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mic22705 1mhz, 7a integrated switch high-efficiency synchronous buck regulator ramp control is a trademark of micrel, inc mlf and micro leadframe are registered trademarks of amkor technology, 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 general description the micrel mic22705 is a high-efficiency, 7a integrated switch synchronous buck (step-down) regulator. the mic22705 is optimized for highest efficiency, achieving more than 95% efficiency while still switching at 1mhz. the ultra-high speed control loop keeps the output voltage within regulation even under the extreme transient load swings commonly found in fpgas and low-voltage asics. the output voltage is pre-bias safe and can be adjusted down to 0.7v to address all low-voltage power needs. the mic22705 offers a full range of sequencing and tracking options. the enable/delay (en/dly) pin, combined with the power good (pg) pin, allows multiple outputs to be sequenced in any way during turn-on and turn-off. the ramp control? (rc) pin allows the device to be connected to another prod uct in the mic22xxx and/or mic68xxx family, to keep the output voltages within a certain ? v on start-up. the mic22705 is available in a 24-pin 4mm x 4mm mlf ? with a junction operating range from ?40 c to +125 c. data sheets and support documentation can be found on micrel?s web site at: www.micrel.com . features ? input voltage range: 2.9v to 5.5v ? output voltage adjustable down to 0.7v ? output load current up to 7a ? safe start-up into a pre-biased output load ? full sequencing and tracking capability ? power good output ? efficiency >95% across a broad load range ? ultra-fast transient response ? easy rc compensation ? 100% maximum duty cycle ? fully-integrated mosfet switches ? thermal-shutdown and cu rrent-limit protection ? 24-pin 4mm x 4mm mlf ? ? ?40 c to +125 c junction temperature range applications ? high power density point-of-load conversion ? servers, routers, and base stations ? dvd recorders / blu-ray players ? computing peripherals ? fpgas, dsp and low-voltage asic power _________________________________________________________________________________________________________________________ typical application mic22705 7a 1mhz synchronous output converter efficiency (v in = 5.0v) vs. output current 50 55 60 65 70 75 80 85 90 95 100 01234567 output current (a) efficiency (%) 2.5v 3.3v v in = 5.0v march 2011 m9999-033111-a
micrel, inc. mic22705 march 2011 2 m9999-033111-a ordering information part number voltage junction temp erature range package lead finish mic22705yml adjustable ?40c to +125c 24-pin 4mm x 4mm mlf ? pb-free note: mlf is a green rohs-compliant package. lead fini sh is nipdau. mold component is halogen free. pin configuration 24-pin 4mm 4mm mlf ? (ml) pin description pin number pin name description 1, 6, 13, 18 pvin power supply voltage (input): the pvin pins are the input supply to the internal p-channel power mosfet. a 22f ceramic is recommended for bypassing at each pvin pin. the svin pin must be connected to a pvin pin. 2 en/dly enable/delay (input): this pin is internally fe d with a 1a current source from svin. a delayed turn on is implemented by adding a capacitor to this pin. the delay is proportional to the capacitor value. the internal circuits are held off until en/dly reaches the enable threshold of 1.24v. this pin is pulled low when the input voltage is lower than the uvlo threshold. 3 nc no connect: leave this pin open. do not connect to ground or route other signals through this pin. 4 rc ramp control: a capacitor from the rc pin-to-ground determines slew rate of output voltage during start-up. the rc pin is internally fed with a 1a current source. the output voltage tracks the rc pin voltage. the slew rate is proporti onal by the internal 1a source and rc pin capacitor. this feature can be used for tr acking capability as well as soft start. 5 pg pg (output): this is an open drai n output that indicates when the output voltage is below 90% of its nominal voltage. the pg flag is asserted wit hout delay when the enable is set low or when the output goes below the 90% threshold. 14 fb feedback: input to the error amplifier. the fb pin is regulated to 0.7v. a resistor divider connecting the feedback to the output is us ed to adjust the desired output voltage.
micrel, inc. mic22705 march 2011 3 m9999-033111-a pin description (continued) pin number pin name description 15 comp compensation pin (input): the mic22705 uses an internal compensation network containing a fixed-frequency zero (phase lead response) and pole (phase lag response) which allows the external compensation network to be much simp lified for stability. the addition of a single capacitor and resistor to the comp pin will a dd the necessary pole and zero for voltage mode loop stability using low-value, low-esr ceramic capacitors. 16 sgnd signal ground: internal signal ground for all low power circuits. 17 svin signal power supply voltage (input): this pin is connected externally to the pvin pin. a 2.2f ceramic capacitor from the svin pin to sgnd must be placed next to the ic. 7, 12, 19, 24 pgnd power ground: internal ground connection to the source of t he internal n-channel mosfets. 8, 9, 10, 11, 20, 21, 22, 23 sw switch (output): this is the connection to the dr ain of the internal p-channel mosfet and drain of the n-channel mosfet. this is a high-fre quency, high-power connection; therefore traces should be kept as short and as wide as practical. ep gnd exposed pad (power): must be connected to the gnd plane for full output power to be realized.
micrel, inc. mic22705 march 2011 4 m9999-033111-a absolute maximum ratings (1, 2) pv in to pgnd .................................................... ?0.3v to 6v sv in to pgnd..................................................?0.3v to pv in v sw to pgnd...................................................?0.3v to pv in v en/dly to pgnd .............................................. -0.3v to pv in v pg to pgnd ................................................... ?0.3v to pv in junction temper ature ................................................ 150c pgnd to sgnd ............................................. ?0.3v to 0.3v storage temperature rang e ....................?65c to +150c lead temperature (soldering, 10s )............................ 260c operating ratings (3) supply volt age ................................................. 2.9v to 5.5v power good voltage (v pg )...................................0v to pv in enable input (v en/dly )...........................................0v to pv in junction temperature (t j ) ..................?40c t j +125c package thermal resistance 4mm x 4mm mlf ? -24 ( jc )................................14c/w 4mm x 4mm mlf ? -24 ( ja )................................40c/w electrical characteristics (4) sv in = pv in = v en/dly = 3.3v, v out = 1.8v, t a = 25c, unless noted. bold values indicate ?40c< t j < +125c. parameter condition min. typ. max. units power input supply input voltage range (pv in ) 2.9 5.5 v undervoltage lockout trip level pv in rising 2.55 2.75 2.9 v uvlo hysteresis 420 mv quiescent supply current v fb = 0.9v (not switching) 0.85 1.3 ma shutdown current v en/dly = 0v 5 10 a reference feedback reference voltage 0.686 0.7 0.714 v load regulation i out = 100ma to 7a 0.2 % line regulation v in = 2.9v to 5.5v; i out = 100ma 0.2 % fb bias current v fb = 0.5v 10 na enable control en/dly threshold voltage 1.14 1.24 1.34 v en hysteresis 10 mv en/dly bias current v en/dly = 0.5v; v in = 2.9v and v in = 5.5v 0.7 1.0 1.3 a rc ramp control rc pin source current v rc = 0.35v 0.7 1.0 1.3 a oscillator switching frequency 0.8 1.0 1.2 mhz maximum duty cycle v fb 0.5v 100 % short-current protection current limit v fb = 0.5v 7 11 21 a notes: 1. exceeding the absolute maximum rating may damage the device. 2. devices are esd sensitive. handling precautions recommended. 3. the device is not guaranteed to function outside its operating rating. 4. specification for packaged product only.
micrel, inc. mic22705 march 2011 5 m9999-033111-a electrical characteristics (4) (continued) v in = pv in = v en/dly = 3.3v, v out = 1.8v, t a = 25c, unless noted. bold values indicate ?40c< t j < +125c. parameter condition min. typ. max. units internal fets top mosfet r ds(on) v fb = 0.5v, i sw = 1a 30 m ? bottom mosfet r ds(on) v fb = 0.9v, i sw = -1a 25 m ? sw leakage current pv in = 5.5v, v sw = 5.5v, v en = 0v 60 v in leakage current pv in = 5.5v, v sw = 0v, v en = 0v 25 a power good (pg) pg threshold threshold % of v fb from v ref ? 7.5 ? 10 ? 12.5 % hysteresis 2.0 % pg output low voltage i pg = 5ma (sinking), v en/dly = 0v 144 mv pg leakage current v pg = 5.5v; v fb = 0.9v 1.0 2.0 a thermal protection over-temperature shutdown t j rising 160 c over-temperature shutdown hysteresis 20 c
micrel, inc. mic22705 march 2011 6 m9999-033111-a typical characteristics v in operating supply current vs. input voltage 0 5 10 15 20 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) supply current (ma) v out = 1.8v i out = 0a switching v in shutdown current vs. input voltage 0 4 8 12 16 20 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) shutdown current (a) v en/dly = 0v feedback voltage vs. input voltage 0.693 0.697 0.700 0.704 0.707 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) feedback voltage (v) v out = 1.8v load regulation vs. input voltage 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) total regulation (%) v out = 1.8v i out = 0a to 7a current limit vs. input voltage 0 5 10 15 20 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) current limit (a) v out = 1.8v switching frequency vs. input voltage 800 900 1000 1100 1200 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) switching frequency (khz) v out = 1.8v i out = 0a enable threshold vs. input voltage 1.0 1.1 1.2 1.3 1.4 1.5 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) enable threshold (v) rising enable input current vs. input voltage 0 1 2 3 4 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) enable input current (a) v en/dly = 0v power good threshold/v ref ratio vs. input voltage 80% 85% 90% 95% 100% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) v pg threshold/v ref (%) v ref = 0.7v
micrel, inc. mic22705 march 2011 7 m9999-033111-a typical characteristics (continued) v in operating supply current vs. temperature 0.0 5.0 10.0 15.0 20.0 -50 -20 10 40 70 100 130 temperature (c) supply current (ma) v in = 5.0v v out = 1.8v i out = 0a switching v in shutdown current vs. temperature 0 5 10 15 20 -50 -20 10 40 70 100 130 temperature (c) supply current (ma) v in = 5.0v i out = 0a v en/dly = 0v v in uvlo threshold vs. temperature 2.2 2.4 2.6 2.8 3.0 -50 -20 10 40 70 100 130 temperature (c) v in threshold (v) rising falling feedback voltage vs. temperature 0.693 0.697 0.700 0.704 0.707 -50 -20 10 40 70 100 130 temperature (c) feedback voltage (v) v in = 5.0v v out = 1.8v i out = 0a load regulation vs. temperature 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% -50 -20 10 40 70 100 130 temperature (c) load regulation (%) v in = 5.0v v out = 1.8v i out = 0a to 7a line regulation vs. temperature 0.0% 0.1% 0.2% 0.3% 0.4% 0.5% -50 -20 10 40 70 100 130 temperature (c) line regulation (%) v in = 2.9v to 5.0v v out = 1.8v i out = 0a switching frequency vs. temperature 800 900 1000 1100 1200 -50 -20 10 40 70 100 130 temperature (c) switching frequency (khz) v in = 5.0v v out = 1.8v i out = 0a enable threshold vs. temperature 1.0 1.1 1.2 1.3 1.4 1.5 -50 -20 10 40 70 100 130 temperature (c) enable threshold (v) v in = 5v rising current limit vs. temperature 0 5 10 15 20 -50 -20 10 40 70 100 130 temperature (c) current limit (a) v in = 5.0v v out = 1.8v
micrel, inc. mic22705 march 2011 8 m9999-033111-a typical characteristics (continued) efficiency vs. output current 80 85 90 95 100 01234567 output current (a) efficiency (%) 3.3v in 5.0v in v out = 1.8v feedback voltage vs. output current 0.693 0.697 0.700 0.704 0.707 01234567 output current (a) feedback voltage (v) v in = 5.0v v out = 1.8v line regulation vs. output current 0.0% 0.1% 0.2% 0.3% 0.4% 0.5% 01234567 output current (a) line regulation (%) v in = 2.9v to 5.5v v out = 1.8v switching frequency vs. output current 800 900 1000 1100 1200 01234567 output current (a) switching frequency (khz) v in = 5.0v v out = 1.8v i out = 0a output voltage (v in = 3.3v) vs. output current 2.8 2.9 3.0 3.1 3.2 3.3 3.4 01234567 output current (a) output voltage (v) v in = 3.3v v fb < 0.7v output voltage (v in = 5.0v) vs. output current 4.6 4.7 4.8 4.9 5.0 5.1 5.2 01234567 output current (a) output voltage (v) v in = 5.0v v fb < 0.7v efficiency (v in = 3.3v) vs. output current 70 75 80 85 90 95 100 0123456789 output current (a) efficiency (%) 2.5v 1.8v 1.5v 1.2v 1.0v 0.9v 0.8v v in = 3.3v ic power dissipation vs. output current (v in = 3.3v) 0 0.5 1 1.5 2 0246 output current (a) power dissipation (w) v in = 3.3v v out = 0.8v, 1.0v, 1.2v, 1.5v, 1.8v, 2.5v case temperature* (v in = 3.3v) vs. output current 0 20 40 60 80 100 01234567 output current (a) case temperature (c) v in = 3.3v v out = 1.8v
micrel, inc. mic22705 march 2011 9 m9999-033111-a typical characteristics (continued) efficiency (v in = 5.0v) vs. output current 70 75 80 85 90 95 100 0123456789 output current (a) efficiency (%) 3.3v 2.5v 1.8v 1.5v 1.2v 1.0v 0.9v 0.8v v in = 5.0v ic power dissipation vs. output current (v in = 5v) 0 0.5 1 1.5 2 0246 output current (a) power dissipation (w) v in = 5v v out = 0.8v, 1.0v, 1.2v, 1.5v, 1.8v, 2.5v, 3.3v case temperature* (v in = 5.0v) vs. output current 0 20 40 60 80 100 01234567 output current (a) case temperature (c) v in = 5v v out = 1.8v die temperature* : the temperature measurement was taken at the hottest point on the mic22705 case and mounted on a five- square inch pcb (see thermal measurement s section). actual results will depend upon the size of the pcb, ambient temperature, and proximity to other heat-emitting components.
micrel, inc. mic22705 march 2011 10 m9999-033111-a functional characteristics
micrel, inc. mic22705 march 2011 11 m9999-033111-a functional characteristics (continued)
micrel, inc. mic22705 march 2011 12 m9999-033111-a functional characteristics (continued)
micrel, inc. mic22705 march 2011 13 m9999-033111-a functional diagram figure 1. mic22705 functional diagram
micrel, inc. mic22705 march 2011 14 m9999-033111-a application information the mic22705 is a 7a sync hronous step-down regulator ic with a fixed 1mhz, voltage-mode pwm control scheme. the other features include tracking and sequencing control for controlling multiple output power systems, and power-on-reset (por). the mic22705 is a voltage mode, pulse-width modulation (pwm) regulator. by controlling the ratio of the on-to-off time, or duty cycle, a regulated dc output voltage is achieved. as load or supply voltage changes, so does the duty cycle to maintain a constant output voltage. in cases where the input supply runs into a dropout condition, the mic2 2705 will run at 100% duty cycle. the mic22705 provides constant switching at 1mhz with synchronous internal mosfets. the internal mosfets include a high-side p-channel mosfet from the input supply to the switch pin and an n-channel mosfet from the switch pin-to-ground. since the low-side n- channel mosfet provides the current during the off cycle, very-low amount of pow er is dissipated during the off period. the pwm control provides fixed-frequency operation. by maintaining a constant switching frequency, predictable fundamental and harmonic frequencies are achieved. other methods of regulation, such as burst and skip modes, have frequency spectrums that change with load that can interfere with sensitive communication equipment. component selection input capacitor a 22f x5r or x7r dielectrics ceramic capacitor is recommended on each of the pvin pins for bypassing. a y5v dielectrics capacitor should not be used. aside from losing most of their capacit ance over temperature, they also become resistive at high frequencies. this reduces their ability to filter out high-frequency noise. output capacitor the mic22705 was designed spec ifically for the use of ceramic output capacitors. the 100f output capacitor can be increased to improve transient performance. since the mic22705 is in voltage mode, the control loop relies on the inductor and output capacitor for compensation. for this reason, do not use excessively large output capacitors. the output capacitor requires either an x7r or x5r dielectric. y5v and z5u dielectric capacitors, aside from the undesirable effect of their wide variation in capacitance over temperature, become resistive at high frequencies. using y5v or z5u capacitors can cause instability in the mic22705. inductor selection inductor selection will be determined by the following (not necessarily in the order of importance): ? inductance ? rated current value ? size requirements ? dc resistance (dcr) the mic22705 is designed for use with a 0.47h to 4.7h inductor. maximum current ratings of the inductor are generally given in two methods: permissible dc current and saturation current. permissible dc current can be rated either for a 40c temperature rise or a 10% loss in inductance. ensure the inductor selected can handle the maximum operating current. when saturation current is specified, make sure that there is enough margin that the peak current will not saturate the inductor. the ripple current can add as much as 1.2a to the output current level. the rms rating should be chosen to be equal or greater than the current limit of the mic22705 to prevent overheating in a fault condition. for best electrical performance, the inductor should be placed very close to the sw nodes of the ic. for this reason, the heat of the inductor is somewhat coupled to the ic (in such cases, the case temperature is not the real dissipation in the regulator), so it offers some level of protection if the inductor gets too hot. it is important to test all operating limits before settling on the final inductor choice. the size requirements refer to the area and height requirements that are necessary to fit a particular design. please refer to the inductor dimensions on their datasheet. dc resistance is also important. while dcr is inversely proportional to size, dcr can represent a significant efficiency loss. refer to t he ?efficiency considerations? sub-section for a more detailed description. efficiency considerations efficiency is defined as the amount of useful output power, divided by the amount of power consumed. 100 iv iv % efficiency inin out out ? ? ? ? ? ? ? ? = maintaining high efficiency se rves two purposes. first, it decreases power dissipation in the power supply, reducing the need for heat sinks and thermal design considerations and it decreas es consumption of current for battery powered applications.
micrel, inc. mic22705 march 2011 15 m9999-033111-a reduced current demand from a battery increases the devices operating time, critical in hand held devices. there are mainly two loss terms in switching converters: static losses and switching losses. static losses are simply the power losses due to vi or i 2 r. for example, power is dissipated in the high-side switch during the on cycle. power loss is equal to the high-side mosfet rds (on) multiplied by the rms switch current squared (i sw 2 ). during the off-cycle, the low-side n-channel mosfet conducts, also dissipating power. similarly, the inductor?s dcr and capacitor?s esr also contribute to the i 2 r losses. device operating current also reduces efficiency by the product of the quiescent (operating) current and the supply voltage. the current required to drive the gates on and off at a constant 1mhz frequency and the switching transitions make up the switching losses. figure 2 illustrates an efficienc y curve. the portion, from 0a to 0.4a, efficiency losses are dominated by quiescent current losses, gate drive, transition and core losses. in this case, lower supply voltages yield greater efficiency in that they require less current to drive the mosfets and have reduced input power consumption. efficiency (v in = 3.3v) vs. output current 70 75 80 85 90 95 100 01234567 output current (a) efficiency (%) v in = 3.3v i out = 1.8v figure 2. efficiency curve the region, 1a to 7a, efficiency loss is dominated by mosfet rds (on) and inductor dc losses. higher input supply voltages will increase the gate-to-source voltage on the internal mosfets, thereby reducing the internal rds (on) . this improves efficiency by decreasing dc losses in the device. all but the inductor losses are inherent to the device. in which case, inductor selection becomes increasingly critical in efficiency calculations. as the inductors are reduced in size, the dc resistance (dcr) can become quite significant. the dcr losses can be calculated as follows: l pd = i out 2 dcr from that, the loss in efficiency due to inductor resistance can be calculated as follows: () 100 liv iv 1loss efficiency pd out out out out ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? + ?= efficiency loss due to dcr is minimal at light loads and gains significance as the load is increased. inductor selection becomes a trade-off between efficiency and size in this case. alternatively, under lighter l oads, the ripple current due to the inductance becomes a significant factor. when light load efficiencies become more critical, a larger inductor value maybe desired. larger inductances reduce the peak-to-peak inductor ripple current, which minimize losses. compensation the mic22705 has a combination of internal and external stability compensation to simplify the circuit for small, high-efficiency designs. in such designs, voltage mode conversion is often the optimum solution. voltage mode is achieved by creating an internal 1mhz ramp signal and using the output of the error amplifier to modulate the pulse width of the switch node, thereby maintaining output voltage regulation. with a typical gain bandwidth of 100khz ? 200khz, the mic22705 is capable of extremely fast transient responses. the mic22705 is designed to be stable with a typical application using a 1h inductor and a 100f ceramic (x5r) output capacitor. these values can be varied dependent upon the tradeoff between size, cost and efficiency, keeping the lc natural frequency ? ? ? ? ? ? ? ? cl 2 1 ideally less than 26 khz to ensure stability can be achieved. the minimum recommended inductor value is 0.47h and minimum recommended output capacitor value is 22f. the tradeoff between changing these values is that with a larger inductor, there is a reduced peak-to-peak current which yields a greater efficiency at lighter loads. a larger output capacitor will improve transient response by providing a larger hold up reservoir of energy to the output.
micrel, inc. mic22705 march 2011 16 m9999-033111-a the integration of one pole-zero pair within the control loop greatly simplifies compensation. the optimum values for c comp (in series with a 20k resistor) are shown below. c ? l 22f ? 47f 47f ? 100f 100f ? 470f 0.47h 0* ? 10pf 22pf 33pf 1h 0 ? ? 15pf 15 ? 22pf 33pf 2.2h 15 ? 33pf 33 ? 47pf 100 ? 220pf * vout > 1.2v, ? vout > 1v feedback the mic22705 provides a feedback pin to adjust the output voltage to the desired level. this pin connects internally to an error amplifier. the error amplifier then compares the voltage at the feedback to the internal 0.7v reference voltage and adjusts the output voltage to maintain regulation. the resistor divider network for a desired v out is given by: ? ? 1 ? 1 v v r1 r2 ref out where v ref is 0.7v and v out is the desired output voltage. a 10k ? or lower resistor value from the output to the feedback (r1) is recommended since large feedback resistor values increase the impedance at the feedback pin, making the feedback node more susceptible to noise pick-up. a small capacitor (50pf ? 100pf) across the lower resistor can reduce noise pick- up by providing a low impedance path to ground. enable/delay (en/dly) pin enable/delay (en/dly) source s 1a out of the ic to allow a startup delay to be implemented. the delay time is simply the time it takes 1a to charge c en/dly to 1.25v. therefore: 6 en/dly en/dly 101 c1.24 t u u rc pin (soft-start) the rc pin provides a trimmed 1a current source/sink for accurate ramp up (soft- start). this allows the mic22705 to be used in systems requiring voltage tracking or ratio-metric voltage tracking at startup. there are two ways of using the rc pin: 1. externally driven from a voltage source 2. externally attached capacitor sets output ramp up/down rate in the first case, driving rc with a voltage from 0v to v ref will program the output voltage between 0 and 100% of the nominal set voltage (as shown in figure 3). in the second case, the external capacitor sets the ramp up and ramp down time of the output voltage. the time is given by 6 rc ramp 101 c0.7 t u u where t ramp is the time from 0 to 100% nominal output voltage. during start-up, a light load condition (i out <1.25a) can lead to negative inductor current. under these conditions, the maximum ramp up time should not exceed the critical ramp up time period to keep regulator in continuous mode operation when v fb reaches 90% of reference voltage. beyond the critical ramp up time, the regulator is in discontinuous mode which leads to prolonged n-channel fet conduction, which in turn causes negative inductor current. the maximum c rc value is calculated as follows: ? ? 1 ? uuuu in out 6 sw out rc v v 1 10flc2.86 c
micrel, inc. mic22705 march 2011 17 m9999-033111-a pre-bias start-up the mic22705 is designed to start-up into a pre-biased output. this prevents large negative inductor currents and excessive output voltage oscillations. the mic22705 starts with the low-side mosfet turned off, preventing reverse inductor current flow. the synchronous mosfet stays off until the power good (pg) goes high after the v fb is above 90% of v ref . a pre-bias condition can occur if the input is turned off then immediately turned back on before the output capacitor is discharged to ground. it is also possible that the output of the mic22705 could be pulled up or pre- biased through parasitic conduction paths from one supply rail to another in multiple voltage (v out ) level ics such as a fpga. figure 3 shows a normal start-up waveform. a 1a current source charges the soft-start capacitor c rc . the c rc capacitor forces the v rc voltage to come up slowly (v rc trace), thereby providing a soft-start ramp. this ramp is used to control the internal reference (v ref ). the error amplifier forces the output voltage to follow the v ref ramp from zero to the final value. figure 3. en turn-on time ? normal start-up if the output is pre-biased to a voltage above the expected value, as shown in figure 4, then neither mosfet will turn on until the ramp control voltage (v rc ) is above the reference voltage (v ref ). then, the high- side mosfet starts switching, forcing the output to follow the v rc ramp. once the feedback voltage is above 90% of the reference voltage, the low-side mosfet will begin switching. figure 4. en turn-on at 1v pre-bias when the mic22705 is turned off, the low-side mosfet will be disabled and the output voltage will decay to zero. during this time, the ramp control voltage (v rc ) will still control the output voltage fall-time with the high-side mosfet if the output voltage falls faster than the v rc voltage. figure 5 shows this operating condition. here a 7a load pulls the output down fast enough to force the high-side mosfet on (v sw trace). figure 5. en turn-off ? 7a load if the output voltage falls slower than the v rc voltage, then the both mosfets will be off and the output will decay to zero as shown in the v out trace in figure 6. with both mosfets off, any resistive load connected to the output will help pull down the output voltage. this will occur at a rate determined by the resistance of the load and the output capacitance.
micrel, inc. mic22705 march 2011 18 m9999-033111-a figure 6. en turn-off ? 200ma load current limit the mic22705 is protected against overload in two stages. the first is to limit the current in the p-channel switch; the second is over temperature shutdown. current is limited by measuring the current through the high-side mosfet during its power stroke and immediately switching off the driver when the preset limit is exceeded. the circuit in figure 7 describes the operation of the current limit circuit. since the actual rds on of the p- channel mosfet varies part-to-part, over temperature and with input voltage, simple ir voltage detection is not employed. instead, a smaller copy of the power mosfet (reference fet) is fed with a constant current which is a directly proportional to the factory set current limit. this sets the current limit as a current ratio and thus, is not dependant upon the rds on value. ? current limit is set to nominal value. variations in the scale factor k between the power pfet and the reference pfet used to generate the limit threshold account for a relatively small inaccuracy. figure 7. current-limit detail thermal considerations the mic22705 is packaged in a mlf ? 4mm x 4mm ? a package that has excellent thermal-performance equaling that of the larger tssop packages. this maximizes heat transfer from the junction to the exposed pad (epad) which connects to the ground plane. the size of the ground plane attached to the exposed pad determines the overall thermal resistance from the junction to the ambient air surrounding the printed circuit board. the junction temperature for a given ambient temperature can be calculated using: t j = t amb + p diss r ja where: ? p diss is the power dissipated within the mlf ? package and is at 7a load. r ja is a combination of junction-to-case thermal resistance (r jc ) and case-to-ambient thermal resistance (r ca ), since thermal resistance of the solder connection from the epad to the pcb is negligible; r ca is the thermal resistance of the ground plane-to-ambient, so r ja = r jc + r ca . ? t amb is the operating ambient temperature. example: the evaluation board has two copper planes contributing to an r ja of approximately 25c/w. the worst case r jc of the mlf ? 4mm x 4mm is 14 o c/w. r ja = r jc + r ca r ja = 14 + 25 = 39 c/w to calculate the junction temperature for a 50c ambient: t j = t amb + p diss r ja t j + 50 + (1.8 39) t j = 120 c
micrel, inc. mic22705 march 2011 19 m9999-033111-a thermal measurements measuring the ic?s case temperature is recommended to ensure it is within its operat ing limits. although this might seem like a very elementary task, it is easy to get erroneous results. the most common mistake is to use the standard thermal couple that comes with a thermal meter. this thermal couple wire gauge is large, typically 22 gauge, and behaves like a heatsink, resulting in a lower case measurement. two methods of temperature measurement are using a smaller thermal couple wire or an infrared thermometer. if a thermal couple wire is used, it must be constructed of 36 gauge wire or higher then (smaller wire size) to minimize the wire heat-sinking effect. in addition, the thermal couple tip must be covered in either thermal grease or thermal glue to make sure that the thermal couple junction is making good contact with the case of the ic. omega brand thermal couple (5sc-tt-k-36-36) is adequate for most applications. whenever possible, an infrared thermometer is recommended. the measurement spot size of most infrared thermometers is too large for an accurate reading on a small form factor ics. however, a ir thermometer from optris has a 1mm spot size, which makes it a good choice for measuring the hottest point on the case. an optional stand makes it easy to hold the beam on the ic for long periods of time. sequencing and tracking there are four variations which are easily implemented using the mic22705. the two sequencing variations are delayed and windowed. the two tracking variants are normal and ratio metric. the following diagrams illustrate methods for connecting two mic22705?s to achieve these requirements.
micrel, inc. mic22705 march 2011 20 m9999-033111-a window sequencing: time (4.0ms/div) delayed sequencing: time (4.0ms/div)
micrel, inc. mic22705 march 2011 21 m9999-033111-a ratio metric tracking: normal tracking: time (4.0ms/div) time (4.0ms/div)
micrel, inc. mic22705 march 2011 22 m9999-033111-a 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 mic22705 converter: ic ? the 2.2f ceramic capacitor, which is connected to the svin pin, must be located right at the ic. the svin pin is very noise sensitive and placement of the capacitor is very critical. use wide traces to connect to the svin and sgnd pins. ? the signal ground pin (sgnd) must be connected directly to the ground planes. do not route the sgnd pin to the pgnd pad on the top layer. ? place the ic 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. input capacitor ? a 22f x5r or x7r dielectrics ceramic capacitor is recommended on each of the pvin pins for bypassing. ? place the input capacitors on the same side of the board and as close to the ic as possible. ? keep both the pvin pin and pgnd connections short. ? place several vias to the ground plane close to the 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. 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. ? to minimize noise, place a ground plane underneath the inductor. ? the inductor can be placed on the opposite side of the pcb with respect to the ic. it does not matter whether the ic or inductor is on the top or bottom as long as there is enough air flow to keep the power components within their temperature limits. the input and output capacitors must be placed on the same side of the board as the ic. 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 divider network must be place close to the ic with the bottom of r2 connected to sgnd. ? the feedback trace should be separate from the power trace and connected as close as possible to the output capacitor. sens ing a long high-current load trace can degrade the dc load regulation. rc snubber ? place the rc snubber on either side of the board and as close to the sw pin as possible.
micrel, inc. mic22705 march 2011 23 m9999-033111-a evaluation board schematic bill of materials item part number manufacturer description qty. c2012x5r0j226m tdk (1) 08056d226mat avx (2) c1, c2, c3, c4 grm21br60j226me39l murata (3) 22f/6.3v, 0805, ceramic capacitor 5 06036d225taat2a avx (2) grm188r7160j225m murata (3) c5 c1608x5r0j225m tdk (1) 2.2f/6.3v, ceramic capaci tor, x5r, size 0805 1 c13 grm188r71h103ka01d murata (3) 10nf, 0603, ceramic capacitor 1 open(06035c102kat2a) avx (2) open(grm188r71h102ka01d) murata (3) 1nf/50v, x7r, 0603, ceramic capacitor c6 open(c1608c0g1h102j) tdk (1) 1nf/50v, cog, 0603, ceramic capacitor 1 06035c471kat2a avx (2) grm188r71h471ka01d murata (3) c7 c1608x7rh471k tdk (1) 470pf/50v, x7r, 0603, ceramic capacitor grm1555c1h390jz01d murata (3) c9 c1005cog1h390j tdk (1) 39pf/50v, cog, 0402, ceramic capacitor 1 c3216x5r0j476m tdk (1) 47f/6.3v, x5r, 1206, ceramic capacitor grm31cr60j476me19 murata (3) 47f/6.3v, x5r, 1206, ceramic capacitor c10, c11 grm31cc80g476me19l murata (3) 47f/4v, x6s, 1206, ceramic capacitor 2
micrel, inc. mic22705 march 2011 24 m9999-033111-a bill of materials (continued) item part number manufacturer description qty. c1608c0g1h101j tdk (1) c12 grm1555c1h101jz01d murata (3) 100pf/50v, cog, 0402, ceramic capacitor 1 spm6530t-1r0m120 tdk (1) 1h, 12a, size 7x6.5x3mm l1 hcp0704-1r0-r coiltronics (5) 1h, 12a, size 6.8x6.8x4.2mm 1 c in ba1851a3477m epcos (6) 470f/10v, elect., 8 11.5 1 r1 crcw06031101fkeye3 vishay (4) resistor, 1.1k, 0603, 1% 1 r2 crcw04026980fkeye3 vishay (4) resistor, 698 ? , 0603, 1% 1 r3 crcw06034752fkeye3 vishay (4) resistor, 47.5k, 0603, 1% 1 r4 crcw04022002fkeye3 vishay (4) resistor, 20k, 0402, 1% 1 r5 open(crcw06031003frt1) vishay (4) resistor, 100k, 0603, 1% 1 r6 crcw060349r9fkea vishay (4) 49.9 ? resistor, 1%, size 0603 1 r7 crcw06032r20fkea vishay (4) 2.2 ? resistor, 1%, size 0603 1 open(2n7002e) vishay (4) q1 open(cmdpm7002a) central semiconductor (7) signal mosfet ? sot23-6 1 u1 mic22705yml micrel, inc. (8) 1mhz, 7a integrated switch high-efficiency synchronous buck regulator 1 notes: 1. tdk: www.tdk.com . 2. avx.: www.avx.com . 3. murata: www.murata.com . 4. vishay tel: www.vishay.com . 5. coiltronics: www.coiltronics.com . 6. epcos: www.epcos.com . 7. central semiconductor: www.centralsemi.com . 8. micrel, inc.: www.micrel.com .
micrel, inc. mic22705 march 2011 25 m9999-033111-a pcb layout recommendations mic22705 evaluation board top layer mic22705 evaluation board top silk
micrel, inc. mic22705 march 2011 26 m9999-033111-a pcb layout recommendations (continued) mic22705 evaluation board mid-layer 1 (ground plane) mic22705 evaluation board mid-layer 2
micrel, inc. mic22705 march 2011 27 m9999-033111-a pcb layout recommendations (continued) mic22705 evaluation board bottom layer mic22705 evaluation board bottom silk
micrel, inc. mic22705 march 2011 28 m9999-033111-a package information 24-pin 4mm 4mm mlf ? (ml)
micrel, inc. mic22705 march 2011 29 m9999-033111-a recommended landing pattern red circle indicates thermal via. size should be .300mm ? .350mm in diameter, 1.00mm pitch, and it should be connected to gnd plane for maximum thermal performance. green rectangle (with shaded area) indicates solder stencil opening on exposed pad area. size should be 1.00mm 1.00mm in size, 1.20mm pitch. 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 the 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 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 impla 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. can nt ? 2010 micrel, incorporated.

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