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1 ? caution: these devices are sensitive to electrosta tic discharge; follow proper ic handling procedures. 1-888-intersil or 1-888-468-3774 | intersil (and design) is a registered trademark of intersil americas inc. copyright ? intersil americas inc. 2007, 2008, 2010. all rights reserved all other trademarks mentioned are the property of their respective owners. ISL8502 2a synchronous buck regulator with integrated mosfets the ISL8502 is a synchronous buck controller with internal mosfets packaged in a small 4mmx4mm qfn package. the ISL8502 can support a continuous load of 2a and has a very wide input voltage range. with the switching mosfets integrated into the ic, the co mplete regulator footprint can be very small and provide a much more efficient solution than a linear regulator. the ISL8502 is capable of stand alone operation or it can be used in a master slave combinat ion for multiple outputs that are derived from the same input rail. multiple slave channels (up to six) can be synchronized. this method minimizes the emi and beat frequencies ef fect 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. the switching regulator provides a maximum static regulation tolerance of 1% over line, load, an d 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 ISL8502 features a bi-directional enable pin that allows the part to pull the enable pin low during fault detection. pinout ISL8502 (24 ld qfn) top view 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 protection ? 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 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) part marking temp. range (c) package (pb-free) pkg. dwg. # ISL8502irz* 85 02irz -40 to +85 24 ld 4x4 qfn l24.4x4d *add ?-t? suffix for tape and reel. please refer to tb347 for details on reel specifications. note: these intersil pb-free plas tic packaged products employ special 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 with both snpb and pb-free soldering operations). intersil pb-f ree products are msl classified at pb-free peak reflow temperatures that meet or exceed the pb-free requirements of ipc/jedec j std-020. data sheet june 29, 2010 fn6389.2
2 fn6389.2 june 29, 2010 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 ISL8502
3 fn6389.2 june 29, 2010 typical application schematics v out + ISL8502 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 1. stand alone regulator: v in 5.5v to 14v v out + ISL8502 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 2. stand alone regulator: v in 4.5v to 5.5v ISL8502
4 fn6389.2 june 29, 2010 ISL8502 with multiple slaved channels ISL8502 en fs m/s synch pvcc gnd master slave phase vin + v out1 v in r t ISL8502 en fs m/s synch gnd phase vin + v out2 r t 5k slave ISL8502 en fs m/s synch gnd phase vin + v outn r t 5k enable ss ISL8502
5 fn6389.2 june 29, 2010 absolute maximum rati ngs 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 1, 2) . . . . . . . . . 39 2.5 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 fo r extended periods of time. exposure to such conditions may adv ersely impact product reliability and result in failures not covered by warranty. notes: 1. ja is measured in free air with the component mounted on a high ef fective thermal conductivity te st board with ?direct attach? fe atures. see tech brief tb379. 2. for jc , the ?case temp? location is the center of the exposed metal pad on the package underside. 3. minimum v in can operate below 5.5v as long as vcc is greater than 4.5v. 4. maximum v in can be higher than 14v voltage stress across the upper and lower do not exceed 15.5v in all conditions. 5. circuit requires 150ns minimum on ti me to detect overcurrent condition. 6. limits established by characteri zation and are not production tested. 7. parameters with min and/or max limits are 100% tested at +25 c, unless otherwise specified. temperature limits established by characterization and are not production tested. electrical specifications refer to block and simplified power system diagram s 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. parameter symbol test conditions min (note 7) typ max (note 7) units v in supply input voltage range v in 5.5 (note 3) 14 (note 4) v v in tied to vcc 4.5 5.5 v input operating supply current i q v fb = 1.0v 7 ma input standby supply current i q_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 ISL8502
6 fn6389.2 june 29, 2010 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 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 t pgood_delay f osc = 500khz 250 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 5, 6) (t a = 0c to +85c) 2.1 3.5 4.5 a ioc from v in to phase (notes 5, 6) (t a = -40c to +0c) 2.0 3.4 4.0 a electrical specifications refer to block and simplified power system diagram s 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. (continued) parameter symbol test conditions min (note 7) typ max (note 7) units ISL8502
7 fn6389.2 june 29, 2010 negative current limit i noc_peak ioc from phase to pgnd (notes 5, 6) (t a = 0c to +85c) 2.2 3.0 3.5 a ioc from phase to pgnd (notes 5, 6) (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 electrical specifications refer to block and simplified power system diagram s 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. (continued) parameter symbol test conditions min (note 7) typ max (note 7) units 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 3. efficiency vs load (v in = 5v) figure 4. efficiency vs load (v in = 12v) figure 5. v out regulation vs load (v out = 0.6v, 500khz) figure 6. v out regulation vs load (v out = 1.2v, 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 ISL8502
8 fn6389.2 june 29, 2010 figure 7. v out regulation vs load (v out = 1.5v, 500khz) figure 8. v out regulation vs load (v out = 1.8v, 500khz) figure 9. v out regulation vs load (v out = 2.5v, 500khz) figure 10. v out regulation vs load (v out = 3.3v, 500khz) figure 11. v out regulation vs load (v out = 5v, 500khz) figure 12. power dissipation vs load (v out = 0.6v, 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) 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 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 ISL8502
9 fn6389.2 june 29, 2010 figure 13. power dissipation vs load (v out = 1.2v, 500khz) figure 14. power dissipation vs load (v out = 1.5v, 500khz) figure 15. power dissipation vs load (v out = 1.8v, 500khz) figure 16. power dissipation vs load (vout = 2.5v, 500khz) figure 17. power dissipation vs load (v out = 3.3v, 500khz) figure 18. power dissipation vs load (v out = 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) 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 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 ISL8502
10 fn6389.2 june 29, 2010 figure 19. v cc load regulation figure 20. vcc regulation vs v in figure 21. master to slave operation f igure 22. master operation at no load figure 23. master operation with full load fi gure 24. master operat ion with negative load 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) 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 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 ISL8502
11 fn6389.2 june 29, 2010 figure 25. soft-start at no load figure 26. start-up with pre-biased figure 27. soft-start at full load figure 28. positive output short circuit figure 29. positive output short circuit (hiccup mode) figure 30. negative output short circuit 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 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 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 ISL8502
12 fn6389.2 june 29, 2010 functional pin descriptions pgood (pin 1) pgood is an open drain output that will pull to low if the output goes out of regulation or a fault is detected. pgood is equipped with a fixed delay upon output power-up. the delay is approximately 250m s at switching frequency 500khz and 108ms at 1.2mhz. sgnd (pin 2) the sgnd terminal of the ISL8502 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 will be pulled low via internal circuitry for a duration of 4 soft -start periods. for automatic start-up, use 10k to 100k pull-up resistor connecting to vcc. synch (pin 4) this is a bi-directional pin t hat is 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 ISL8502 will be 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 this pin and ground. as a master or a stand alone dev ice, tie this pin directly to the vcc pin. do not short m/s pin to gnd. fs (pin 6) this pin provides oscillator switching frequency adjustment. by placing a resistor (r t ) from this 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 will force the switching frequency to 800khz. using resistors with values below 40k (1.2mhz) or with values higher than 97k (500khz) may damage the ISL8502. comp (pin 7) and fb (pin 8) the switching regulator employs a single voltage control loop. fb is the negative input to the voltage loop error amplifier. the output voltage is set by an external resistor divider connected to fb. with a properly selected divider, the output voltage can be set to any voltage between the power rail (reduced by converter losses) and the 0.6v reference. loop compensation is achieved by connecting an ac network across comp and fb. the fb pin is also monitored for undervoltage events. ss (pin 9) connect a capacitor from this pin to ground. this capacitor, along with an internal 30a current source, sets the soft-start interval of the converter, t ss as shown in equation 2. figure 31. recover from positive short circuit figure 32. 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) 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 r t k [] 48000 f osc khz [] ------------------------------ = (eq. 1) c ss f [] 50 t ss s [] ? = (eq. 2) ISL8502
13 fn6389.2 june 29, 2010 pgnd (pins 10-13) these pins are used as the ground connection of the power train. phase (pins 14-17) these 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 these 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 recommendation allows for transient spikes and voltage ringing to occur while not exceeding absolute maximum ratings. boot (pin 22) this pin provides ground refe renced 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 ISL8502. pvcc (pin 23) this pin is the output of the internal linear regulator that supplies the bias and gate voltage for the ic. a minimum 4.7f decoupling capacitor is recommended. vcc (pin 24) this 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 is recommended. functional description initialization the ISL8502 automatically initializes upon receipt of input power. the power-on reset (por) function continually monitors the voltage on the vcc pin. if the voltage on the en pin exceeds its rising threshold, then the por function initiates soft-start operation after the bias voltage has exceeded the por threshold. stand alone operation the ISL8502 can be configured to function as a stand alone single channel voltage mode synchronous buck pwm voltage regulator. the ?typical application schematics? on page 3 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 will create 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 ISL8502 will use 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 ISL8502 can be configured to function in a multi-channel system. the ?ISL8502 with multiple slaved channels? on page 4 shows a typical configuration for the multi-channel system. in the multi-channel system, each ISL8502 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 ma ster, the m/s pin must be shorted to the vcc pin. the synch pins of all the ISL8502 controller ics in the mult i-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 from m/s pin to ground. the master device and all the slave devices can have their en pins tied to an enable ?bus?. since the en pin is bi-directional, this allows for options on how each ic is tied to the enable ?bus?. if the en pin of any ISL8502 is tied directly to the enable bus, then that device will be capable of disabling all the other devices that have their en pins ti ed directly to the enable bus. if the en pin of an ISL8502 is tied to the enable bus through a diode (anode tied to ISL8502 en pin, cathode tied to enable bus) then this part will 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 will continue to send the clock signal from the synch pin. this allows slave devices to continue operating. fault protection the ISL8502 monitors the output of the regulator for overcurrent and undervoltage events. the ISL8502 also provides protection from exce ssive 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 over current condition, the upper mosfet will be immediately turned off and will not be turned on again until the next switching cycle. upon detection of the initial overcurr ent 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 overcurrent condition is detected, the oc fault counter will be incremented. if there are eight sequential oc fault detections, the regulator will be shut down under an ISL8502
14 fn6389.2 june 29, 2010 overcurrent fault condition and the en pin will be pulled low. an overcurrent fault condition will result with the regulator attempting to restart in a hiccup mode with the delay between restarts being 4 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 overcurrent condition goes away prior to the oc fault counter reaching a count of four, the overcurrent condition flag will set back to low. if the overcurrent condition flag is high and the overcurrent fault counter is less than four and an undervoltage event is detected, the regulator will be shut down immediately. undervoltage protection if the voltage detected on the fb pin falls 18% below the internal reference voltage and the overcurrent condition flag is low, then the regulator will be shutdown immediately under an undervoltage fault condition and the en pin will be pulled low. an undervoltage fault condition will result with the regulator attempting to restart in a hiccup mode with the delay between restarts being 4 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 ISL8502 ic junction temperature reaches a nominal temperature of +150c, the regulator will be disabled. the ISL8502 will not re-enable the regulator until the junction temperature drops below +130c. shoot-through protection a shoot-through condition oc curs 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 is l8502 incorporates specialized circuitry, which insures that the complementary mosfets are not on simultaneously. application guidelines operating frequency the ISL8502 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 regulator 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. refer to figure 34. the output voltage programming resistor, r 4 , will depend on the value chosen for the feedback resistor and the desired output 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 curren t. the filtering requirements are a function of the switching freq uency 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 board 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 waveform during a load transient that represents the worst case loading conditions will ultimately determine 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 vo ltage. 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). r t k [] 48000 f osc khz [] ------------------------------ = (eq. 3) figure 33. 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 ISL8502
15 fn6389.2 june 29, 2010 after the initial spike, attributable 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 the 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 33 shows a typical response to a load transient. the amplitudes of the differen t 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 the esl are typically the major contributing factors in determining the output capacitanc e. the number of output capacitors can be determined by using equation 6, which relates the esr and esl of the capacitors to the transient load step and the voltage limit (dvo): if dv sag and/or dv hump are 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 no t usually listed in databooks. practically, it can be approximated using equation 7 if an impedance vs frequency curve is given for a specific capacitor: where: f res is the frequency where the lowest impedance is achieved (resonant frequency). the esl of the capacitors becomes a concern when designing circuits that supply power to loads with high rates of change in the current. output inductor selection the output inductor is selected to meet the output voltage ripple requirements and minimize the converter?s response time to the load transient. th e inductor value determines the converter?s ripple current and the ripple voltage is a function of the ripple current. the ripple voltage and current are approximated by using equation 8: increasing the value of inductance reduces the ripple current and voltage. however, the large inductance values reduce the converter?s response time to a load transient. one of the parameters limiting the converter?s response to a load transient is the time required to change the inductor current. given a sufficiently fast control loop design, the ISL8502 will provide either 0% or 100% duty cycle in response to a load transient. the response time is the time required to slew the inductor cu rrent from an initial current value to the transient current level. during this interval the difference between the inductor current and the transient 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. equation 9 gives the approximate response time interval for application 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 removal of load. be sure to check both of these equations at the minimum and maximum output levels for the worst case response time. input capacitor selection use a mix of input bypass capaci tors to control the voltage overshoot across the mosfets. use small ceramic capacitors for high frequency decoupling and bulk capacitors to supply the current needed ea ch time the upper mosfet turns on. place the small cerami c 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 the 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 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 di tran ? dt --------------------------------- esr i tran ? + v o ----------------------------------------------------------------------- = (eq. 6) esl 1 c2 ? f res ? () 2 ---------------------------------------- = (eq. 7) i= v in - v out fs x l v out v in v out = i x esr x (eq. 8) t rise = l x i tran v in - v out t fall = l x i tran v out (eq. 9) ISL8502
16 fn6389.2 june 29, 2010 a voltage rating of 1.5x is a conservative guideline. for most cases, the rms current rati ng requirement for the input capacitor of a buck regulator is approximately 1/2 the dc load current. the maximum rms current through the input capacitors may be closely approximated using equation 10: for a through-hole design, seve ral electrolytic capacitors may be needed. for surface mount designs, 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 34 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 ISL8502 incorporates a feed forward loop that accounts for changes in the input voltage. this maintains a constant modulator gain. modulator break frequency equations the compensation network consists of the error amplifier (internal to the ISL8502) and the impedance networks z in and z fb . the goal of the compensation network is to provide a closed loop transfer function with the highest 0db crossing frequency (f 0db ) and adequate phase margin. phase margin is the difference between the closed loop phase at f 0db and 180. equation 12 relates the 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 34. 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 1 st zero below filter?s double pole (~75% f lc ). 3. place 2 nd zero at filter?s double pole. 4. place 1 st pole at the esr zero. 5. place 2 nd pole at half the switching frequency. 6. check gain against error amplifier?s open-loop gain. 7. estimate phase margin - repeat if necessary. 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 34. 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 ISL8502 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 ------ - + ?? ?? ?? = f lc 1 2 x l o x c o ------------------------------------------ - = f esr 1 2 x esr x c o ------------------------------------------- - = (eq. 11) ISL8502
17 fn6389.2 june 29, 2010 compensation break frequency equations figure 35 shows an asymptotic plot of the dc/dc converter?s gain vs frequency. the actual modulator gain has a high gain peak due to the high q factor of the output filter and is not shown in figure 35. using the guidelines provided should give a compensation gain similar to the curve plotted. the open loop error amplifier gain bounds the compensation gain. check the compensation gain at f p2 with the capabilities of the e rror amplifier. the closed loop gain is constructed on the graph of figure 35 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 crossing with -20db/decade slope and a phase margin greater than +45. include worst case component variations when determining phase margin. a more detailed explanation of voltage mode 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 switching 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 minimizes 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, cu rrent 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 component selection, tight layout of the critical components, and short, wide traces minimi zes the magnitude of voltage spikes. there are two sets of critic al components in the ISL8502 switching converter. the switching components are the most critical because t hey switch large amounts of energy, and therefore tend to generate larg e amounts of noise. next, are the small signal components, which connect to sensitive nodes or supply critical bypass current and signal coupling. a multi-layer printed circuit board is recommended. figure 36 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 filled 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 mosfet 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 the heat to move away 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 ISL8502 first. minimize the length of the 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. 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 35. asymptotic bode plot of converter gain ISL8502
18 all intersil u.s. products are manufactured, asse mbled and tested utilizing iso9000 quality systems. intersil corporation?s quality certifications ca n be viewed at www.intersil.com/design/quality intersil products are sold by description only. intersil corpor ation 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 furnishe d by intersil is believed to be accurate and reliable. however, no responsibility is assumed by intersil or its subsidiaries for its use; nor for any infringements of paten ts 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 intersil or its subsidiari es. for information regarding intersil corporation and its products, see www.intersil.com fn6389.2 june 29, 2010 figure 36. printed circuit board power planes and islands vin pvcc ISL8502 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 ISL8502
19 fn6389.2 june 29, 2010 ISL8502 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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