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1 2a synchronous buck regulator with integrated mosfets isl8502a the isl8502a is a synchronous buck controller with internal mosfets packaged in a small 4mmx4mm qfn package. the isl8502a can support a continuous load of 2a and has a very wide input voltage range. with the switching mosfets integrated into the ic, the complete regulator footprint can be very small and provide a much mo re efficient solution than a linear regulator. the isl8502a is capable of stand- alone operation or it can be used in a master slave combination for multiple outputs that are derived from the same input rail. multiple slave channels (up to six) can be synchroniz ed. this method minimizes the emi and beat frequencies effect with multi-channel operation. the switching pwm controller drives two internal n-channel mosfets in a synchronous-rectified buck converter topology. the synchronous buck converter uses voltage-mode control with fast transient response. th e switching regulator provides a maximum static regulation tolerance of 1% over line, load, and temperature ranges. the output is user-adjustable by means of external resistors down to 0.6v. the output is monitored for undervoltage events. the switching regulator also has overcurrent protection. thermal shutdown is integrated. the isl8502a features a bi-directional enable pin that allows the part to pull the enable pin low during fault detection. pgood delay for isl8502a has be en decreased to 1ms typical (at 500khz switching frequency) compared to 250ms (at 500khz) for isl8502. features ? up to 2a continuous output current ? integrated mosfets for small regulator footprint ? adjustable switching frequency, 500khz to 1.2mhz ? tight output voltage regulation, 1% over-temperature ? wide input voltage range, 5v 10% or 5.5v to 14v ? wide output voltage range, from 0.6v ? simple single-loop voltage-mode pwm control design ? input voltage feed-forward for constant modulator gain ? fast pwm converter transient response ? lossless r ds(on) high side and low side overcurrent protections ? undervoltage detection ? integrated thermal shutdown protection ? power-good indication ? adjustable soft-start ? start-up with pre-bias output ? pb-free (rohs compliant) applications ? point of load applications ? graphics cards - gpu and memory supplies ? asic power supplies ? embedded processor and i/o supplies ? dsp supplies figure 1. stand-alone regulator: v in 5.5v to 14v figure 2. stand-alone regulator: v in 4.5v to 5.5v v out + isl8502a pvcc sgnd synch boot vin phase pgnd m/s fs pgood en ss fb comp + v in enable power good 5.5v to 14v vcc v out + isl8502a pvcc sgnd synch boot vin phase pgnd m/s fs pgood en ss fb comp + v in enable power good 4.5v to 5.5v vcc caution: these devices are sensitive to electrostatic discharge; follow proper ic handling procedures. 1-888-intersil or 1-888-468-3774 | copyright intersil americas inc. 2011. all rights reserved intersil (and design) is a trademark owned by intersil corporation or one of its subsidiaries. all other trademarks mentioned are the property of their respective owners. october 21, 2011 fn7940.0
isl8502a 2 fn7940.0 october 21, 2011 block diagram gate drive and adaptive shoot thru protection pgood ss fb boot phase (x4) oc monitor synch m/s sgnd en fs comp pvcc clock and oscillator generator vin series regulator 0.6v reference pvcc voltage monitor vin (x4) pvcc pgnd (x4) oc monitor 30 a vcc bias fault monitoring por monitor
isl8502a 3 fn7940.0 october 21, 2011 pin configuration* isl8502a (24 ld qfn) top view *see ?functional pin descriptions? beginning on page 13 for pin descriptions. 1 2 3 4 5 6 18 17 16 15 14 13 24 23 22 21 20 19 789101112 vin phase phase phase phase pgnd pgood sgnd en synch m/s fs comp fb ss pgnd pgnd pgnd vcc pvcc boot vin vin vin gnd 25 ordering information part number (note 2) part marking temp. range (c) package (pb-free) pkg. dwg. # isl8502airz (notes 1, 3) 85 02airz -40 to +85 24 ld 4x4 qfn l24.4x4d ISL8502AEVAL1Z evaluation board notes: 1. add ?-t*? suffix for tape and reel. please refer to tb347 for details on reel specifications. 2. these intersil pb-free plastic packaged products employ spec ial pb-free material sets, molding compounds/die attach materials , and 100% matte tin plate plus anneal (e3 termination finish , which is rohs compliant and compatible wi th both snpb and pb-free soldering opera tions). intersil pb-free products are msl classified at pb-fr ee peak reflow temperatures that meet or exceed the pb-free requirements of ipc/jed ec j std-020. 3. for moisture sensitivity level (msl), please see device information page for isl8502a . for more information on msl please see tech brief tb363 .
isl8502a 4 fn7940.0 october 21, 2011 typical application schematics v out + isl8502a pvcc sgnd synch boot vin phase pgnd m/s fs pgood en ss fb comp + v in enable power good 5.5v to 14v vcc figure 3. stand-alone regulator: v in 5.5v to 14v v out + isl8502a pvcc sgnd synch boot vin phase pgnd m/s fs pgood en ss fb comp + v in enable power good 4.5v to 5.5v vcc figure 4. stand-alone regulator: v in 4.5v to 5.5v
isl8502a 5 fn7940.0 october 21, 2011 isl8502a with multiple slaved channels isl8502a en fs m/s synch pvcc gnd master slave phase vin + v out1 v in r t isl8502a en fs m/s synch gnd phase vin + v out2 r t 5k slave isl8502a en fs m/s synch gnd phase vin + v outn r t 5k enable ss
isl8502a 6 fn7940.0 october 21, 2011 absolute maximum rating s thermal information vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . gnd - 0.3v to +16.5v vcc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . gnd - 0.3v to +6.0v absolute boot voltage, v boot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +22.0v upper driver supply voltage, v boot - v phase . . . . . . . . . . . . . . . . . . . +6.0v all other pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . gnd - 0.3v to vcc + 0.3v recommended operating conditions supply voltage on vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5v to 14v ambient temperature range . . . . . . . . . . . . . . . . . . . . . . . -40c to +85c junction temperature range . . . . . . . . . . . . . . . . . . . . . . -40c to +125c thermal resistance ja (c/w) jc (c/w) qfn package (notes 4, 5) . . . . . . . . . . . . . 38 2 maximum junction temperature (plastic package) . . . . . . . . . . . +150c maximum storage temperature range . . . . . . . . . . . . . -65c to +150c pb-free reflow profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . see link below http://www.intersil.com/pbfree/pb-freereflow.asp caution: do not operate at or near the maximum ratings listed for extended periods of time. exposure to such conditions may adv ersely impact product reliability and result in failures not covered by warranty. notes: 4. ja is measured in free air with the compon ent mounted on a high effective thermal conductivity test board with ?direct attach? fe atures. see tech brief tb379 . 5. for jc , the ?case temp? location is the center of the exposed metal pad on the package underside. electrical specifications refer to ?block diagram?and ?typical application schematics?. operating conditions unless otherwise noted: v in = 12v, or v cc = 5v 10%, t a = -40c to +85c. typical are at t a = +25c. boldface limits apply over the operating temperature range, -40c to +85c parameter symbol test conditions min (note 6) typ max (note 6) units v in supply input voltage range v in 5.5 (note 7) 14 (note 8) v v in tied to v cc 4.5 5.5 v input operating supply current i q v fb = 1.0v 7 ma input standby supply current iq_sby en tied to gnd, v in = 14v 1.25 2 ma series regulator vcc voltage v pvcc v in > 5.6v 4.5 5.0 5.5 v maximum output current i pvcc v in = 12v 50 ma vcc current limit v in = 12v, vcc shorted to pgnd 300 ma power-on reset rising vcc por threshold 4.2 4.4 4.49 v falling vcc por threshold 3.85 4.0 4.10 v enable rising enable threshold voltage v en_rising 2.7 v falling enable threshold voltage v en_fall 2.3 v enable sinking current i en 500 a oscillator pwm frequency f osc r t = 96k 400 500 600 khz r t = 40k 960 1200 1440 khz fs pin tied to vcc 800 khz ramp amplitude v osc v in = 14v 1.0 v ramp amplitude v osc v in = 5v 0.470 v modulator gain v vin / v osc by design 8 - maximum duty cycle d max f osc = 500khz 88 % maximum duty cycle d max f osc = 1.2mhz 76 % reference voltage reference voltage v ref 0.600 v
isl8502a 7 fn7940.0 october 21, 2011 system accuracy -1.0 +1.0 % fb pin bias current 80 200 na soft-start soft-start current i ss 20 30 40 a enable soft-start threshold 0.8 1.0 1.2 v enable soft-start threshold hysteresis 12 mv enable soft-start voltage high 2.8 3.2 3.8 v error amplifier dc gain 88 db gain-bandwidth product gbwp 15 mhz maximum output voltage 3.9 4.4 v slew rate sr 5 v/s internal mosfets upper mosfet r ds(on) r ds_upper v cc = 5v 180 m lower mosfet r ds(on) r ds_lower v cc = 5v 90 m pgood pgood threshold v fb/ v ref rising edge hysteresis 1% 107 111 115 % falling edge hysteresis 1% 86 90 93 % pgood rising delay (note 11) t pgood_delay f osc = 500khz 1 ms pgood leakage current v pgood = 5.5v 5 a pgood low voltage v pgood 0.10 v pgood sinking current i pgood 0.5 ma protection positive current limit i poc_peak ioc from v in to phase (notes 9, 10) (t a = 0c to +85c) 2.1 3.5 4.5 a ioc from v in to phase (notes 9, 10) (t a = -40c to +0c) 2.0 3.4 4.0 a negative current limit i noc_peak ioc from phase to pgnd (notes 9, 10) (t a = 0c to +85c) 2.2 3.0 3.5 a ioc from phase to pgnd (notes 9, 10) (t a = -40c to +85c) 1.9 2.8 3.7 a undervoltage level v fb /v ref 76 80 84 % thermal shutdown setpoint t sd 150 c thermal recovery setpoint t sr 130 c notes: 6. parameters with min and/or max limits are 100% tested at +25c, unless otherwise specified. temperature limits established by characterization and are not production tested. 7. minimum v in can operate below 5.5v as long as vcc is greater than 4.5v. 8. maximum v in can be higher than 14v voltage stress across the u pper and lower do not exceed 15.5v in all conditions. 9. circuit requires 150ns minimum on ti me to detect overcurrent condition. 10. limits established by characterization and are not production tested. 11. pgood rising delay is measured from the point where v out reaches regulation to the point where pgood rises. it does not include the external soft-start time. the pgood rising delay specification is measured at 500khz. electrical specifications refer to ?block diagram?and ?typical application schematics?. operating conditions unless otherwise noted: v in = 12v, or v cc = 5v 10%, t a = -40c to +85c. typical are at t a = +25c. boldface limits apply over the operating temperature range, -40c to +85c parameter symbol test conditions min (note 6) typ max (note 6) units
isl8502a 8 fn7940.0 october 21, 2011 typical performance curves v in = 12v, v out = 2.5v, i o = 2a, fs = 500khz, l = 4.7h, c in = 20f, c out = 100f + 22f, t a = +25 c, unless otherwise noted. figure 5. efficiency vs load (v in = 5v) figure 6. efficiency vs load (v in = 12v) figure 7. v out regulation vs load (v out = 0.6v, 500khz) figure 8. v out regulation vs load (v out = 1.2v, 500khz) figure 9. v out regulation vs load (v out = 1.5v, 500khz) figure 10. v out regulation vs load (v out = 1.8v, 500khz) 40 50 60 70 80 90 100 0.0 0.5 1.0 1.5 2.0 2.5 output load (a) efficiency (%) v out = 2.5v v out = 3.3v v out = 1.8v 40 50 60 70 80 90 100 0.00.51.01.52.02.5 output load (a) efficiency (%) v out = 1.8v v out = 2.5v v out = 5.0v v out = 3.3v 0.6019 0.6020 0.6021 0.6022 0.6023 0.6024 0.6025 0.6026 012 output load (a) output voltage (v) 14v in 9v in 5v in 1.200 1.201 1.202 1.203 1.204 1.205 1.206 012 output load (a) output voltage (v) 9v in 5v in 14v in 1.510 1.512 1.514 1.516 1.518 1.520 012 output load (a) output voltage (v) 5v in 9v in 14v in 1.810 1.811 1.811 1.812 1.812 1.813 1.813 1.814 1.814 1.815 1.815 01 2 output load (a) output voltage (v) 14v in 9v in 5v in
isl8502a 9 fn7940.0 october 21, 2011 figure 11. v out regulation vs load (v out = 2.5v, 500khz) figure 12. v out regulation vs load (v out = 3.3v, 500khz) figure 13. v out regulation vs load (v out = 5v, 500khz) figure 14. power dissipation vs load (v out = 0.6v, 500khz) figure 15. power dissipation vs load (v out = 1.2v, 500khz) figure 16. power dissipation vs load (v out = 1.5v, 500khz) typical performance curves v in = 12v, v out = 2.5v, i o = 2a, fs = 500khz, l = 4.7h, c in = 20f, c out = 100f + 22f, t a = +25 c, unless otherwise noted. (continued) 2.505 2.507 2.509 2.511 2.513 2.515 012 output load (a) output voltage (v) 14v in 5v in 9v in 3.345 3.346 3.347 3.348 3.349 3.350 3.351 3.352 3.353 3.354 3.355 012 output load (a) output voltage (v) 9v in 14v in 5v in 5.020 5.022 5.024 5.026 5.028 5.030 output load (a) output voltage (v) 012 7v in 14v in 9v in 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 012 output load (a) power dissipation (w) 5v in 9v in 14v in 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 output load (a) power dissipation (w) 012 5v in 9v in 14v in 0.0 0.5 1.0 1.5 2.0 2.5 01 2 output load (a) power dissipation (w) 14v in 9v in 5v in
isl8502a 10 fn7940.0 october 21, 2011 figure 17. power dissipation vs load (v out = 1.8v, 500khz) figure 18. power dissipation vs load (vout = 2.5v, 500khz) figure 19. power dissipation vs load (v out = 3.3v, 500khz) figure 20. power dissipation vs load (v out = 5v, 500khz) figure 21. v cc load regulation figure 22. v cc regulation vs v in typical performance curves v in = 12v, v out = 2.5v, i o = 2a, fs = 500khz, l = 4.7h, c in = 20f, c out = 100f + 22f, t a = +25 c, unless otherwise noted. (continued) 0.0 0.5 1.0 1.5 2.0 2.5 012 output load (a) power dissipation (w) 14v in 9v in 5v in 0.0 0.5 1.0 1.5 2.0 2.5 01 2 output load (a) power dissipation (w) 14v in 9v in 5v in 0.0 0.5 1.0 1.5 2.0 2.5 01 2 output load (a) power dissipation (w) 5v in 9v in 14v in 0.0 0.5 1.0 1.5 2.0 2.5 01 2 output load (a) power dissipation (w) 9v in 7v in 14v in 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 0 50 100 150 200 250 300 i v cc (ma) vcc (v) 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 5.5 3 5 7 9 10 11 12 13 14 15 v in (v) vcc (v) 468 100ma load no load
isl8502a 11 fn7940.0 october 21, 2011 figure 23. master to slave operation figure 24. master operation at no load figure 25. master operation with full load fi gure 26. master operation with negative load figure 27. soft-start at no load figure 28. start-up with pre-biased typical performance curves v in = 12v, v out = 2.5v, i o = 2a, fs = 500khz, l = 4.7h, c in = 20f, c out = 100f + 22f, t a = +25 c, unless otherwise noted. (continued) v out1 ripple 20mv/div v out2 ripple 20mv/div 0.5s 5v phase1 5v/div phase2 5v/div phase1 5v/div v out1 ripple 20mv/div synch1 2v/div il1 0.5a/div phase1 5v/div vout1 ripple il1 1a/div synch1 5v/div 20mv/div phase1 10v/div vout1 ripple 20mv/div il1 1a/div synch1 5v/div en1 5v/div v out1 1v/div il1 2a/div ss1 2v/div en1 5v/div vout1 0.5v/div il1 1a/div ss1 2v/div 2v pre-biased
isl8502a 12 fn7940.0 october 21, 2011 figure 29. soft-start at full load figure 30. positive output short circuit figure 31. positive output short circuit (hiccup mode) figure 32. negative output short circuit figure 33. recover from positive short circuit figure 34. load transient typical performance curves v in = 12v, v out = 2.5v, i o = 2a, fs = 500khz, l = 4.7h, c in = 20f, c out = 100f + 22f, t a = +25 c, unless otherwise noted. (continued) en1 5v/div vout1 1v/div il1 1a/div ss1 2v/div phase1 10v/div v out1 1v/div il1 1a/div pgood1 5v/div phase1 10v/div ss1 2v/div il1 2a/div v out1 2v/div phase1 10v/div pgood1 5v/div il1 2a/div v out1 2v/div phase1 10v/div vout1 1v/div il1 1a/div pgood1 5v/div phase1 5v/div iout1 2a/div il1 2a/div vout1 ripple 50mv/div
isl8502a 13 fn7940.0 october 21, 2011 functional pin descriptions pgood (pin 1) pgood is an open drain output th at pulls to low if the output goes out of regulation or a faul t is detected. pgood is equipped with a fixed delay upon output power-up. the pgood rising delay specification is measured at 500 khz from the point where v out reaches regulation to the point where pgood rises. this delay is reversely proportional to the switching frequency. sgnd (pin 2) the sgnd terminal of the isl8502a provides the return path for the control and monitor portions of the ic. en (pin 3) the enable pin is a bi-directional pin. if the voltage on this pin exceeds the enable threshold voltage, the part is enabled. if a fault is detected, the en pin is pulled low via internal circuitry for a duration of four soft-start periods. for automatic start-up, use 10k to 100k pull-up resistor connecting to vcc. synch (pin 4) synch is a bi-directional pin used to synchronize slave devices to the master device. as a master device, this pin outputs the clock signal to which the slave devices synchronize. as a slave device, this pin is an input to receive the clock signal from the master device. if configured as a slave device, the isl8502a is disabled if there is no clock signal from the master device on the synch pin. leave this pin unconnected if the ic is used in stand-alone operation. m/s (pin 5) as a slave device, tie a 5k resistor between the m/s pin and ground. as a master or a stand-alone device, tie the m/s pin directly to the vcc pin. do not short the m/s pin to gnd. fs (pin 6) the fs pin provides oscillator switching frequency adjustment. by placing a resistor (r t ) from the fs pin to gnd, the switching frequency can be programmed as desired between 500khz and 1.2mhz as shown in equation 1. tying the fs pin to the vcc pin forces the switching frequency to 800khz. using resistors with values below 40k (1.2mhz) or with values higher than 97k (500khz) may damage the isl8502a. comp (pin 7) and fb (pin 8) the switching regulator employs a single voltage control loop. the fb pin is the negative input to the voltage loop error amplifier. the output voltage is se t by an external resistor divider connected to fb. with a properly selected divider, the output voltage can be set to any vo ltage between the power rail (reduced by converter losses) and the 0.6v reference. loop compensation is achieved by co nnecting an ac network across the comp pin and the fb pin. the fb pin is also monitored for undervoltage events. ss (pin 9) connect a capacitor from the ss pin to ground. this capacitor, along with an internal 30a curre nt source, sets the soft-start interval of the converter, t ss , as shown in equation 2. pgnd (pins 10-13) the pgnd pins are used as the ground connection of the power train. phase (pins 14-17) the phase pins are the phase node connections to the inductor. these pins are connected to the source of the control mosfet and the drain of the synchronous mosfet. vin (pins 18-21) connect the input rail to the vin pins. these pins are the input to the regulator as well as the source for the internal linear regulator that supplies the bias for the ic. it is recommended that the dc voltage applied to the vin pins does not exceed 14v. this reco mmendation allows for transient spikes and voltage ringing to occur while not exceeding absolute maximum ratings. boot (pin 22) the boot pin provides ground-referenced bias voltage to the upper mosfet driver. a bootstrap circuit is used to create a voltage suitable to drive the internal n-channel mosfet. the boot diode is included within the isl8502a. pvcc (pin 23) the pvcc pin is the output of th e internal linear regulator that supplies the bias and gate voltage for the ic. a minimum 4.7f decoupling capacitor is recommended. vcc (pin 24) the vcc pin supplies the bias voltage for the ic. this pin should be tied to the pvcc pin through an rc low pass filter. a 10 resistor and 0.1f capacitor are recommended. functional description initialization the isl8502a automatically initializes upon receipt of input power. the power-on reset (por) function continuously monitors the voltage on the vcc pin. if the voltage on the en pin exceeds its rising threshold, then the po r function initiates soft-start operation after the bias voltage has exceeded the por threshold. r t k [] 48000 f osc khz [] ------------------------------ = (eq. 1) c ss f [] 50 t ss s [] ? = (eq. 2)
isl8502a 14 fn7940.0 october 21, 2011 stand-alone operation the isl8502a can be configured to function as a stand-alone single channel voltage mode synchronous buck pwm voltage regulator. the ?typical application schematics? on page 4 show the two configurations for stand-alone operation. the internal series linear regulator requires at least 5.5v to create the proper bias for the ic. if the input voltage is between 5.5v and 15v, simply connect the vin pins to the input rail, and the series linear regulator creates the bias for the ic. the vcc pin should be tied to a capacitor for decoupling. if the input voltage is 5v 10%, then tie the vin pins and the vcc pin to the input rail. the isl8502a uses the 5v rail as the bias. a decoupling capacitor should be placed as close as possible to the vcc pin. multi-channel (master/slave) operation the isl8502a can be configured to function in a multi-channel system. ?isl8502a with multiple slaved channels? on page 5 shows a typical configuration for the multi-channel system. in the multi-channel system, each isl8502a ic regulates a separate rail while sharing the same input rail. by configuring the devices in a master/slave configuration, the clocks of each ic can be synchronized. there can only be one master ic in a multi-channel system. to configure an ic as the master, the m/s pin must be shorted to the vcc pin. the synch pins of a ll the isl8502a controller ics in the multi-channel system must be tied together. the frequency set resistor value (r t ) used on the master device must be used on every slave device. each slave device must have a 5k resistor connecting it fr om m/s pin to ground. the master device and all slave devices can have their en pins tied to an enable ?bus.? since the en pin is bi-directional, it allows for options on how each ic is tied to the enable bus. if the en pin of any isl8502a is tied directly to the enable bus, then that device is capable of disabling all the other devices that have their en pins tied directly to the enable bus. if the en pin of an isl8502a is tied to the enable bus through a diode (ano de tied to isl8502a en pin, cathode tied to enable bus), then the part does not disable other devices on the enable bus if it disables itself for any reason. if the master device is disabled via the en pin, it continues to send the clock signal from the synch pin. this allows slave devices to continue operating. fault protection the isl8502a monitors the output of the regulator for overcurrent and undervoltage events. the isl8502a also provides protection from excessive junction temperatures. overcurrent protection the overcurrent function protects the switching converter from a shorted output by monitoring the current flowing through both the upper and lower mosfets. upon detection of any overcurrent condition, the upper mosfet is immediately turned off and is not turned on again until the next switching cycle. upon detection of the initial overcurrent condition, the overcurrent fault counter is set to 1, and the overcurrent condition flag is set from low to high. if, on the subsequent cycle, another overcu rrent condition is detected, the oc fault counter is incremented. if there are eight sequential oc fault detections, the regulator is shut down under an overcurrent fault condition, and the en pin is pulled low. an overcurrent fault condition results, with the re gulator attempting to restart in hiccup mode. the delay between restarts is four soft-start periods. at the end of the fourth soft-start wait period, the fault counters are reset, the en pin is released, and soft-start is attempted again. if the overcurren t condition goes away prior to the oc fault counter reaching a count of four, the overcurrent condition flag is set back to low. if the overcurrent condition flag is high, the overcurrent fault counter is less than four, and an undervoltage event is detected, the regulator shuts down immediately. undervoltage protection if the voltage detected on the fb pin falls 18% below the internal reference voltage, and if the over current condition flag is low, then the regulator is shut down immediately under an undervoltage fault condition, and the en pin is pulled low. an undervoltage fault condition resu lts in the regulator attempting to restart in hiccup mode, with the delay between restarts being four soft-start periods. at the end of the fourth soft-start wait period, the fault counters are reset, the en pin is released, and soft-start is attempted again. thermal protection if the isl8502a ic junction temperature reaches a nominal temperature of +150c, the regulator is disabled. the isl8502a does not re-enable the regulator until the junction temperature drops below +130c. shoot-through protection a shoot-through condition occurs when both the upper and lower mosfets are turned on simultaneously, effectively shorting the input voltage to ground. to protect from a shoot-through condition, the isl8502a incorporates specialized circuitry, which ensures that the complementary mosfets are not on simultaneously. application guidelines operating frequency the isl8502a can operate at switching frequencies from 500khz to 1.2mhz. a resistor tied from the fs pin to ground is used to program the switching frequency (equation 3). output voltage selection the output voltage of the regula tor can be programmed via an external resistor divider that is used to scale the output voltage relative to the internal reference voltage and feed it back to the inverting input of the error amplifier (see figure 36). the output voltage programming resistor, r 4 , depends on the value chosen for the feedback resistor and the desired output r t k [] 48000 f osc khz [] ------------------------------ = (eq. 3)
isl8502a 15 fn7940.0 october 21, 2011 voltage of the regulator. the value for the feedback resistor is typically between 1k and 10k . if the output voltage desired is 0.6v, then r 4 is left unpopulated. output capacitor selection an output capacitor is required to filter the inductor current and supply the load transient current. the filtering requirements are a function of the switching frequency and the ripple current. the load transient requirements are a function of the slew rate (di/dt) and the magnitude of the transient load current. these requirements are generally met with a mix of capacitors and careful layout. high frequency capacitors initially supply the transient and slow the current load rate seen by the bulk capacitors. the bulk filter capacitor values are generally determined by the esr (effective series resistance) and voltage rating requirements rather than actual capacitance requirements. high frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible. be careful not to add inductance in the circuit boar d wiring that could cancel the usefulness of these low inductance components. consult with the manufacturer of the load on specific decoupling requirements. the shape of the output voltage wa veform during a load transient that represents the worst-case loading conditions ultimately determines the number of output capacitors and their type. when this load transient is applied to the converter, most of the energy required by the load is initially delivered from the output capacitors. this is due to the finite amount of time required for the inductor current to slew up to the level of the output current required by the load. this phenomenon results in a temporary dip in the output voltage. at the very edge of the transient, the equivalent series inductance (esl) of each capacitor induces a spike that adds on top of the existing voltage drop due to the equivalent series resistance (esr). after the initial spike, attribut able to the esr and esl of the capacitors, the output voltage experiences sag. this sag is a direct consequence of the amount of capacitance on the output. during removal of the same output load, the energy stored in the inductor is dumped into the output capacitors. this energy dumping creates a temporary hump in the output voltage. this hump, as with the sag, can be attributed to the total amount of capacitance on the output. figure 35 shows a typica l response to a load transient. the amplitudes of the different types of voltage excursions can be approximated using equation 5. where: i tran = output load current transient, and c out = total output capacitance. in a typical converter design, the esr of the output capacitor bank dominates the transient response. the esr and esl typically are the major contributing factors in determining the output capacitance. the number of output capacitors can be determined by using equation 6, which relates the esr and esl of the capacitors to the transien t load step and the voltage limit (dvo): if dv sag or dv hump is found to be too large for the output voltage limits, then the amount of capacitance may need to be increased. in this situation, a trade-off between output inductance and output capacitance may be necessary. the esl of the capacitors, which is an important parameter in the previous equations, is not usually listed in databooks. practically, it can be approxim ated using equation 7 if an impedance vs frequency curve is given for a specific capacitor: where f res is the frequency at which the lowest impedance is achieved (resonant frequency). the esl of the capacitors beco mes a concern when designing circuits that supply power to load s with high rates of change in the current. output inductor selection the output inductor is selected to meet the output voltage ripple requirements and to minimize the converter?s response time to the load transient. the inductor value determines the converter?s ripple current, and the ripple voltage is a function of the ripple current. the ripple voltage and cu rrent are approximated by using equation 8: figure 35. typical transient response r 4 r 1 0.6v v out 0.6v ? ---------------------------------- = (eq. 4) v out i out dv esl dv esr dv sag dv hump i tran v esr esr i tran ? = v esl esl di tran dt --------------- ? = v sag l out i tran 2 ? c out v in v out ? () ? ------------------------------------------------- - = v hump l out i tran 2 ? c out v out ? -------------------------------- = (eq. 5) number of capacitors esl d i tran ? dt --------------------------------- esr i tran ? + v o ----------------------------------------------------------------------- = (eq. 6) esl 1 c2 ? f res ? () 2 ---------------------------------------- = (eq. 7) di = vin - vout fs x l vout vin dvout = di x esr x (eq. 8)
isl8502a 16 fn7940.0 october 21, 2011 increasing the value of inductan ce reduces the ripple current and voltage. however, the large in ductance values reduce the converter response time to a load transient. one of the parameters limiting converter response to a load transient is the time required to change the inductor current. given a sufficiently fast control loop design, the isl8502a provides either 0% or 100% duty cycle in response to a load transient. the response time is the time required to slew the inductor current from an initia l current value to the transient current level. during this interval, the difference between the inductor current and the tran sient current level must be supplied by the output capacitor. minimizing the response time can minimize the output capacitance required. the response time to a transient is different for the application of load and the removal of load. eq uation 9 gives the approximate response time interval for applic ation and removal of a transient load: where: i tran is the transient load current step, t rise is the response time to the application of load, and t fall is the response time to the removal of load. the worst-case response time can be either at the application or remo val of load. be sure to check both of these equati ons at the minimum and maximum output levels for the worst-case response time. input capacitor selection use a mix of input bypass capacitors to control the voltage overshoot across the mosfets. use small ceramic capacitors for high-frequency decoupling, and bulk capacitors to supply the current needed each time the upper mosfet turns on. place the small ceramic capacitors physically close to the mosfets and between the drain of the upper mosfet and the source of the lower mosfet. the important parameters for bulk input capacitance are the voltage rating and the rms current rating. for reliable operation, select bulk capacitors with voltage and current ratings above the maximum input voltage and largest rms current required by the circuit. their voltage rating should be at least 1.25x greater than the maximum input voltage, while a voltage rating of 1.5x is a conservative guideline. for most cases, the rms current rating requirement for the input capacitor of a buck regulator is approximately one-half the dc load current. the maximum rms current through the input capacitors can be closely approximated using equation 10: for a through-hole design, several electrolytic capacitors may be needed. for surface mount design s, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating. these capacitors must be capable of handling the surge current at power-up. some capacitor series available from reputable manufacturers are surge current tested. feedback compensation figure 36 highlights the voltage-mode control loop for a synchronous-rectified buck converter. the output voltage (v out ) is regulated to the reference voltage level. the error amplifier output (v e/a ) is compared with the oscillator (osc) triangular wave to provide a pulse-width modulated (pwm) wave with an amplitude of v in at the phase node. the pwm wave is smoothed by the output filter (l o and c o ). the modulator transfer function is the small-signal transfer function of v out /v e/a . this function is dominated by a dc gain and the output filter (l o and c o ), with a double pole break frequency at f lc and a zero at f esr . the dc gain of the modulator is simply the input voltage (v in ) divided by the peak-to-peak oscillator voltage, dv osc . the isl8502a incorporates a feed-forward loop that accounts for changes in the input voltage. this configuration maintains a constant modulator gain. modulator break frequency equations the compensation network consists of the error amplifier (internal to the isl8502a) and the impedance networks, z in and z fb . the goal of the compensation network is to provide a closed loop transfer function with th e highest 0db crossing frequency t rise = l x i tran vin - vout t fall = l x i tran vout (eq. 9) v out v in ------------- - i out max 2 1 v out v in ----------- ? ?? ?? 1 12 ------ v in v out ? lf osc ---------------------------- - v out v in ------------- - ?? ?? ?? 2 + ?? ?? ?? (eq. 10) figure 36. voltage-mode buck converter compensation design and output voltage selection v out reference l o c o esr v in v osc error amp pwm driver (parasitic) z fb + - reference r 1 r 3 r 2 c 3 c 1 c 2 comp v out fb z fb z in comparator driver detailed compensation components phase v e/a + - + - z in osc r 4 v out 0.6 1 r 1 r 4 ------ - + ?? ?? ?? = isl8502a f lc 1 2 x l o x c o ------------------------------------------ - = f esr 1 2 x esr x c o ------------------------------------------- - = (eq. 11)
isl8502a 17 fn7940.0 october 21, 2011 (f 0db ) and adequate phase margin. phase margin is the difference between the closed loop phase at f 0db and 180 degrees. equation 12 relates th e compensation network?s poles, zeros, and gain to the components (r 1 , r 2 , r 3 , c 1 , c 2 and c 3 ) in figure 36. use these guidelines for locating the poles and zeros of the compensation network: 1. pick gain (r 2 /r 1 ) for desired converter bandwidth. 2. place first zero below filt er?s double pole (~75% f lc ). 3. place second zero at filter?s double pole. 4. place first pole at esr zero. 5. place second pole at half the switching frequency. 6. check gain against error amplifier?s open-loop gain. 7. estimate phase margin; repeat if necessary. compensation break frequency equations figure 37 shows an asymptotic plot of the dc/dc converter gain vs frequency. the actual modula tor gain has a high gain peak due to the high q factor of the output filter and is not shown in figure 37. using the guidelines provided should give a compensation gain similar to th e curve plotted. the open loop error amplifier gain bounds th e compensation gain. check the compensation gain at f p2 with the capabilities of the error amplifier. the closed loop gain is constructed on the graph of figure 37 by adding the modulator gain (in db) to the compensation gain (in db). this is equivalent to multiplying the modulator transfer function to the compensation transfer function and plotting the gain. the compensation gain uses external impedance networks, z fb and z in , to provide a stable, high bandwidth (bw) overall loop. a stable control loop has a gain cr ossing with -20db/decade slope and a phase margin greater than +45. include worst-case component variations when dete rmining phase margin. a more detailed explanation of voltage mo de control of a buck regulator can be found in tech brief tb417 , entitled ?designing stable compensation networks for single phase voltage mode buck regulators.? layout considerations layout is very important in high frequency switching converter design. with power devices swit ching efficiently between 500khz and 1.2mhz, the resulting current transitions from one device to another cause voltage spikes across the interconnecting impedances and parasitic circuit elements. these voltage spikes can degrade efficiency, radiate noise into the circuit, and lead to device overvoltage stress. careful component layout and printed circuit board design minimize these voltage spikes. as an example, consider the turn-off transition of the control mosfet. prior to turn-off, the mosfet is carrying the full load current. during turn-off, current stops flowing in the mosfet and is picked up by the lower mosfet . any parasitic inductance in the switched current path generates a large voltage spike during the switching interval. careful compon ent selection, tight layout of the critical components, and shor t, wide traces minimize the magnitude of voltage spikes. there are two sets of critical components in the isl8502a switching converter. the switch ing components are the most critical because they switch large amounts of energy and therefore tend to generate large amounts of noise. next are the small signal components, which connect to sensitive nodes or supply critical bypass curr ent and signal coupling. a multi-layer printed circuit board is recommended. figure 38 shows the connections of the critical components in the converter. note that capacitors c in and c out could each represent numerous physical capacitors. dedicate one solid layer (usually a middle layer of the pc board) for a ground plane, and make all critical component ground connections with vias to this layer. dedicate another solid layer as a power plane, and break this plane into smaller islands of common voltage levels. keep the metal runs from the phase terminals to the output inductor short. the power plane should support the input power and output power nodes. use copper-f illed polygons on the top and bottom circuit layers for the phase nodes. use the remaining printed circuit layers for small signal wiring. the wiring traces from the gate pins to the mosf et gates should be kept short and wide enough to easily handle the 1a of drive current. in order to dissipate heat generated by the internal v tt ldo, the ground pad, pin 29, should be connected to the internal ground plane through at least five vias. this allows heat to move away f z1 1 2 x r 2 x c 1 ------------------------------------ = f z2 1 2 x r 1 r 3 + () x c 3 ------------------------------------------------------ - = f p1 1 2 x r 2 x c 1 x c 2 c 1 c 2 + --------------------- - ?? ?? ?? -------------------------------------------------------- - = f p2 1 2 x r 3 x c 3 ------------------------------------ = (eq. 12) 100 80 60 40 20 0 -20 -40 -60 f p1 f z2 10m 1m 100k 10k 1k 100 10 open loop error amp gain f z1 f p2 20log f lc f esr compensation gain (db) frequency (hz) gain 20log (v in / v osc ) modulator gain (r 2 /r 1 ) closed loop gain figure 37. asymptotic bode plot of converter gain
isl8502a 18 fn7940.0 october 21, 2011 from the ic and also ties the pad to the ground plane through a low impedance path. the switching components should be placed close to the isl8502a first. minimize the le ngth of connections between the input capacitors, c in , and the power switches by placing them nearby. position both the ceramic and bulk input capacitors as close to the upper mosfet drain as possible. position the output inductor and output capacitors between the upper and lower mosfets and the load. make the pgnd and the output capacitors as short as possible. the critical small signal components include any bypass capacitors, feedback components, and compensation components. place the pwm converter compensation components close to the fb and comp pins. the feedback resistors should be located as close as possible to the fb pin, with vias tied straight to the ground plane as required. figure 38. printed circuit board power planes and islands vin pvcc vcc phase pgnd comp fb gnd pad r 4 r 3 c 3 r 1 c 1 c 2 r 2 c out1 v out1 c in v in l 1 c bp2 r bp c bp1 5v island on power plane layer island on circuit and/or power plane layer via connection to ground plane key load isl8502a
isl8502a 19 intersil products are manufactured, assembled and tested utilizing iso9000 quality systems as noted in the quality certifications found at www.intersil.com/design/quality intersil products are sold by description only. intersil corporat ion reserves the right to make changes in circuit design, soft ware and/or specifications at any time without notice. accordingly, the reader is cautioned to verify that data sheets are current before placing orders. information furnished by intersil is believed to be accurate and reliable. however, no responsi bility is assumed by intersil or its subsid iaries for its use; nor for any infringem ents of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of i ntersil or its subsidiaries. for information regarding intersil corporation and its products, see www.intersil.com fn7940.0 october 21, 2011 for additional products, see www.intersil.com/product_tree products intersil corporation is a leader in the design and manufacture of high-performance analog semico nductors. the company's product s address some of the industry's fastest growing markets, such as , flat panel displays, cell phones, handheld products, and noteb ooks. intersil's product families address power management and analog sign al processing functions. go to www.intersil.com/products for a complete list of intersil product families. for a complete listing of applications, rela ted documentation and related parts, please see the respective device information p age on intersil.com: isl8502a to report errors or suggestions for this datasheet, please go to: www.intersil.com/askourstaff fits are available from our website at: http://rel.intersil.co m/reports/search.php revision history the revision history provided is for inform ational purposes only and is believed to be accurate, but not warranted. please go t o web to make sure you have the latest revision. date revision change 10/21/2011 fn7940.0 initial release
isl8502a 20 fn7940.0 october 21, 2011 package outline drawing l24.4x4d 24 lead quad flat no-lead plastic package rev 2, 10/06 0 . 90 0 . 1 5 c 0 . 2 ref typical recommended land pattern 0 . 05 max. ( 24x 0 . 6 ) detail "x" ( 24x 0 . 25 ) 0 . 00 min. ( 20x 0 . 5 ) ( 2 . 50 ) side view ( 3 . 8 typ ) base plane 4 top view bottom view 7 12 24x 0 . 4 0 . 1 13 4.00 pin 1 18 index area 24 19 4.00 2.5 0.50 20x 4x see detail "x" - 0 . 05 + 0 . 07 24x 0 . 23 2 . 50 0 . 15 pin #1 corner (c 0 . 25) 1 seating plane 0.08 c 0.10 c c 0.10 m c a b a b (4x) 0.15 located within the zone indicated. the pin #1 indentifier may be unless otherwise specified, tolerance : decimal 0.05 tiebar shown (if present) is a non-functional feature. the configuration of the pin #1 identifier is optional, but must be between 0.15mm and 0.30mm from the terminal tip. dimension b applies to the metallized terminal and is measured dimensions in ( ) for reference only. dimensioning and tolerancing conform to amse y14.5m-1994. 6. either a mold or mark feature. 3. 5. 4. 2. dimensions are in millimeters. 1. notes:

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