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mic2177 2.5a synchronous buck regulator micrel inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel +1 ( 408 ) 944-0800 ? fax + 1 (408) 474-1000 ? http://www.micrel.com april 2008 m9999-042108 general description the micrel mic2177 is a 200khz synchronous buck (step- down) switching regulator designed for high-efficiency, battery-powered applications. the mic2177 operates from a 4.5v to 16.5v input and features internal power mosfets that can supply up to 2.5a output current. it can operate with a maximum duty cycle of 100% for use in low-dropout conditions. it also features a shutdown mode that reduces quiescent current to less than 5a. the mic2177 achieves high efficiency over a wide output current range by switching between pwm and skip mode. operating mode is automatically selected according to output conditions. switching frequency is preset to 200khz and can be synchronized to an external clock signal of up to 300khz. the mic2177 uses current-mode control with internal current sensing. current-mode control provides superior line regulation and makes the regulator control loop easy to compensate. the output is protected with pulse-by-pulse current limiting and thermal shutdown. undervoltage lockout turns the output off when the input voltage is less than 4.5v. the mic2177 is packaged in a 20-pin wide power so package with an operating temperature range of ?40c to +85c. see the mic2178 for externally selected pwm or skip-mode operation. data sheets and support documentation can be found on micrel?s web site at: www.micrel.com. features ? 4.5v to 16.5v input voltage range ? dual-mode operation for high efficiency (up to 96%) ? pwm mode for > 200ma load current ? skip mode for < 200ma load current ? 100m ? internal power mosfets at 12v input ? 200khz preset switching frequency ? low quiescent current ? 1.0ma in pwm mode ? 500a in skip mode ? < 5a in shutdown mode ? 100% duty cycle for low dropout operation ? current-mode control ? simplified loop compensation ? superior line regulation ? current limit ? thermal shutdown ? undervoltage lockout applications ? high-efficiency, battery-powered supplies ? buck (step-down) dc-to-dc converters ? cellular telephones ? laptop computers ? hand-held instruments ? battery charger _________________________________________________________________________________________________________ typical application bias sgnd en comp pgnd fb out vin c2 100f 10v v out 5v/1a l1, 50h c c 6.8nf c3 r1 0.01f 10k mic 2177-5.0 auto sync r1 10k v in 5.4v to 18v c1 22f 35v u1 20 18 11 13 14?17 19 12 4?7 3,8 1,2,9 d1 mbrs130l sw 10 enable shutdown 2.2 nf 70 75 80 85 90 95 100 10 100 1000 2500 efficiency (%) output current (ma) 5v output efficiency v in =6v skip pwm
micrel, inc. mic2177 april 2008 2 m9999-042108 ordering information part number output voltage switching frequency temperature range package lead finish mic2177-3.3bwm 3.3v 200khz ?40c to +85c 20-pin wide soic standard mic2177-5.0bwm 5.0v 200khz ?40c to +85c 20-pin wide soic standard mic2177bwm adj. 200khz ?40c to +85c 20-pin wide soic standard mic2177-3.3ywm 3.3v 200khz ?40c to +85c 20-pin wide soic pb-free pin configuration 2 vin 3 sw 4 pgnd 5 pgnd 6 pgnd 7 pgnd 1 vin 8 sw 9 vin en 20 bias 19 syn c 18 sgnd 17 sgnd 16 sgnd 15 10 out 14 13 12 11 auto fb com p sgnd 20-pin wide soic (wm)
micrel, inc. mic2177 april 2008 3 m9999-042108 pin description pin number pin name pin function 1, 2, 9 vin supply input: controller and switch supply. unregulated supply input to internal regulator, output switches, and control circuitry. requires bypass capacitor to pgnd. all three pins must be connected to vin. 3, 8 sw switch (output): internal power mosfet switch output. both pins must be externally connected together. 4, 5, 6, 7 pgnd power ground: output stage ground connections. connect all pins to a common ground plane. 10 out output voltage sense (input): senses output voltage to determine minimum switch current for pwm operation. connect directly to vout. 11 auto automatic mode: connect 2.2nf timing capacitor for automatic pwm-/skip-mode switching. regulator operates exclusiv ely in pwm mode when pin is pulled low. 12 fb feedback (input): error amplifier inverti ng input. for adjustable output version, connect fb to external resistive divider to set output voltage. for 3.3v and 5v fixed output versions, connect fb directly to output. 13 comp compensation: internal error amplifier output. connect to capacitor or series rc network to compensate the regulator control loop. 14, 15, 16, 17 sgnd signal ground: ground connection of contro l section. connect all pins to common ground plane. 18 sync frequency synchronization (input): optional clock input. connect to external clock signal to synchronize oscillator. leading edge of signal above 1.7v terminates switching cycle. connect to sgnd if not used. 19 bias bias supply: internal 3.3v bias supply output. decouple with 0.01f bypass capacitor and 10k ? to sgnd. do not apply any external load. 20 en enable (input): logic high enables operation. logic low shuts down regulator. do not allow pin to float.
micrel, inc. mic2177 april 2008 4 m9999-042108 absolute maximum ratings supply voltage [100ms transient] (v in ) ..........................18v output switch voltage (v sw ). .........................................18v output switch current (i sw )...........................................6.0a enable, output-sense voltage (v en , v out ). ...................18v sync voltage (v sync )........................................................6v operating ratings supply voltage (v in )....................................... 4.5v to 16.5v junction temperature (t j ) ........................ ?40c to +125c electrical characteristics v in = 7.0v; t a = 25c, bold values indicate ?40c< t a < +85c, unless noted. symbol parameter condition min typ max units pwm mode, output not switching, 4.5v v in 16.5v 1.0 1.5 ma skip mode, output not switching, 4.5v v in 16.5v 500 650 a i ss input supply current v en = 0v, 4.5v v in 16.5v 1 25 a v bias bias regulator output voltage v in = 16.5v 3.10 3.30 3.40 v v fb feedback voltage mic2177 [adj.]: v out = 3.3v, i load = 0 1.22 1.245 1.27 v mic2177 [adj.]: v out = 3.3v, 5v v in 16v, 10ma i load 2a 3.20 3.14 3.3 3.40 3.46 v v mic2177-5.0: i load = 0 4.85 5.0 5.15 v mic2177-5.0: 6v v in 16v, 10ma i load 2a 4.85 4.75 5.0 5.15 5.25 v v mic2177-3.3: i load = 0 3.20 3.3 3.40 v v out output voltage mic2177-3.3: 5v v in 16v, 10ma i load 2a 3.20 3.14 3.3 3.40 3.46 v v v th upper threshold 4.25 4.35 v v tl undervoltage lockout lower threshold 3.9 4.15 v mic2177 [adj.] 60 150 na i fb feedback bias current mic2177-5.0, mic2177-3.3 20 40 a a vol error amplifier gain 0.6v v comp 0.8v 15 18 20 v upper limit 0.9 1.5 v error amplifier output swing lower limit 0.05 0.1 v error amplifier output current source and sink 15 25 35 a f o oscillator frequency 160 200 240 khz d max maximum duty cycle v fb = 1.0v 100 % t on min minimum on-time v fb = 1.5v 300 400 ns sync frequency range 220 300 khz sync threshold 0.8 1.6 2.2 v sync minimum pulse width 500 ns i sync sync leakage v sync = 0v to 5.5v ?1 0.01 1 a pwm mode, v in = 12v 3.8 4.7 5.7 a i lim current limit skip mode 600 ma high-side switch, v in = 12v 90 250 m ? r on switch on-resistance low-side switch, v in = 12v 110 250 m ?
micrel, inc. mic2177 april 2008 5 m9999-042108 symbol parameter condition min typ max units i sw output switch leakage v sw = 16.5v 1 10 a enable threshold 0.8 1.6 2.2 v i en enable leakage v en = 0v to 5.5v ?1 0.01 1 a auto threshold 0.8 1.6 v auto source current v fb = 1.5v, vauto < 0.8v 7 11 15 a v in ? v out = 0v 220 ma minimum switch current for pwm operation v in ? v out = 3v 420 ma general note: devices are esd sensitive. handling precautions recommended.
micrel, inc. mic2177 april 2008 6 m9999-042108 typical characteristics 175 180 185 190 195 200 205 -60 -30 0 30 60 90 120 150 frequency (khz) temperature (c) oscillator frequenc y vs. temperature 1.238 1.240 1.242 1.244 1.246 1.248 1.250 1.252 reference voltage (v) mic2177 [adj.] -60 -30 0 30 60 90 120 150 temperature (c) reference voltage vs. temperature 3.280 3.285 3.290 3.295 3.300 3.305 3.310 3.315 3.320 reference voltage (v) mic2177-3.3 -60 -30 0 30 60 90 120 150 temperature (c) reference voltage vs. temperature 4.970 4.980 4.990 5.000 5.010 5.020 5.030 reference voltage (v) mic21775.0 -60 -30 0 30 60 90 120 150 temperature (c) reference voltage vs. temperature 16.0 16.5 17.0 17.5 18.0 18.5 19.0 amplifier voltage gain -60 -30 0 30 60 90 120 150 temperature (c) error-amplifier gain vs. temperature 0 20 40 60 80 100 120 bias current (na) -60 -30 0 30 60 90 120 150 temperature (c) feedback input bias current vs. temperature 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 current limit (a) -60 -30 0 30 60 90 120 150 temperature (c) current limit vs. temperature 0 50 100 150 200 250 24681012141618 input voltage (v) high-side switc h on-resistance 125c 85c 25c 0c 0 50 100 150 200 250 300 350 24681012141618 input voltage (v) low-side switch on-resistance 125c 85c 25c 0c 0 2 4 6 8 10 12 2 4 6 8 10 12 14 16 18 supply current (ma) input voltage (v) pwm-mode supply current output switching 60 65 70 75 80 85 90 95 100 10 100 1000 2500 efficiency (%) output current (ma) v in =5v 8v 12v skip pwm 3.3v output efficiency 70 75 80 85 90 95 100 10 100 1000 2500 efficiency (%) output current (ma) 8v 12v v in =6v skip pwm 5v output efficiency
micrel, inc. mic2177 april 2008 7 m9999-042108 functional diagram v out = 1.245 r1 r2 + 1 sw pgnd i sense amp. pwm/ skip-mode select logic v ref 1.245v fb 100m n-channel 100m p-channel comp vin error amp. skip-mode comp. i min comp. i limit comp. output control logic i min thrshld. low output comp. 40mv r s q 200khz oscillator pwm comp. 3.3v regulator uvlo, thermal shutdown v out l1 out sgnd 2.2nf c c auto 0.01f en bias v in 4.5v to 16.5v c in sync c out mic2177 [adjustable] internal supply voltage enable shutdown auto-mode pwm 20 19 18 11 13 2 1 3 8 d 4 5 6 7 r1 r2 12 10 14 15 16 17 bold lines indicate high current traces r c 10k corrective ramp reset pulse 10a 3.3v 9 ( )
micrel, inc. mic2177 april 2008 8 m9999-042108 functional description micrel?s mic2177 is a synchronous buck regulator that operates from an input volt age of 4.5v to 16.5v and provides a regulated output voltage of 1.25v to 16.5v. it has internal power mosfets that supply up to 2.5a of load current and operates with up to 100% duty cycle to allow low-dropout operation. to optimize efficiency, the mic2177 operates in pwm and skip mode. skip mode provides the best efficiency when load current is less than 200ma, while pwm mode is more efficient at higher current. a patented technique allows the mic2177 to automatically select the co rrect operating mode as the load current changes. during pwm operation, the mic2177 uses current-mode control which provides superior line regulation and makes the control loop easier to compensate. the pwm switching frequency is set internally to 200khz and can be synchronized to an external clock frequency up to 300khz. other features incl ude a low-current shutdown mode, current limit, undervoltage lockout, and thermal shutdown. see the following sections for details. switch output the switch output (sw) is a half h-bridge consisting of a high-side p-channel and low-side n-channel power mosfet. these mosfets have a typical on-resistance of 100m ? when the mic2177 operates from a 12v supply. anti-shoot-through circuitry prevents the p- channel and n-channel from turning on at the same time. current limit the mic2177 uses pulse-by- pulse current limiting to protect the output. during each switching period, a current limit comparator dete cts if the p-ch annel current exceeds 4.7a. when it does, the p-channel is turned off until the next switching period begins. undervoltage lockout undervoltage lockout (uvlo) turns off the output when the input voltage (v in ) is too low to provide sufficient gate drive for the output mosfet s. it prevents the output from turning on until v in exceeds 4.3v. once operating, the output will not shut off until v in drops below 4.2v. thermal shutdown thermal shutdown turns off the output when the mic2177 junction temperature exceeds the maximum value for safe operation. afte r thermal shutdown occurs, the output will not turn on until the junction temperature drops approximately 10c. shutdown mode the mic2177 has a low-current shutdown mode that is controlled by the enable input (en). when a logic 0 is applied to en, the mic2177 is in shutdown mode and its quiescent current drops to less than 5a. internal bias regulator an internal 3.3v regulator provides power to the mic2177 control circuits. this internal supply is brought out to the bias pin for bypas sing by an external 0.01f capacitor. do not connect any external load to the bias pin. it is not designed to provide an external supply voltage. frequency synchronization the mic2177 operates at a pr eset switching frequency of 200khz. it can be synchronized to a higher frequency by connecting an external cl ock to the sync pin. the sync pin is a logic level i nput that synchronizes the oscillator to the rising edge of an external clock signal. it has a frequency range of 220khz?300khz, and can operate with a minimum pulse-width of 500ns. if synchronization is not required, connect sync to ground. low-dropout operation output regulation is maintained in pwm or skip mode even when the difference between v in and v out decreases below 1v. as v in ? v out decreases, the duty cycle increases until it reaches 100%. at this point, the p-channel is kept on for several cycles at a time, and the output stays in regulation until v in ? v out falls below the dropout voltage (dropout voltage = p-channel on resistance load current). pwm-mode operation refer to ?pwm-mode functional diagram? which is a simplified block diagram of the mic2177 operating in pwm mode with its associated waveforms. when operating in pwm mode, the output p-channel and n-channel mosfets are alternately switched on at a constant frequency and variable duty cycle. a switching period begins when the oscillator generates a reset pulse. this pulse resets the rs latch which turns on the p-channel and turns off the n-channel. during this time, inductor current (i l1 ) increases and energy is stored in the inductor. the current sense amplifier (i sense amp) measures the p-channel drain-to-source voltage and outputs a voltage proportional to i l1 . the output of i sense amp is added to a saw tooth waveform (corrective ramp) generated by the oscillator, creating a composite waveform labeled i sense on the timing diagram. when i sense is greater than the error amplifier output, the pwm comparator will set the rs latch which turns off the p- channel and turns on the n-channel. energy is then
micrel, inc. mic2177 april 2008 9 m9999-042108 discharged from the inductor and i l1 decreases until the next switching cycle begins. by varying the p-channel on-time (duty cycle), the average inductor current is adjusted to whatever value is required to regulate the output voltage. the mic2177 uses current-mode control to adjust the duty cycle and regulate t he output voltage. current- mode control has two signal loops that determine the duty cycle. one is an outer l oop that senses the output voltage, and the other is a fast er inner loop that senses the inductor current. signals from these two loops control the duty cycle in the following way: v out is fed back to the error amplifier which compares the feedback voltage (v fb ) to an internal reference voltage (v ref ). when v out is lower than its nominal value, the error amplifier output voltage increases. this voltage then intersects the current-sense waveform later in switching period which increases the duty cycle and average inductor current. if v out is higher than nominal, the error amplifier output voltage dec reases, reducing the duty cycle. the pwm control loop is stabilized in two ways. first, the inner signal loop is compensated by adding a corrective ramp to the out put of the current sense amplifier. this allows the regulator to remain stable when operating at greater than 50% duty cycle. second, a series resistor-capacitor load is connected to the error amplifier output (comp pin). this places a pole-zero pair in the regulator control loop. one more important item is sy nchronous rectification. as mentioned earlier, the n-channel output mosfet is turned on after the p-channel turns off. when the n- channel turns on, its on-resistance is low enough to create a short across the output diode. as a result, inductor current flows thr ough the n-channel and the voltage drop across; it is significantly lower than a diode forward voltage. this reduces power dissipation and improves efficiency to greater than 95% under certain operating conditions. to prevent shoot through current, the output stage employs break-before-make circuitry that provides approximately 50ns of delay from the time one mosfet turns off and the other turns on. as a result, inductor current briefly flows through the output diode during this transition. skip-mode operation refer to ?skip-mode functional diagram? which is a simplified block diagram of the mic2177 operating in skip mode and its associated waveforms. skip-mode operation turns on the output p-channel at a frequency and duty cycle that is a function of v in , v out , and the output inductor value. while in skip mode, the n- channel is kept off to optimize efficiency by reducing gate charge dissipation. v out is regulated by skipping switching cycles that turn on the p-channel. to begin analyzing mic2177 skip-mode operation, assume the skip-mode comparator output is high and the latch output has been reset to a logic 1. this turns on the p-channel and causes i l1 to increase linearly until it reaches a current limit of 600ma. when i l1 reaches this value, the current limit comp arator sets the rs latch output to logic 0, turning off the p-channel. the output switch voltage (v sw ) then swings from v in to 0.4v below ground, and i l1 flows through the schottky diode. l1 discharges its energy to the output and i l1 de-creases to zero. when i l1 = 0, v sw swings from ?0.4v to v out , and this triggers a one-shot t hat resets the rs latch. resetting the rs latch turns on the p-channel, which begins another switching cycle. the skip-mode comparator regulates v out by controlling when the mic2177 skips cycles. it compares v fb to v ref and has 10mv of hysteresis to prevent oscillations in the control loop. when v fb is less than v ref ? 5mv, the comparator output is logic 1, allowing the p-channel to turn on. conversely, when v fb is greater than v ref + 5mv, the p-channel is turned off. note that this is a self-osc illating topology which explains why the switching frequency and duty cycle are a function of v in , v out , and the value of l1. it has the unique feature (for a pul se-skipping regulator) of supplying the same value of maximum load current for any value of v in , v out , or l1. this allows the mic2177 to always supply up to 300ma of load current (i load ) when operating in skip mode. changing from pwm to skip mode refer to ?block diagram? for circuits described in the following sections. the mic2177 automatically changes from pwm to skip mode operation when i load drops below a minimum value. i min is determined indirectly by detecting when the peak inductor current (i l(peak) ) is less than 420ma. this is done by the minimum current comparator which detects if the output p-channel current equals 420ma during each switching cycle. if it does not, the pwm/skip-mode select logic places the mic2177 into skip-mode operation. the value of i min that corresponds to i l1(peak) = 420ma is given by the following equation: 2 ? i 420ma i l1 min ? = where: ? i l1 = inductor ripple current this equation shows i min varies as a function of ? i l . therefore, the user must se lect an inductor value that results in i min = 200ma when i l(peak) = 420ma. the formulas for calculating the correct inductor value are
micrel, inc. mic2177 april 2008 10 m9999-042108 given in the ?applications info rmation? section. note that ? i l varies as a function of input voltage, and this also causes i min to vary. in applications where the input voltage changes by a factor of two, i min will typically vary from 130ma to 250ma. during low-dropout operati on, the minimum current thresh-old circuit reduces the minimum value of i l1(peak) for pwm operation. this compensates for ? i l1 decreasing to almost zero when the difference between v in and v out is very low. changing from skip to pwm mode the mic2177 will aut omatically change from skip to pwm mode when i load exceeds 300ma. during skip- mode operation, it can supply up to 300ma, and when i load exceeds this limit, v out will fall below its nominal value. at this point, the mi c2177 begins operating in pwm mode. note that the maximum value of i load for skip mode is greater than the minimum value required for pwm mode. this current hysteresis prevents the mic2177 from toggling between modes when i load is in the range of 100ma to 300ma. the low output comparator determines when v out is low enough for the regulator to change operating modes. it detects when the feedback voltage is 3% below nominal, and pulls the auto pin to ground. when auto is less than 1.6v, the pwm/skip-mode select logic places the mic2177 into pwm operation. the external 2.2nf capacitor connected to auto is charged by a 10a current source after the regulator begins operating in pwm mode. as a result, auto stays below 1.6v for several switching cycles after pwm operation begins, forcing the mic2177 to remain in pwm mode during this transition. external pwm-mode selection the mic2177 can be forced to operate in only pwm mode by connecting auto to ground. this prevents skip-mode operation in applications that are sensitive to switching noise.
micrel, inc. mic2177 april 2008 11 m9999-042108 pwm-mode functional diagram sw pgnd i sense amp. v ref 1.245v 100m n-channel 100m p-channel comp vin error amp. r s q 200khz oscillator pwm comp. v out l1 fb sgnd c c v in 4.5v to 16.5v c in syn c c out mic2177 [adjustable] pwm-mode signal path stop 18 13 2 1 3 8 d 4 5 6 7 r1 r2 12 14 15 16 17 r c corrective ram p reset pulse i l1 9 v out = 1.245 r1 r2 + 1 ( ) v sw i l1 reset pulse i sense i load i l1 error amp. ou t p u t
micrel, inc. mic2177 april 2008 12 m9999-042108 skip-mode functional diagram s r q one shot sw pgnd i sense amp. v ref 1.245v 100m p-channel vin skip-mode comp. i limit comp. v out l1 fb sgnd v in 4.5v to 16.5v c in c out mic2177 [adjustable] skip-mode signal pat 2 1 3 8 d 4 5 6 7 r1 r2 12 14 15 16 17 i limit thresh. voltage output control logic i l1 9 v out = 1.245 r1 r2 + 1 ( ) v sw i l1 one-shot pulse v fb v ref + 5mv v ref ? 5mv 0 i lim 0 v out v in
micrel, inc. mic2177 april 2008 13 m9999-042108 application information feedback resistor selection (adjustable version) the output voltage is conf igured by connecting an external resistive divider to the fb pin as shown in ?mic2177 block diagram.? the ratio of r1 to r2 determines the output voltage. to optimize efficiency during low output current operation, r2 should not be less than 20k ? . however, to prevent feedback error due to input bias current at the fb pin, r2 should not be greater than 100k ? . after selecting r2, calculate r1 using the following formula: ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? = 1 1.245v v r2 r1 out input capacitor selection the input capacitor is selected for its rms current and voltage rating and should be a low esr (equivalent series resistance) electrolytic or tantalum capacitor. as a rule-of-thumb, the voltage rating for a tantalum capacitor should be twice the value of v in , and the voltage rating for an electrolytic should be 40% higher than v in . the rms current rating must be equal or greater than the maximum rms input ripple current. a simple, worst-case formula for calculating this rms current is: 2 i i load(max) rms(max) = tantalum capacitors are a better choice for applications that require the most compac t layout or operation below 0c. the input capacitor must be located very close to the v in pin (within 0.2 inches, 5mm). also place a 0.1f ceramic bypass capacitor as close as possible to v in . inductor selection the inductor must be at least a minimum value in order for the mic2177 to change from pwm to skip mode at the correct value of output current. this minimum value ensures the inductor ripple current never exceeds 600ma, and is calculated using the following formula: 8.3 .3 v v 1 v l in(max) out out min ? ? ? ? ? ? ? ? ? = where: v in(max) = maximum input voltage in general, a value at least 20% greater than l min should be selected because inductor values have a tolerance of 20%. two other parameters to consider in selecting an inductor are winding resistance and peak current rating. the inductor must have a pea k current rating equal or greater than the peak inductor current. otherwise, the inductor may saturate, causing excessive current in the output switch. also, the inductor?s core loss may increase significantly. both of these effects will degrade efficiency. the formula for peak inducto rcurrent is: i l(peak) = i load(max) + 300ma to maximize efficiency, the inductor?s resistance must be less than the output switch on-resistance (preferably 50m ? or less). output capacitor selection select an output capacitor that has a low value of esr. this parameter determines a regulator?s output ripple voltage (v ripple ) which is generated by ? i l esr. as mentioned in ?inductor selection,? the maximum value for ? i l is 600ma. therefore, the maximum value of esr is: ripple max v 600ma esr = where: v ripple < 1% of v out typically, capacitors in the range of 100f to 220f have esr less than this maximum value. the output capacitor can be either a low esr electrolytic or tantalum capacitor, but tant alum is a better choice for compact layout and operation at temperatures below 0c. the voltage rating of a t antalum capacitor must be 2 v out , and the voltage rating of an electrolytic must be 1.4 v out . output diode selection in pwm operation, inductor current flows through the output diode approximately 50ns during the dead time when one output mosfet turns off and the other turns on. in skip-mode, the inductor current flows through the diode during the entire p-channel off time. the correct diode for both of these conditions is a 1a diode with a reverse voltage rating greater than v in . it must be a schottky or ultra fast-recovery diode (t r <100ns) to minimize power dissipation from the diode?s reverse- recovery charge. compensation compensation is provided by connecting a series rc load to the comp pin. this creates a pole-zero pair in the regulator control loop, allowing the regulator to remain stable with enough low frequency loop-gain for good load and line regulation. at higher frequencies pole-zero reduces loop-gain to a level referred to as the mid-band gain. the mid-band gain is low enough so that the loop gain crosses 0db with sufficient phase margin. typical values for the rc load are 4.7nf ? 10nf for the capacitor and 5k ? ? 20k ? for the resistor. printed circuit board layout a well designed pc board will prevent switching noise and ground bounce from interfering with the operation of
micrel, inc. mic2177 april 2008 14 m9999-042108 the mic2177. a good design takes into consideration component placement and routing of power traces. the first thing to consider is the locations of the input capacitor, inductor, output diode, and output capacitor. the input capacitor must be placed very close to the v in pin, the inductor and output di ode very close to the sw pin, and the output capacito r near the inductor. these components pass large high-frequency current pulses, so they must use short, wide power traces. in addition, their ground pins and pgnd are connected to a ground plane that is nearest the power supply ground bus. the feedback resistors, rc compensation network, and bias pin bypass capacitor s hould be located near their respective pins. to prevent ground bounce, their ground traces and sgnd should not be in the path of switching currents returning to the power supply ground bus. sgnd and pgnd should be tied together by a ground plane that extends und er the mic2177. bias sgnd auto comp pgnd fb sw vin c2 100f 10v v out 3.3v/1a l1, 50h c4 6.8nf c3 0.01f mic2177 sync en r4 10k r4 10k v in 4 .5v to 16.5v c1 22f 35v u1 20 18 10 11 13 14?17 19 12 4?7 3,8 1,2,9 d1 mbrs130l u1 micrel mic2177-3.3bwm c1 avx c2 avx c3 z5uorx7r ceramic dielectric material c4 x7rornp0 ceramic dielectric material d1 motorola mbrs130lt3 l1 coiltronics ctx50-4p, dcr = 0.097 l1 coilcraft l1 bi hm77-11003, dcr = 0.07 3 c5 0.01 f out bill of materials inductors capacitors diodes transistors coilcraft 1102 silver lake rd. cary, il 60013 tel: (708) 639-2361 fax: (708) 639-1469 avx 801 17 th ave. myrtle beach, sc 29577 tel: (803) 448-9411 fax: (803) 448-1973 general instruments (gi) 10 melville park rd. melville, ny 11747 tel: (516) 847-3222 fax: (516) 847-3150 siliconix 2201 laurelwood rd. santa clara, ca 96056 tel: (800) 554-5565 coiltronics 6000 park of commerce blvd. boca raton, fl 33487 tel: (407) 241-7876 fax: (407) 241-9339 sanyo video components corp. 2001 sanyo ave. san diego, ca 92173 tel: (619) 661-6835 fax: (619) 661-1055 international rectifier corp. 233 kansas st. el segundo, ca 90245 tel: (310) 322-3331 fax: (310) 322-3332 bi technologies 4200 bonita place fullerton, ca 92835 tel: (714) 447-2345 fax: (714) 447-2500 sprague electric 60005 lower main st. sanford, me 04073 tel: (207) 324-4140 motorola, inc. ms 56-126 3102 north 56 th st. phoenix, az 85018 tel: (602) 244-3576 fax: (602) 244-4015
micrel, inc. mic2177 april 2008 15 m9999-042108 package information 20-pin wide soic (wm) 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 the information furnished by micrel in this data sheet is belie ved to be accurate and reliable. however, no responsibility is a ssumed by micrel for its use. micrel reserves the right to change circuitry and specifications at any time without notification to the customer. 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 personal injury. life suppo rt devices or systems 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. ? 1999 micrel, incorporated.

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