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mic2165 adaptive on-time dc-dc controller featuring hyperlight load ? hyper speed control? family mlf and micro leadframe are registered trademarks of amkor technology, inc. hyperlight load is a registered trademark of micrel, inc. hyper speed control and any capacitor are trademarks of micrel, inc. micrel inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel +1 ( 408 ) 944-0800 ? fax + 1 (408) 474-1000 ? http://www.micrel.com september 2010 m9999-092410-e general description the micrel mic2165 is a synchronous adaptive on-time buck controller targeting high- performance, cost-sensitive applications such as set-t op boxes, gateways, routers, computing peripherals, and telecom/networking equipment. the mic2165 operates over a supply range of 4.5v to 28v. it has an internal linear regulator which provides a regulated 5v supply to power t he internal control circuitry. mic2165 operates at a constant 600khz switching frequency and can be used to drive up to 25a of output current. the output voltage is adjustable from 0.8v to 5.5v. a unique hyper speed control? architecture enables ultra-fast transient response while reducing the output capacitance and also makes high v in /low v out operation possible. a uvlo feature is provided to ensure proper operation under power-sag conditions to prevent the external power mosfet from over heating. also, a soft start feature is provided to reduce the inrush current. short current sensing on the bottom mosfet with hiccup mode current limiting ensures protection in case of an output short circuit. further, the mic2165 includes an en pin to shut down the converter and a power good (pgood) pin to allow simple sequencing. the mic2165 is available in a 10-pin msop epad package with a junction oper ating temperature ranging from ?40 c to +125 c. all support documentation can be found on micrel?s web site at: www.micrel.com . features ? hyper speed control? architecture enables ? high v in low v out operation (v in =28v & v out =0.8v) ? small output capacitance ? hyperlight load ? efficiency ? built-in 5v regulator for single-supply operation ? any capacitor tm stable - zero esr to high esr ? power-good output ? input voltage range: 4.5v to 28v ? 5 a typical shutdown current ? 25a output current drive capability ? adjustable output from 0.8v to 5.5v with 1% fb accuracy ? 600khz switching frequency ? internal 5ms digital soft start ? thermal shutdown and hiccup current-limit protection ? no external current-sense resistor required ? safe start-up into pre-biased loads ? 10-pin msop epad package ? ?40 c to +125 c junction temperature range applications ? set-top box, gateways, routers and dsl modems ? printers, scanners, graphic and video cards ? servers, pcs and processor core supply ? low-voltage distributed power typical application mic2165 adjustable output 600khz buck converter 12v to 3.3v efficiency 0 10 20 30 40 50 60 70 80 90 100 0.01 0.10 1.00 10.00 output current (a) efficiency (%)
micrel, inc. mic2165 september 2010 2 m9999-092410-e ordering information part number voltage switching frequency junction temp. range package lead finish MIC2165YMME adj. 600khz ?40 to +125c 10-pin epad msop pb-free pin configuration 10-pin epad msop (mme) pin description pin number pin name pin function 1 fb feedback (input): input to the tran sconductance amplifier of the contro l loop. the fb pin is regulated to 0.8v. a resistor divider connecting the output to fb is used to set the desired output voltage. 2 pgood power good (output): open drain ou tput. the pgood pin is externally tied with a resistor to vdd. high output when v out >90% nominal. 3 en enable (input): a logic level control of the output. the en pin is cmos-compatible. logic high = enable, logic low = shutdown. in the off state, supply current of the device is greatly reduced (typically 5a). the en pin should not be left open. connect to vin if sequencing not required. 4 vin supply voltage: input voltage for the internal +5v linear regulator. the vin operating voltage range is from 4.5v to 28v. a 0.1f capacitor be tween vin and the ground is required. 5 vdd 5v internal linear regulator (output): vdd is t he external mosfet gate drive supply voltage and an internal supply bus for the ic. vdd is created by internal ldo from vin. when vin <+5.5v, vdd should be tied to vin. a 2.2f (minimum) ceramic capacitor from vdd to gnd is recommended for clean operation. 6 dl low-side gate drive (output): high- current driver output for external low-side mosfet. the dl driving voltage swings from ground to vdd. 7 pgnd power ground. pgnd is the ground path for the mic2165 buck converter power stage. the pgnd pin connects to the sources of low-side n-channel mosf ets, the negative terminal s of input capacitors, and the negative terminals of output capacitors. t he loop for the power ground should be as small as possible and separate from the signal ground (gnd) loop. 8 dh high-side gate drive (output): high-current driv er output for external high-side mosfet. the dh driving voltage is floating on the switch node volt age (sw). it swings from ground-to-vdd minus the diode drop. 9 sw switch node (input): high current output driver return. the sw pin connects directly to the switch node. due to the high speed switching on this pin, the sw pin should be routed away from sensitive nodes. current-sense input (input): sw pin also senses t he current by monitoring the voltage across the low- side mosfet during off-time. the current sensing is necessary for short circuit protection and zero current cross comparator. in order to sense the curre nt accurately, connect the low-side mosfet drain to sw using a kelvin connection. 10 bst boost (output): bootstrappe d voltage to the high-side n-channel mosfet driver. a schottky diode is connected between the vdd pin and t he bst pin. a boost capacitor of 0.1 f is connected between the bst pin and the sw pin. adding a small resistor at bs t pin can slow down the turn-on time of high-side n-channel mosfets. ep gnd thermal pad and signal ground. gnd is the ground path for vdd and the control circuitry. the loop for the signal ground should be separate from the power ground (pgnd) loop.
micrel, inc. mic2165 september 2010 3 m9999-092410-e absolute maximum ratings (1) v in to gnd ..................................................... ? 0.3v to +29v v dd , v fb , v pgood to gnd ................................. ? 0.3v to +6v v bst to v sw ...................................................... ? 0.3v to +6v v bst to gnd .................................................. ? 0.3v to +35v v en to gnd............................................ ? 0.3v to (v in +0.3v) v dh to v sw ......................................... ? 0.3v to (v bst + 0.3v) v dl to gnd ......................................... ? 0.3v to (v dd + 0.3v) pgnd to gnd .............................................. ? 0.3v to +0.3v junction temperature .............................................. +150c storage temperature (t s )......................... ? 65 c to +150 c lead temperature (solder ing, 10sec)........................ 260c operating ratings (2) supply voltage (v in ).......................................... 4.5v to 28v enable input voltage (v en )..................................... 0v to v in junction temperature (t j ) ........................ ? 40 c to +125 c package thermal resistance (3) msop-10l epad ( ja ) ......................................77 c/w msop-10l epad ( jc ) ......................................10 c/w electrical characteristics v in = v en = 12v; v bst ? v sw = 5v; t j = 25c, unless noted. bold values indicate ? 40c t j 125c. parameter condition min. typ. max. units power input supply input voltage range (v in ) 4.5 28 v quiescent supply current v fb = 1.5v (non-switching) 450 750 a shutdown current v en = 0v 5 10 a vdd supply vdd output voltage v in = 7v to 28v, i dd = 40ma 4.8 5.2 5.4 v vdd uvlo threshold v dd rising 3.7 4.2 4.5 v vdd uvlo hysteresis 400 mv dropout voltage (v in ? v dd ) i dd = 25ma 380 600 mv dc-dc controller output-voltage adjust range (v out ) 0.8 5.5 v reference t j = 25c 0.792 0.8 0.808 v 0c t j 85c 0.788 0.812 v feedback regulation voltage ? 40c t j 125c 0.784 0.816 v load regulation i out = 2a to 10a (continuous mode) depends on external components 0.25 % line regulation v in = 4.5v to 28v depends on external components 0.25 % fb bias current v fb = 0.8v 50 500 na enable control (5) enable logic level high 1.6 v enable logic level low 0.6 v enable hysteresis 100 mv enable bias current v en = 12v 6 30 a
micrel, inc. mic2165 september 2010 4 m9999-092410-e electrical characteristics (continued) v in = v en = 12v; v bst ? v sw = 5v; t j = 25c, unless noted. bold values indicate ? 40c t j 125c. parameter condition min. typ. max. units on timer switching frequency 450 600 750 khz minimum off-time 200 300 400 ns maximum duty cycle results from switching frequency and minimum off-time 82 % minimum duty cycle v fb = 1.0v 0 % bottom fet active zero crossing comparator offset -16 0.5 16 mv short-current protection current-limit threshold v fb = 0.79v 98 133 182 mv short-circuit current v fb = 0v 24 48 72 mv fet drives dh, dl output low voltage i sink = 10ma 0.1 v dh, dl output high voltage i source = 10ma v dd ? 0.1v or v bst ? 0.1v v pull up, i source = 20ma 2 3 dh on-resistance pull down, i sink = 20ma 1.5 3 ? pull up, i source = 20ma 2 3 dl on-resistance pull down, i sink = 20ma 1 2 ? sw, bst leakage current v sw = v bst = 0 30 a power good (pgood) pgood threshold voltage sweep v fb from low to high 85 90 95 %v out pgood hysteresis sweep v fb from high to low 6.0 %v out pgood delay time sweep v fb from low to high 100 s pgood low voltage v fb <0.9 v nom , i pgood = 1ma 70 200 mv thermal protection over-temperature shutdown t j rising 160 c over-temperature shutdown hysteresis 15 c notes: 1. exceeding the absolute maximum rating may damage the device. 2. the device is not guaranteed to function outside its operating rating. 3. the maximum allowable power dissipation of any t a (ambient temperature) is p d(max) = (t j(max) ? t a ) / ja . exceeding the maximum allowable power dissipation will result in excessive die temperatur e, and the regulator will go into thermal shutdown. 4. devices are esd sensitive. handli ng precautions recommended. human body mo del, 1.5k in series with 100pf. 5. enable pin should not be left open.
micrel, inc. mic2165 september 2010 5 m9999-092410-e typical characteristics 12v to 3.3v efficiency 0 10 20 30 40 50 60 70 80 90 100 0.01 0.10 1.00 10.00 output current (a) efficiency (%) 12v to 1.2v efficiency 0 10 20 30 40 50 60 70 80 90 100 0.01 0.10 1.00 10.00 output current (a) efficiency (%) output voltage vs. load 1.10 1.12 1.14 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 0246810 output current (a) output voltage (v) output voltage vs. input voltage 1.10 1.12 1.14 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 4 8 12 16 20 24 28 input voltage (v) output voltage (v) feedback voltage vs. temperature 0.788 0.792 0.796 0.800 0.804 0.808 0.812 -4004080120 temperature (oc) feedback voltage (v ) switching frequency vs. load (continuous mode) 450 480 510 540 570 600 630 660 690 720 750 0246810 output current (a) switching frequency (khz) switching frequency vs. temperature 450 480 510 540 570 600 630 660 690 720 750 -40 0 40 80 120 temperature (c) switching frequency (khz) switching frequency v s. input voltage 450 480 510 540 570 600 630 660 690 720 750 4 8 12 16 20 24 28 input voltage (v) switching frequency (khz) current limit threshold vs. feedback voltage percentage 0 20 40 60 80 100 120 140 160 0 20406080100 feedback voltage percentage (%) current limit threshold (mv) v in = 12v v out = 1.2v i out = 100ma i out = 10a v in = 12v v out = 2.5v v in = 12v v out = 2.5v i out = 5a
micrel, inc. mic2165 september 2010 6 m9999-092410-e typical characteristics (continued) current limit threshold vs. temperature 0 20 40 60 80 100 120 140 160 -40 0 40 80 120 temperature (c) current limit threshold (mv) vdd vs. temperature 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 -40 0 40 80 120 temperature (c) vdd voltage (v ) v dd dropout vs. temperature 250 300 350 400 450 500 550 -40 0 40 80 120 temperature (c) vdd dropout (mv) enable threshold v s. input voltage 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.40 4 8 12 16 20 24 28 input voltage (v) enable threshold (v) enable threshold vs. temperature 0.0 0.3 0.6 0.9 1.2 1.5 -40 0 40 80 120 temperature (c) enable threshold (v) power good threshold vs. temperature 89 90 91 92 93 94 95 96 97 -40 0 40 80 120 temperature (c) power good threshold (%) quiescent current vs. input voltage 300 320 340 360 380 400 420 440 460 480 500 4 8 12 16 20 24 28 input voltage (v) quiescent current (a) shutdown current vs. input voltage 0 10 20 30 40 50 60 70 80 90 100 4 8 12 16 20 24 28 input voltage (v) shutdown current (a) vdd dropout vs. vdd load 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 vdd output current (ma) vdd dropout (mv) v fb = 0.79v v fb = 0v i dd = 25ma
micrel, inc. mic2165 september 2010 7 m9999-092410-e functional characteristics
micrel, inc. mic2165 september 2010 8 m9999-092410-e functional characteristics (continued)
micrel, inc. mic2165 september 2010 9 m9999-092410-e functional characteristics (continued)
micrel, inc. mic2165 september 2010 10 m9999-092410-e functional diagram figure 1. mic2165 block diagram
micrel, inc. mic2165 september 2010 11 m9999-092410-e functional description the mic2165 is an adaptive on-time buck controller built for low cost and high performance. featuring an internal 5v linear regulator and pgood output, it is designed for a wide input voltage range from 4.5v to 28v, high output power buck converters. an estimated on-time method is used in the mic2165 to obtain a constant switching frequency and to simplify the control compensation. over-current protection is implemented without the use of an external sense resistor. it includes an internal soft-start function which reduces the power supply input surge current at start-up by controlling the output voltage rise time. theory of operation the mic2165 is an adaptive on-time buck controller. figure 1 illustrates the block di agram for the control loop. mic2165 is able to operate in two modes: continuous mode and discontinuous mode. the operation mode of mic2165 is determined by t he output of zero cross comparator (zc), as shown in figure 1. continuous mode in the continuous mode, the output voltage variation will be sensed by the mic2165 feedback pin fb via the voltage divider. the fb voltage v fb is compared to a 0.8v reference voltage v ref at the error comparator through a low gain transconductance (g m ) amplifier at switching frequency. this g m amplifier improves the mic2165 converter output voltage regulation. if the fb voltage v fb decreases and the output of the g m amplifier is below 0.8v, the error comparator will trigger the control logic and generate an on-time period, in which dh pin is logic high and dl pin is logic low. the on-time period length is predetermined by the ?fixed t on estimator? circuitry: sw in out ed) on(estimat f v v t = (1) where v out is the output voltage, v in is the power stage input voltage, and f sw is the switching frequency (600khz for mic2165). after an on-time period, the mic2165 goes into the off-time period, in which dh pin is logic low and dl pin is logic high. the off-time period length depends on v fb in most cases. when v fb decreases and the output of the g m amplifier is below 0.8v, the on-time period is triggered and the off-time period ends. if the off-time period determined by v fb is less than the minimum off time t off(min) , which is about 300ns typical, then the mic2165 control logic will apply the t off(min) instead. t off(min) is required to maintain enough energy in the boost capacitor (c bst ) to drive the high-side mosfet. the maximum duty cycle is obtained from the 300ns t off(min) : s s off(min) s max t 300ns 1 t t t d ? = ? = where t s = 1/f sw . it is not recommended to use mic2165 with an off-time close to t off(min) during steady state operation. also, as v out increases, the internal ripple injection will increase and reduce the line regulation performance. therefore, the maximum output voltage of the mic2165 should be limited to 5.5v. please refer to ?setting output voltage? subsection in ?application information? for more details. the estimated on-time method results in a constant switching frequency in the mic2165. the actual on-time varies slightly with the different rising and falling times of the external mosfets. therefore, the type of the external mosfets and the output load current will modify the actual on-time and the switching frequency. also, the minimum t on results in a lower switching frequency in high v in and low v out applications, such as 24v to 1.0v. the minimum t on measured on the mic2165 eval uation board is about 100ns. during the load transient, the switching frequency is changed due to the varying off-time. to illustrate the control loop, the steady-state scenario and the load transient scenario are analyzed. for easy analysis, the gain of the g m amplifier is assumed to be 1. with this assumption, the inverting input of the error comparator is the same as v fb . figure 2 shows the mic2165 control loop timing during steady-state operation in continuous mode. during steady-state, the g m amplifier senses v fb ripple, which is proportional to the output voltage (v out ) ripple and the inductor current ripple, to trigger the on-time period. the on-time is predetermined by the estimation. the ending of off-time is controlled by v fb . at the valley of v fb ripple, which occurs when v fb falls below v ref , the off period ends and the next on-time period is triggered through the control logic circuitry.
micrel, inc. mic2165 september 2010 12 m9999-092410-e figure 2. mic2165 control loop timing (continuous mode) figure 3 shows the load transient operation of the mic2165 converter. the output voltage drops due to the sudden load increase, which causes v fb to be less than v ref . this will cause the error comparator to trigger an on-time period. at the end of the on-time period, a minimum off-time t off(min) is generated to charge c bst since v fb is still below v ref . then, the next on-time period is triggered due to the low v fb . therefore, the switching frequency changes during the load transient. with the varying duty-cycle and switching frequency, the output recovery time is fast and the output voltage deviation is small in mic2165 converter. figure 3. mic2165 load-transient response unlike in current-mode control, the mic2165 uses the output voltage ripple, which is proportional to the inductor current ripple if t he esr of the output capacitor is large enough, to trigger an on-time period. the predetermined on-time makes mic2165 control loop have the advantage of consta nt on-time mode control and eliminates the need for slope compensation. the mic2165 has its ow n stability concern: v fb ripple should be in phase with the inductor current ripple and large enough to be sensed by the g m amplifier and the error comparator. the recommended v fb ripple is 20mv~100mv. if a low esr output capacitor is selected, the v fb ripple may be too small to be sensed by the g m amplifier and the error comparator. also, the v out ripple and the v fb ripple are not in phase with the inductor current ripple if the esr of the output capacitor is very low. therefore, ripple injection is required for a low esr output capacitor. please refer to ?ripple injection? subsection in ?application information? for more details. discontinuous mode in continuous mode, the inductor current is always greater than zero; however, at light loads the mic2165 is able to force the inductor current to operate in discontinuous mode. discontinuous mode is where the inductor current falls to zero, as indicated by trace (i l ) shown in figure 4. during this period, the efficiency is optimized by shutting down all the non-essential circuits and minimizing the supply current. the mic2165 wakes up and turns on the high-side mosfet when the feedback voltage v fb drops below 0.8v. the mic2165 has a zero crossing comparator that monitors the inductor current by sensing the voltage drop across the low-side mosfet during its on-time. if the v fb > 0.8v and the inductor current goes sli ghtly negative, then the mic2165 automatically powers down most of the ic circuitry and goes into a low-power mode. once the mic2165 goes into discontinuous mode, both dl and dh are low, which turns off the high-side and low-side mosfets. the load current is supplied by the output capacitors and v out drops. if the drop of v out causes v fb to go below v ref , then all the circuits will wake up into normal continuous mode. first, the bias currents of most circuits reduced during the discontinuou s mode are restored, then a t on pulse is triggered before the drivers are turned on to avoid any possible glitches. fina lly, the high-side driver is turned on. figure 4 shows the control loop timing in discontinuous mode.
micrel, inc. mic2165 september 2010 13 m9999-092410-e figure 4. mic2165 control loop timing (discontinuous mode) an external schottky diode d2 is recommended in parallel with the low-side mosfet for high efficiency performance as shown in the typical application schematic. please refer to ?external schottky diode? subsection in ?application information? for more details. during discontinuous mode, the zero crossing comparator and the current limit comparator are turned off. the bias current of most circuits are reduced. as a result, the total power supply current during discontinuous mode is only about 450 a, allowing the mic2165 to achieve high efficiency in light load applications. soft-start soft-start reduces the power supply input surge current at startup by cont rolling the output voltage rise time. the input surge appears while the output capacitor is charged up. a slower output ri se time will draw a lower input surge current. the mic2165 implements an internal digital soft-start by making the 0.8v reference voltage v ref ramp from 0 to 100% in about 5ms. therefor e, the output voltage is controlled to increase slowly by a stair-case v ref ramp. once the soft-start cycle ends, the related circuitry is disabled to reduce current consumption. during soft-start, the discontinuous mode is disabled in mic2165. current limit the mic2165 uses the r ds(on) of the low-side power mosfet to sense over-current conditions. this method will avoid adding cost, boar d space and power losses taken by discrete current sense resistors. the low-side mosfet is used because it displays much lower parasitic oscillations during switching than the high-side mosfet. in each switching cycle of the mic2165 converter, the inductor current is sensed by monitoring the low-side mosfet in the off period. the sensed voltage is compared with a current-l imit threshold voltage v cl after a blanking time of 150ns. if the sensed voltage is over v cl , which is 133mv typical at 0.8v v fb , then the mic2165 turns off the high-side and low-side mosfets and a soft-start sequence is triggered. this mode of operation is called ?hiccup mode? and its purpose is to protect the downstream load in case of a hard short. the current limit threshold v cl has a foldback characteristic related to the fb voltage. please refer to the ?typical char acteristics? for the curve of current limit threshold vs. fb voltage percentage. the circuit in figure 5 illustrates the mic2 165 current limiting circuit. figure 5. mic2165 curre nt limiting circuit using the typical v cl value of 133mv, the current limit value is roughly estimated as: ds(on) cl r 133mv i for designs where the current ripple is significant compared to the load current i out , or for low duty-cycle operation, calculating the current limit i cl should take into account that one is sensing the peak inductor current and that there is a blanking delay of approximately 150ns.
micrel, inc. mic2165 september 2010 14 m9999-092410-e 2 i l t v r 133mv i l(pp) dly out ds(on) cl ? + = (2) l f d) (1 v i sw out l(pp) ? = (3) where: v out = the output voltage t dly = current limit blanking time, 150ns typical i l(pp) = inductor current ripple peak-to-peak value d = duty cycle f sw = switching frequency the mosfet r ds(on) varies between 30% to 40% with temperature; therefore, it is recommended to add 50% margin to i cl in the above equation to avoid false current limiting due to increased mosfet junction temperature rise. it is also recommended to connect sw pin directly to the drain of the low-side mosfet to accurately sense the mosfets r ds(on) . mosfet gate drive the mic2165 high-side drive circuit is designed to switch an n-channel mosfet. the typical application schematic shows a bootstrap circuit, consisting of d1 (a schottky diode is recommended) and c bst . this circuit supplies energy to the high-side drive circuit. capacitor c bst is charged while the low-side mosfet is on and the voltage on the sw pin is approximately 0v. when the high-side mosfet driver is turned on, energy from c bst is used to turn the mosfet on. as the high-side mosfet turns on, the voltage on the sw pin increases to approximately v in . diode d1 is reversed biased and c bst floats high while continuing to keep the high-side mosfet on. the bias current of the high-side driver is less than 10ma so a 0.1 f to 1 f is sufficient to hold the gate voltage with minimal droop for the power stroke (high-side switching) cycle, i.e., bst = 10ma x 1.67 s/0.1 f = 167mv. when the low-side mosfet is turned back on, c bst is recharged through d1. a small resistor r g at bst pin can be used to slow down the turn-on time of the high-side n-channel mosfet. the drive voltage is derived from the internal linear regulator v dd . the nominal low-side gate drive voltage is v dd and the nominal high-side gate drive voltage is approximately v dd ? v diode , where v diode is the voltage drop across d1. a dead time of approximate 30ns delay between the high-side and low-side driver transitions is used to prevent current from simultaneously flowing unimpeded through both mosfets.
micrel, inc. mic2165 september 2010 15 m9999-092410-e application information mosfet selection the mic2165 controller works from an input voltage of 4.5v to 28v and has an internal 5v v dd to provide power to turn the external n-channel power mosfets for the high-side and low-side switches. for applications where v in < 5.5v, it is recommended to connect vdd to vin to bypass the internal linear r egulator. the external power mosfets should be logic-level mosfets, whose operation is specified at v gs = 4.5v. there are different criteria for choosing the high-side and low-side mosfets. these differences are more significant at lower duty cycles such as 24v to 1.2v conversion. in such an application, the high-side mosfet is required to switch as quickly as possible to minimize transition losses, whereas the low-side mosfet can switch slower, but must handle larger rms currents. when the duty cycle approaches 50%, the current carrying capability of the high-side mosfet starts to become critical. it is important to note that the on-resistance of a mosfet increases with increasing temperature. for a mosfet with a 0.4%/c thermal coefficient a 75c rise in junction temperature will increase the channel resistance of the mosfet by 30% resistance specified at 25c. this change in resistance must be accounted for when calculating mosfet power dissipation and the value of current limit. total gate charge is the charge required to turn the mosfet on and off under specified operating conditions (v ds and v gs ). the gate charge is supplied by the mic2165 gate-drive circuit. at 600khz switching frequency, the gate charge can be a significant source of power dissipation in the mic216 5. at light output load, this power dissipation is noticeable as a reduction in efficiency. the average current required to drive the high-side mosfet is: sw g g[hs] f q (avg) i = (4) where: i g[hs] (avg) = average high-side (hs) mosfet gate current q g = total gate charge for the high-side mosfet taken from the manufacturer?s data sheet for v gs = v dd . f sw = switching frequency the low-side mosfet is turned on and off at v ds = 0v because an internal body diode or external freewheeling diode is conducting during this time. the switching loss for the low-side mosfet is usually negligible. also, the gate- drive current for the low-side mosfet is more accurately calculated using c iss at v ds = 0 instead of gate charge. for the low-side (ls) mosfet: sw gs iss g[ls] f v c (avg) i = (5) since the current from the gate drive comes from the v dd , which is the output of the inte rnal linear regulator power by v in , the power dissipated in the mic2165 due to gate drive is: (avg)) i (avg) (i v p side] - g[low side] g[high- in gatedrive + = (6) a convenient figure of merit for switching mosfets is the on resistance times the total gate charge (r ds(on) q g ). lower numbers translate into higher efficiency. low gate- charge logic-level mosfets are a good choice for use with the mic2165. also, the r ds(on) of the low-side mosfet will determine the current limit value. please refer to ?current limit? subsection is ?functional description? for more details. parameters that are important to mosfet switch selection are: ? voltage rating ? on-resistance ? total gate charge the voltage ratings for the high-side and low-side mosfets are essentially equal to the power stage input voltage v in . a safety factor of 20% should be added to the v ds (max) of the mosfets to account for voltage spikes due to circuit parasitic elements. the power dissipated in the mosfets is the sum of the conduction losses during the on-time (p conduction ) and the switching losses during the period of time when the mosfets turn on and off (p ac ).
micrel, inc. mic2165 september 2010 16 m9999-092410-e ac conduction sw p p p + = (7) ds(on) 2 sw(rms) conduction r i p = (8) ac(on) ) ac(off ac p p p + = (9) where: r ds(on) = on-resistance of the mosfet switch d = duty cycle = v out / v in making the assumption that the turn-on and turn-off transition times are equal; the transition times can be approximated by: t t = g in oss dd iss i v c v c + (10) where: c iss and c oss are measured at v ds = 0 i g = gate-drive current the total high-side mosfet switching loss is: sw t pk d in ac f t i ) v (v p + = (11) where: t t = switching transition time v d = diode drop f sw = switching frequency the high-side mosfet switching losses increase with the switching frequency and the input voltage v in . the low- side mosfet switching losses are negligible and can be ignored for these calculations. inductor selection values for inductance, peak, and rms currents are required to select the output inductor. the input and output voltages and the inductance value determine the peak-to- peak inductor ripple current. generally, higher inductance values are used with higher input voltages. larger peak-to- peak ripple currents w ill increase the powe r dissipation in the inductor and mosfets. larger output ripple currents will also require more output capacitance to smooth out the larger ripple current. smaller peak-to-peak ripple currents require a larger inductance value and therefore a larger and more expensive inductor. a good compromise between size, loss and cost is to set the inductor ripple current to be equal to 20% of the maximum output current. the inductance value is calculated in equation 12. ( ) ? out (max) ? out i % 20 f v v v v l ? = (12) where: f sw = switching frequency 20% = ratio of ac ripple current to dc output current v in(max) = maximum power stage input voltage the peak-to-peak inductor current ripple is: l f v ) v v ( v i sw (max) in out (max) in out ) pp ( l ? = (13) the peak inductor current is equal to the average output current plus one half of the peak -to-peak inductor current ripple. l(pp) out(max) l(pk) i 0.5 i i + = (14) the rms inductor current is used to calculate the i 2 r losses in the inductor. 12 i i i 2 l(pp) 2 out(max) l(rms) + = (15) maximizing efficiency requires the proper selection of core material and minimizing the winding resistance. the high frequency operation of the mi c2165 requires the use of ferrite materials for all but the most cost sensitive applications. lower cost iron powder cores may be used but the increase in core loss will redu ce the efficiency of the power supply. this is especially noticeable at low output power. the winding resistance decreases efficiency at the higher output current levels. the winding resistance must be minimized although this usually comes at the expense of a larger inductor. the power dissipated in the inductor is equal to the sum of the core and copper losses. at higher output loads, the core losses are usually insignificant and can be ignored. at lower output currents, the core losses can be a significant contributor. core loss information is usually available from the m agnetics vendor. copper loss in the inductor is calculated by in equation 16:
micrel, inc. mic2165 september 2010 17 m9999-092410-e p inductorcu =i l(rms) 2 r winding (16) the resistance of the copper wire, r winding , increases with the temperature. the value of the winding resistance used should be at the operating temperature. r winding = r winding(20c) (1+ 0.0042 (t h ? t 20c )) (17) where: t h = temperature of wire under full load t 20c = ambient temperature r winding(20c) = room temperature winding resistance (usually specified by the manufacturer) output capacitor selection the type of the output capacitor is usually determined by its esr (equivalent series resistance). voltage and rms current capability are two other impo rtant factors for selecting the output capacitor. recommended capacitors are tantalum, low-esr aluminum electrolytic, os-con, poscaps, and ceramic. the output capacitor?s esr is usually the main cause of the output ripple. the output capacitor esr also affects the control loop from a stability point of view. the maximum value of esr is calculated: l(pp) out(pp) c i v esr out (18) where: v out(pp) = peak-to-peak output voltage ripple i l(pp) = peak-to-peak inductor current ripple the total output ripple is a combination of the esr and output capacitance. the total ripple is calculated in equation 19: () 2 c l(pp) 2 sw out l(pp) out(pp) out esr i 8 f c i v + ? ? ? ? ? ? ? ? = (19) where: c out = output capacitance value f sw = switching frequency as described in the ?theory of operation? subsection in ?functional description?, mic2165 requires at least 20mv peak-to-peak ripple at the fb pin to make the g m amplifier and the error comparator to behavior properly. also, the output voltage ripple should be in phase with the inductor current. therefore, the output voltage ripple caused by the output capacitor c out should be much smaller than the ripple caused by the output capacitor esr. if low esr capacitors, such as ceramic capacitors, are selected as the output capacitors, then a ripple injection method should be applied to provide the enough fb voltage ripples. please refer to the ?ripple injection? subsection for more details. the voltage rating of the capacitor should be twice the output voltage for a tantalum and 20% greater for aluminum electrolytic or os-con. the output capacitor rms current is calculated in equation 20: 12 i i l(pp) (rms) c out = (20) the power dissipated in the output capacitor is: out out out c 2 (rms) c ) diss(c esr i p = (21) input capacitor selection the input capacitor for the power stage input v in should be selected for ripple current rating and voltage rating. tantalum input capacitors may fail when subjected to high inrush currents, caused by turning on a ?hot-plugging?. a tantalum input capacitor?s voltage rating should be at least two times the maximum input voltage to maximize reliability. aluminum electrolytic, os-con, and multilayer polymer film capacitors can handle the higher inrush currents without voltage de-rat ing. the input voltage ripple will primarily depend upon the input capacitor?s esr. the peak input current is equal to the peak inductor current, so: in c ) pk ( l in esr i v = (22) the input capacitor must be rated for the input current ripple. the rms value of i nput capacitor current is determined at the maximum output current. assuming the peak-to-peak inductor current ripple is low: () d 1 d i i (max) out ) rms ( c in ? (23)
micrel, inc. mic2165 september 2010 18 m9999-092410-e the power dissipated in the input capacitor is: in in in c 2 ) rms ( c ) c ( diss esr i p = (24) external schottky diode an external freewheeling diode, which is recommended to improve the efficiency in discontinuous mode, can be used to keep the inductor current flow continuous while both mosfets are turned off. in continuous mode, the di ode conducts current during the dead-time. the dead-time prevents current from flowing unimpeded through both mosfets and is typically 30ns. the diode conducts twice during each switching cycle. although the average current through this diode is small, the diode must be able to handle the peak current. sw out d(avg)cm f 30ns 2 i i = (25) in the discontinuous mode, the average current through the diode is large. ) 2 i ) on ( rds v ( ) d 1 ( i ) pp ( l zc d(avg)dm ? ? (26) where v zc is the zero cross comparator offset. the reverse voltage requirement of the diode is: in diode(rrm) v v = the power dissipated by the schottky diode is: f d(avg) diode v i p = (27) where, v f = forward voltage at the peak diode current. an external schottky diode is recommended, even though the low-side mosfet contains a parasitic body diode since the schottky diode has much less forward voltage than the body diode. the external diode will improve efficiency and reduce the high frequency noise. if the mosfet body diode is used, it must be rated to handle the peak and average current. the body diode has a relatively slow reverse recovery time and a relatively high forward voltage drop. the power lost in the diode is proportional to the forward voltage drop of the diode. as the high-side mosfet starts to turn on, the body diode becomes a short circuit for the reverse recovery period, dissipating additional power. the diode recovery and the circuit inductance will cause ringing during the high-side mosfet turn-on. an external schottky diode conducts at a lower forward voltage preventing the body diode in the mosfet from turning on. the lower forward voltage drop dissipates less power than the body diode. the lack of a reverse recovery mechanism in a schottky diode causes less ringing and less power loss. snubber design a snubber is used to damp out high frequency ringing caused by parasitic inductance and capacitance in the buck converter circuit. fi gure 6 shows a simplified schematic of the buck converter. stray capacitance consists mostly of the two mosfets? output capacitance (c oss ). the stray inductance consists mostly package inductance and trace inductance. the arrows show the resonant current path when the high side mosfet turns on. this ringing causes stress on the semiconductors in the circuit as well as increased emi. q1 q2 c out c in l stray1 l stray2 l l stray3 l stray4 sync_buck controller v dc c oss1 c oss2 + ? figure 6. output parasitics
micrel, inc. mic2165 september 2010 19 m9999-092410-e one method of reducing the ringing is to use a resistor and capacitor to lower the q of the resonant circuit, as shown in figure 7. capacitor c s is used to block dc and minimize the power dissipation in the resistor. this capacitor value should be between 2 and 10 times the parasitic capacitance of the mosfet c oss . a capacitor that is too small will have high impedance and prevent the resistor from damping the ringing. a capacitor that is too large causes unnecessary power dissipation in the resistor, which lowers efficiency. c s r s r ds l stray1 l stray2 l stray3 l stray4 c oss2 figure 7. snubber circuit the snubber components should be placed as close as possible to the low-side mosfet and/or external schottky diode since it contributes to mo st of the stray capacitance. placing the snubber too far from the mosfet or using trace that is too long or thin will add inductance to the snubber and diminishes its effectiveness. a proper snubber design requires that the parasitic inductance and capacitance be known. a method of determining these values and calculating the damping resistor value is outlined below. 1. measure the ringing frequen cy at the switch node which is determined by parasitic l p and c p . define this frequency as f 1 . 2. add a capacitor c s (such as 2 times as big as the c oss of the fet) from the switch node to ground and measure the new ringing frequency. define this new (lower) frequency as f 2 . l p and c p can now be solved using the values of f 1 , f 2 and c s . 3. add a resistor r s in series with c s to generate critical damping. step 1: first measure the ringing frequency on the switch node voltage when the high-side mosfet turns on. this ringing is characterized by equation 28: p p 1 c l 2 1 f = (28) where c p and l p are the parasitic capacitance and inductance. step 2: add a capacitor, c s , in parallel with the synchronous mosfet, q2. the capacitor value should be approximately 2 times the c oss of q2. measure the frequency of the switch node ringing, f 2 : ) cp cs ( lp 2 1 f 2 + = (29) define f? as: 2 1 ' f f f = combining the equations for f 1 , f 2 and f? to derive c p , the parasitic capacitance: 1 ) f ( c c 2 ' s p ? = (30) l p is solved by re-arranging the equation for f 1 : () 2 1 p 2 p ) f ( c 2 1 l = (31) step 3: calculate the damping resistor. critical damping occurs at q = 1: 1 l c r q p p s = = (32)
micrel, inc. mic2165 september 2010 20 m9999-092410-e solving for r s : p p s c l r = (33) figure 7 shows the snubber in the circuit and the damped switch node waveform. the snubber capacitor, c s , is charged and discharged each switching cycle. the energy stored in c s is dissipated by the snubber resistor, r s , two times per switching period. this power is calculated in equation 34: 2 in s sw snubber v c f p = (34) ripple injection the v fb ripple required for proper operation of the mic2165 g m amplifier and error comparator is 20mv to 100mv. however, the output voltage ripple is generally designed as 1% to 2% of the voltage. for a low output voltage, such as a 1v output, the output voltage ripple is only 10mv to 20mv, and the v fb ripple is less than 20mv. if the v fb ripple is so small that the g m amplifier and error comparator cannot sense it, the mic2165 will lose control and the output voltage is not regulated. in order to have some amount of v fb ripple, a ripple injection method is applied for low output voltage ripple applications. the applications are divided into three situations according to the amount of the v fb ripple: 1) enough ripple at v out due to the large esr of the output capacitors. as shown in figure 8a, the converter is stable without any ripple injection. the v fb ripple is: out fb(pp) v r2 r1 r2 v + = (35) where: (pp) l c out i esr v out ? = , i l(pp) is the peak-to- peak value of the inductor current ripple. 2) inadequate ripple at v out due to the small esr of the output capacitors. the output voltage ripple is fed into the fb pin through a feedforward capacitor c ff in this situation, as shown in figure 8b. the typical c ff value is between 1nf and 100nf. with the feedforward capacitor, v fb ripple is very close to the output voltage ripple: out fb(pp) v v = sw div in fb(pp) f 1 d) - (1 d k v v (37) r1//r2 r r1//r2 k inj div + = (38) where: v in = power stage input voltage at vin pin d = duty cycle f sw = switching frequency ff inj c ) r // 2 r // 1 r ( ? = in equations 37 and 38, it is assumed that the time constant associated with c ff must be much greater than the switching period: 1 t f 1 sw << = if the voltage divider resistors r1 and r2 are in the k ? range, a c ff of 1nf to 100nf can easily satisfy the large time constant consumption. also, a 100nf injection capacitor c inj is used in order to be considered as short for a wide range of the frequencies. figure 8a. mv 20 v r2 r1 r2 out > +
micrel, inc. mic2165 september 2010 21 m9999-092410-e figure 8b. mv 20 v r2 r1 r2 out < + and mv 20 v out > figure 8c. mv 20 v out < the process of sizing the ripple injection resistor and capacitors is: step 1. select c ff to feed all output ripples into the feedback pin and make sure the large time constant assumption is satisfied. typical choice of c ff is 1nf to 100nf if r1 and r2 are in k ? range. step 2. select r inj according to the expected feedback voltage ripple. according to equation 37: ) d 1 ( d f v v k sw in ) pp ( fb div ? = (39) then the value of r inj is obtained as: ) 1 k 1 ( ) 2 r // 1 r ( r div inj ? = (40) step 3. select c inj as 100nf, which could be considered as short for a wide range of the frequencies. setting output voltage the mic2165 requires two resistors to set the output voltage, as shown in figure 9. figure 9. voltage-divider configuration the output voltage is determined by the equation: ) r2 r1 (1 v v ref out + = (41) where v ref = 0.8v. if r1 is too large, it may allow noise to be introduced into the voltage feedback loop. if r1 is too small, it will decrease the efficiency of the power supply, especially at light loads. the total voltage divider resistance r1+r2 is recommended to be 7.5k ? . once r1 is selected, r2 can be calculated using: ref out ref v v r1 v r2 ? = (42) in addition to the external ripple injection added at the fb pin, internal ripple injection is added at the inverting input of the comparator inside the mic2165, as shown in figure 10. the inverting input voltage v inj is clamped to 1.2v. as v out is increased, the swing of v inj will be clamped. the clamped v inj reduces the line regulation because it is reflected back as a dc error on the fb terminal. therefore, the maximum output voltage of the mic2165 should be limited to 5.5v to avoid this problem. figure 10. internal ripple injection
micrel, inc. mic2165 september 2010 22 m9999-092410-e pcb layout guidelines warning!!! to minimize emi and output noise, follow these layout recommendations. pcb layout is critical to achieve reliable, stable and efficient performance. a ground plane is required to control emi and minimize the inductance in power, signal and return paths. the following guidelines should be followed to insure proper operation of the mic2165 converter. ic ? place the ic and mosfets close to the point of load (pol). ? use fat traces to route the input and output power lines. ? signal and power grounds should be kept separate and connected at only one location. ? the exposed pad (epad) on the bottom of the ic must be connected to the ground through several vias. ? the feedback resistors should be placed close to the fb pin. the top feedback resistor should connect directly to the output node. run this trace away from the switch node (sw). input capacitor ? place the vin input capacitor next. ? place the vin input capacitors on the same side of the board and as close to the mosfets as possible. ? keep both the vin and pgnd connections short. ? place several vias to the ground plane close to the vin input capacitor ground terminal. ? use either x7r or x5r dielectric input capacitors. do not use y5v or z5u type capacitors. ? do not replace the ceramic input capacitor with any other type of capacitor. any type of capacitor can be placed in parallel with the input capacitor. ? if a tantalum input capacitor is placed in parallel with the input capacitor, it must be recommended for switching regulator applications and the operating voltage must be derated by 50%. ? in ?hot-plug? applications, a tantalum or electrolytic bypass capacitor must be used to limit the over- voltage spike seen on the input supply with power is suddenly applied. ? the 2.2f (minumum) capacitors, which connect to the v dd terminal, must be located right at the ic. the vdd terminal is very noise sensitive and placement of the capacitor is very critical. connections must be made with wide trace. inductor ? keep the inductor connection to the switch node (sw) short. ? do not route any digital lines underneath or close to the inductor. ? keep the switch node (sw) away from the feedback (fb) pin. ? the sw pin should be connected directly to the drain of the low-side mosfet to accurate sense the voltage across the low-side mosfet. ? to minimize noise, place a ground plane underneath the inductor. output capacitor ? use a wide trace to connect the output capacitor ground terminal to the input capacitor ground terminal. ? phase margin will change as the output capacitor value and esr changes. contact the factory if the output capacitor is different from what is shown in the bom. ? the feedback trace should be separate from the power trace and connected as close as possible to the output capacitor. sensing a long high current load trace can degrade the dc load regulation. schottky diode ? place the schottky diode on the same side of the board as the mosfets and vin input capacitor. ? the connection from the schottky diode?s anode to the input capacitors ground terminal must be as short as possible. ? the diode?s cathode connection to the switch node (sw) must be keep as short as possible. rc snubber ? place the rc snubber on the same side of the board and as close to the mosfets as possible. mosfets ? low-side mosfet gate drive trace (dl pin to mosfet gate pin) must be short and routed over a ground plane. the ground plane should be the connection between the mosfet source and pgnd. ? chose a low-side mosfet with a high c gs /c gd ratio and a low internal gate resistance to minimize the effect of dv/dt inducted turn-on. ? do not put a resistor between the lsd output and the gate. ? use a 4.5v v gs rated mosfet. its higher gate threshold voltage is more immune to glitches than a 2.5v or 3.3v rated mosfet. mosfets that are rated for operation at less than 4.5v v gs should not be used.
micrel, inc. mic2165 september 2010 23 m9999-092410-e evaluation board schematic figure 11. schematic of mic2165 8-24 v in to 1.2 v out /10a evaluation board
micrel, inc. mic2165 september 2010 24 m9999-092410-e bill of materials item part name manufacturer description qty b41125a7227m epcos (1) c1 222215095001e3 vishay (2) 220f aluminum capacitor, smd, 35v 1 12105c475kaz2a avx (3) c2,c3 grm32er71h475ka88l murata (4) 4.7f ceramic capacitor, x7r, size 1210, 50v 2 06035c104kat2a avx (3) grm188r71h104ka93d murata (4) c6, c8, c10 c1608x7r1h104k tdk (5) 0.1f ceramic capacitor, x7r, size 0603, 50v 3 0805zd106kat2a avx (3) c7 grm21br61a106ke19l murata (4) 10f ceramic capacitor, x5r, size 0805, 10v 1 06035c102kat2a avx (3) grm188r71h102ka01d murata (4) c11 c1608x7r1h102k tdk (5) 1nf ceramic capacitor, x7r, size 0603, 50v 1 06035c472kat2a avx (3) grm188r71h472ka01d murata (4) c12 c1608x7r1h472k tdk (5) 4.7nf ceramic capacitor, x7r, size 0603, 50v 1 12106d107mat2a avx (3) c13 grm32er60j107me20l murata (4) 100f ceramic capacitor, x5r, size 1210, 6.3v 1 c15 6sepc560mx sanyo (6) 560f oscon capacitor, 6.3v 1 sd103bws-7 diodes inc (7) d1 sd103bws vishay (2) small signal schottky diode 1 l1 hcf1305-1r0-r cooper bussmann (8) 1.0h inductor, 24a saturation current 1 q1 fds6298 fairchild (9) 30v 13a n-channel mosfet 12m ? rds(on) @ 4.5v 1 q2 fds8672s fairchild (9) 30v 18a n-channel mosfet 7m ? rds(on) @ 4.5v 1 r1, r14 crcw06032r21fkea vishay/dale (2) 2.21 ? resistor, size 0603, 1% 2 r2 crcw08051r21fkea vishay/dale (2) 1.21 ? resistor, size 0805, 1% 1 r3 crcw060319k6fkea vishay/dale (2) 19.6k ? resistor, size 0603, 1% 1 r4 crcw06032k49fkea vishay/dale (2) 2.49k ? resistor, size 0603, 1% 1 r5 crcw06034k99fkea vishay/dale (2) 4.99k ? resistor, size 0603, 1% 1 r13 crcw060320r0fkea vishay/dale (2) 20 ? resistor, size 0603, 1% 1 r15, r16 crcw060310k0fkea vishay/dale (2) 10k ? resistor, size 0603, 1% 2 r17 crcw060349r9fkea vishay/dale (2) 49.9 ? resistor, size 0603, 1% 1 u1 MIC2165YMME micrel inc. (10) 600khz buck controller 1 notes: 1. epcos: www.epcos.com . 2. vishay: www.vishay.com . 3. avx: www.avx.com . 4. murata: www.murata.com . 5. tdk: www.tdk.com . 6. sanyo: www.sanyo.com . 7. diode inc.: www.diodes.com . 8. cooper bussmann: www.cooperbussmann.com . 9. fairchild: www.fairchildsemi.com . 10. micrel, inc: www.micrel.com .
micrel, inc. mic2165 september 2010 25 m9999-092410-e pcb layout recommendations figure 12. mic2165 10a evaluation board top layer figure 13. mic2165 10a evaluation board bottom layer
micrel, inc. mic2165 september 2010 26 m9999-092410-e pcb layout recommendations (continued) figure 14. mic2165 10a evaluati on board mid-layer 1 (gnd plane) figure 15. mic2165 10a evaluation board mid-layer 2
micrel, inc. mic2165 september 2010 27 m9999-092410-e package information 10-pin epad msop (mme)
micrel, inc. mic2165 september 2010 28 m9999-092410-e recommended landing pattern micrel, inc. 2180 fortune drive san jose, ca 95131 usa tel +1 (408) 944-0800 fax +1 (408) 474-1000 web http://www.micrel.com micrel makes no representations or warranties with respect to t he accuracy or completeness of the information furnished in this data sheet. this information is not intended as a warranty and micrel does not assume responsibility for it s use. micrel reserves the right to change circuitry, specifications and descriptions at any time without notice. no license, whether express, implied, arising by estoppel or other wise, to any intellectual property rights is granted by this document. except as provided in micrel?s terms and conditions of sale for such products, mi crel assumes no liability whatsoever, and micrel disclaims any express or implied warranty relating to the sale and/or use of micrel products including l iability or warranties relating to fitness for a particular purpose, merchantability, or infringement of an y patent, copyright or other intellectual p roperty right micrel products are not designed or authori zed for use as components in life support app liances, devices or systems where malfu nction of a product can reasonably be expected to result in pers onal injury. life support devices or system s are devices or systems that (a) are in tended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significan t injury to the user. a purchaser?s use or sale of micrel produc ts for use in life support app liances, devices or systems is a purchaser?s own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2010 micrel, incorporated.

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