 1/5th of f co provides sufficient phase margin. co z1 c c f 1 f 2 c r 5 = therefore: c co c 5 c 2 f r if the esr output zero is located at less than one-half the switching frequency, use the (optional) secondary compensation capacitor, c cc , to cancel it, as follows: p3 z2 cc c out 1 1 f f 2 c r 2 c esr = = = therefore: out cc c c esr c r = if the esr zero exceeds 1/2 the switching frequency, use the following equation: sw p3 cc c f 1 f 2 c r 2 = = therefore: cc sw c 2 c 2 f r = overall c cc detracts from the overall system phase margin. place this third pole well beyond the desired crossover frequency to minimize the interaction with the system loop response at crossover. ignore c cc in these calculations if c cc is smaller than 10pf. power dissipation the ic is available in a 20-bump wlp package and can dissipate up to 745.5mw at t a = +70 n c. when the die temperature exceeds +160 n c, the thermal-shutdown protection is activated. see the thermal shutdown protection section.
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 ______________________________________________________________________________________ 17 layout procedure careful pcb layout is critical to achieve clean and stable operation. it is highly recommended to duplicate the MAX15108 evaluation kit layout for optimum perfor - mance. if deviation is necessary, follow these guidelines for good pcb layout: 1) connect input and output capacitors to the power ground plane. 2) place bypass capacitors as close to in and the soft- start capacitor as close to ss as possible. 3) keep the high-current paths as short and wide as possible. keep the path of switching current short and minimize the loop area formed by lx, the output capacitors, and the input capacitors. 4) connect in, lx, and pgnd separately to a large copper area to help cool the ic to further improve efficiency. 5) ensure all feedback connections are short and direct. place the feedback resistors and compensa - tion components as close as possible to the ic. 6) route high-speed switching nodes (such as lx) away from sensitive analog areas (such as fb, comp, sgnd, and ss). see the MAX15108 ev kit layout for a tested layout example. typical application circuit skip skip 2.7v to 5.5v r pull 100ki r 1 8.06ki r 2 5.36ki r ea 2.43ki c ea 4700pf c ea2 100pf r ext_ref 1ki c in2 22f c out1 47f c out2 47f c out1 0.1f c in2 22f c ss 33nf en in lx pgnd fb comp output l out 33h inx pgood ss max15018
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 18 _____________________________________________________________________________________ chip information process: bicmos package information for the latest package outline information and land patterns (footprint), go to www.maxim-ic.com/packages . note that a +, #, or - in the package code indicates rohs status only. package drawings may show a different suffix character, but the drawing pertains to the package regardless of rohs status. package type package code outline no. land pattern no. 20 wlp w202d2z+1 21-0505 refer to application note 1891
high-efficiency, 8a, current-mode synchronous step-down switching regulator MAX15108 maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circuit patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. the parametric values (min and max limits) shown in the electrical characteristics table are guaranteed. other parametric values quoted in this data sheet are provided for guidance. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 19 ? 2011 maxim integrated products maxim is a registered trademark of maxim integrated products, inc. revision history revision number revision date description pages changed 0 6/11 initial release

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