View NX9548_9083090.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

copyright ? 2013 microsemi page 1 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet description the NX9548 is a synchronous buck switching regulator with 21m ? internal n-channel mosfets, primarily intended for portable (mobile) applications. the NX9548 operates from 4.5v to 24v, and the output voltage range is from 0.75v to 5v, with output currents as high as 8a. it can be selected to operate in synchronous mode, or non-synchronous (diode emulation or psm) mode at light loads, to improve efficiency. adaptive constant on time (cot) control provides extremely fast transient response to line and load steps, while at the same time providing near-constant switching frequency over a wide input voltage range. the frequency is also externally adjustable. the NX9548 features overcurrent protection (ocp), feedback under-voltage lockout (fb uvlo), and overvoltage protection (ovp). it also includes an integrated bootstrap schottky diode, and provides low-voltage (5v) gate-drive capability. in addition it provides a power good indicator and has adaptive deadtime . features ? adaptive cot control ? adjustable, constant switching frequency up to 1mhz ? extremely low-r ds on n-mosfets ? bus voltage 4.5v to 24v ? selectable diode emulation mode (psm mode) ? current limit, uvlo, ovp ? gate resistor provision for emi reduction ? -40c to +85c ambient temperature ? -40c to +150c junction temperature ? rohs compliant ? 55 mm very thin profile qfn (vqfn) package applications ? ultramobile/notebook pcs ? tablets/slates ? hand-held portable instruments figure 1: typical application diagram for low-input voltages downloaded from: http:///
copyright ? 2013 microsemi page 2 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet ot her typical application diagram figure 2: typical application diagram for wide input range pin configuration and pinout 1 hg gnd 9 6 3 4 8 10 11 12 13 5 7 28 s1 2 14 15 16 24 19 22 21 17 20 18 23 32 31 30 29 28 27 26 25 s1s1 d2 d2 d2 d2 s2s2 bst d2 hdrv nc ton vout en_mode d1 d1 fb vcc s2s2 d1 s2s2 pvcc ocp gnd pgood d1 pin 33 (gnd) pin 34 (d1) pin 35 (d2) top view (5x5 mcm, vqfn C 32l) NX9548 part marking: line 1: * msc line 2: 9548 line 3: date / lot code figure 3 : pinout note: all pins and pads are at the bottom of the chip. * is the pin one dot. downloaded from: http:///
copyright ? 2013 microsemi page 3 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet ordering information ambient temperature type package part number packaging type -40c to 85c rohs compliant, pb -free vqfn-32l (mcm) 5 5mm NX9548ilq bulk NX9548ilq- tr tape and reel pin description pin number pin designator description 1, 2, 3 s1 source of the high side n-channel mosfet. these pins must be connected directly to the drain of low side mosfet via a pcb plane connection. 4, 30, 31, 32 34 d1 drain of high side mosfet. 5, 6, 7, 8, 19 35 d2 drain of low side mosfet and the controller pin out sw 9, 10, 11, 12, 13, 14 s2 source of low side mosfet and needs to be directly connected to power ground via multiple vias. 15 pvcc this pin provides the voltage supply to the lower mosfet drivers. place a high f euey deouplig apaito f/x5r from this pin to gnd. 16 ocp this pin is the input of the over current protection (ocp) comp arator , and it should be connected to the drain of the low side mosfet vi a a resistor . an internal current source is supplied from this pin to an external resistor which sets the ocp voltage across the r dson of the low side mosfet. the current limit level is this voltage divided by the r dson . once this threshold is reached the chip shuts off. 17 nc not connected internally. 18 hdrv high side gate driver output which needs to be connected to hi gh side mosfet gate hg pin. a small value resistor may be placed in series to slow down the high side mosfet, reducing the ringing on sw node. 20 bst this pin supplies voltage to high side fet driver. a high frequency . f ceramic capacitor should be placed as close as possible and connected pin 19. a 4.7 ? resistor is recommended in series with this capacitor. 21 , 28, 33 gnd ground for the ic and the buck topology. downloaded from: http:///
copyright ? 2013 microsemi page 4 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet 22 en_mode switching converter enable input. connect to vcc for psm/non synchronous mode, connect to an external resistor divider equaling 70% of vcc for ultrasonic mode, connect to gnd for shutdown mode , floating or connected to 2v for synchronous mode. 23 vout this pin is directly connected to the output of the switching regulator and senses the vout voltage. an internal mosfet discharges the output during turn off. 24 ton vin sensing input. a resistor connect ed from this pin to vin will program the frequency. a 1nf capacitor from this pin to gnd is recommended to ensure the proper operation. 25 vcc this pin supplies the internal 5v bias circuit. a f/x5r ceramic capacitor is placed as close as possible to this pin and ground pin . 26 fb this pin is the error amplifier s inverting input. this pin is connected via resistor divider to the output of the switching regulator to set the output dc voltage from 0.75v to 5v. 27 pgood pgood indicator for switching regulator. it requires a pull up resisto r to vcc or lower voltage. when fb pin reaches 90% of the reference voltage pgood transitions from lo to hi state. 29 hg the gate of the high side switching mosfet. ca oet typially ? in series to improve emi, at the expense of about 2-3 % in efficienc y downloaded from: http:///
copyright ? 2013 microsemi page 5 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet functional block diagram figure 4 : block diagram downloaded from: http:///
copyright ? 2013 microsemi page 6 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet absolute maximum ratings performance is not necessarily guaranteed over this entire range. these are maximum stress ratings only. exceeding these ratings, even momentarily, can cause immediate damage, or negatively impac t long-term operating reliability. min max units vcc, pvcc to gnd -0.3 6.5 v bst and hdrv to sw (s1-d2 node) -0.3 6.5 v ton to gnd -0.3 28 v d1 to s1 and d2 to s2 30 v all other pins to gnd -0.3 vcc+0.3 v output current 9 a junction temperature - 40 1 50 c storage temperature - 65 150 c lead soldering temperature (40s, reflow) 260 c note: pin 33 is connected by copper plane on pcb to gnd (pins 21 and 28), pin 34 i s similarly connected to d1, and pin 35 is similarly connected to d2. operating ratings performance is generally guaranteed over this range as further detailed belo w under electrical characteristics. min max units v in 4.5 24 v vcc, pvcc 4.5 5.5 v ambient temperature - 40 85 c output current 0 8 a note: corresponding maximum junction temperature of 150c thermal properties thermal resistance typ units ja 35 c /w jc 29 c /w jl 1.2 c /w note: the ? ja numbers assume no forced airflow. junction temperature is calcul ated using t j = t a + (pd x ? ja ). i patiula, ja is a function of the pcb construction. the stated number above is for a fo ur-layer board in accordance with jesd-51 (jedec). for jl , the lead temperature is measured at the center of pad2 at the bottom of the package. for jc , the case temperature is measured at the center point of pad2 on top of the package on the plastic with infinitely large heat sink on top of the device. downloaded from: http:///
copyright ? 2013 microsemi page 7 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet electrical characteristics the following specifications apply over the operating ambient temperature of -40c t a 85c except where otherwise noted, with the following test conditions: vcc = pvcc = 5v, 4.5v < v in < 24v. typical parameter refers to t j =25c, v in =12v. symbol parameter conditions min typ max units v in i d1 + i ton shutdown current v en_mode = gnd 20 na vcc, pvcc supply i vcc quiescent current (switching no-load) v fb = 0.85v, v en_mode = 5v 1.5 ma i vcc + i pvcc shutdown current v en_mode = v sw = v hg = gnd, note 1 45 a vcc uvlo v cc _uvlo_hi undervoltage lockout threshold (rising) 3.87 4.1 4.5 v v cc _uvlo _lo undervoltage lockout threshold (falling) 3.67 3.9 4.3 v on and o ff time i to n_op ton operating current v in = 15v, r ton = 1m ? 17 25 a t on on -time v in = 9v , r ton = 1m ?, v out = 0.75v 312 456 620 ns t off_min min off-time 380 496 700 ns fb voltage v ref feedback voltage 0.739 0.75 0.761 v i offset feedback pin bias current (into pin) 75 na line regulation vcc from 4.5v to 5.5v, v in = 12v, note 1 0.1 % vin from 4.5v to 24v, vcc = 5v, note 1 0.3 load regulation 0 < l load < 8a, v in = 12v, v out = 1v, v en_mode = 2.5v, note 1. 0.07 %/a downloaded from: http:///
copyright ? 2013 microsemi page 8 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet symbol parameter conditions min typ max units output voltage v out output voltage range v in = 10.5v 0.75 5 v r vo ut_dis output voltage discharge resistance v en_mode = gnd 30 ? t ss soft-start time 1.5 ms pgood v fb_g ood _hi power good threshold (rising voltage on fb pin) 90 %v ref t pgood_delay power good deglitch time after soft-start completed note 1 1.6 ms t pgood_speed propagation delay of power good signal note 1 2 s v pgood_hys power good hysteresis 5 % r pgood_lo power good low impedance 13 ? i pgood power good leakage current v fb > 0.9 v ref 1 a en_mode threshold and bias current v en_mode_skip en_mode pin threshold for psm 81 100 %vcc v en_mode_us en_mode pin threshold for ultrasonic mode 61 79 %vcc v en _synch en_mode pin voltage to enable in synchronous mode (or leave pin floating) 2 59 % vcc v v en_sd en_mode pin voltage to disable switching 0 0.8 v downloaded from: http:///
copyright ? 2013 microsemi page 9 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet symbol parameter conditions min typ max units i en_mode_hi en_mode pin bias current, held high v en_mode = vcc 5 a i en_mode_hi en_mode pin bias current, held low v en_mode = gnd -5 a sw zero cross comparator v zero sw node zero cross comparator offset voltage 5 mv current limit i clim ocp pin current (out of pin) 19 24 30 a over temperature t sd over-temperature shutdown threshold (t j ) note 1 138 152 165 c t sd_hys over-temperature shutdown threshold hysteresis (t j ) note 1 26 c under voltage v fb_uvlo v fb undervoltage lockout threshold t a = 25 c 70 75 %v ref over voltage v fb_ovp v fb overvoltage trip threshold t a = 25 c 114 125 %v ref internal schottky diode v bst_diode internal bootstrap diode forward drop i bst_diode = 50ma 660 mv reverse leakage current t a = 25 c , v bst C v pvcc = 22v 20 a output stage r ds_hi high-side fet r ds i sw = - 2a 23 35 ? r ds_ lo low-side fet r ds i sw = - 2a 21 33 ? note1: this parameter is guaranteed by design but not tested in production(gbnt). downloaded from: http:///
copyright ? 2013 microsemi page 10 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet typical performance curves step response figure 5: step response in psm mode when v in = 5v figure 6: step response in psm mode when v in = 20v start up figure 7: start up and shut down, no load figure 8: startup when 12v bus is present and 5v is started up downloaded from: http:///
copyright ? 2013 microsemi page 11 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet typical performance curves short circuit inductor current output voltage input voltage ~65s uvp switching stops 1.8v 20v short applied on output figure 9: behavior under short circuit start up into full load inductor current output voltage input voltage startup behavior (into full load ) figure 10: start up into full load downloaded from: http:///
copyright ? 2013 microsemi page 12 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet typical performance curves efficiency & converter losses v o =1.8v v in =5v v in =12v v in =24v in pulse skip mode load current (a) efficiency (%) total converter losses (w) v in =5v v in =12v v in =24v figure 11: efficiency and converter losses in psm mode v out = 1.8v, fs = 300khz downloaded from: http:///
copyright ? 2013 microsemi page 13 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet typical performance curves efficiency output voltage 1.0v 1.2v 1.5v 1.8v 2.5v 3.3v inductor value 1.0h 1.5 h 1.5 h 2.2 h 3.3 h 3.3 h figure 12: efficiency in psm mode with 12v input @ 600khz switching frequency 0 0.5 1 1.5 2 2.5 3 3.5 50 55 60 65 70 75 80 85 90 95 100 0 1 2 3 4 5 6 7 8 total converter losses (w) efficiency (%) load current (a) efficiency and losses / vin = 12v, fs = 600khz vout = 1.0v eff vout = 1.2v eff vout = 1.5v eff vout = 1.8v eff vout = 2.5v eff vout = 3.3v eff vout = 1.0v loss vout = 1.2v loss vout = 1.5v loss vout = 1.8v loss vout = 2.5v loss vout = 3.3v loss downloaded from: http:///
copyright ? 2013 microsemi page 14 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet typical performance curves efficiency figure 13 : efficiency in psm mode with 12v input @ 300khz switching fre quency 0 10 20 30 40 50 60 70 80 90 100 1 10 100 1000 efficiency (%) load current (ma) efficiency (log) / vin = 12v, fs = 300khz vout=1v vout=1.2v vout=1.5v vout=1.8v vout=2.5v vout=3.3v downloaded from: http:///
copyright ? 2013 microsemi page 15 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet v o =1.8v vo -1.8 regulation = 100 1.8 regulation (%) load current (a) pulse skip mode activated v in =5v v in =12v v in =24v 1m timing resistor figure 14: regulation characteristics figure 15: frequency stability downloaded from: http:///
copyright ? 2013 microsemi page 16 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory symbol used in application information: v in - input voltage v out - output voltage i out - output current ? v ripple - output voltage ripple f s - working frequency ? i ripple - inductor current ripple design example the following is typical application for NX9548, th e schematic is figure 1. v in = 8 to 20v v out = 1.5v f s = 220khz i out = 7a ? v ripple <=60mv ? v droop <=60mv @ 3a step on -time and frequency calculation the constant on time control technique used in NX9548 delivers high efficiency, excellent transien t dynamic response making it a good candidate for step-down notebook applications. an internal one-shot timer turns on the high side driver with an on time which is proportional to the input supply v in as well inversely proportional to the output voltage v out . during this time, the output inductor charges the output capacitor increasing th e output voltage by the amount equal to the output ripple. once the timer turns off, the hdrv turns o ff and causes the output voltage to decrease until reaching the internal fb voltage of 0.75v on the ps m comparator. at this point the comparator trips causing the cycle to repeat itself. a minimum off time of 400ns is internally set. t = . 0 ? r v v i ? 0. v fs = v v i t l = v i ? v t i i i i = k i the equations setting the on time in second and frequency in hertz are as follows: in this application example, the r ton is chosen to be 1m ? , when v in = 20v, the t on is 3 42 ns and f s is around 220khz. output inductor selection the value of inductor is decided by the inductor ripple current and working frequency. larger inductor value normally means smaller ripple current , however if the inductance is chosen too large, it results in slow response and lower efficiency. the ripple current is a design freedom which can be de termined by the design engineer according to various application requirements. the inductor val ue can be calculated by using the following equations: where k is percentage of output current. in this example, inductor from coilcraft do5010h- 332 with l=3.3h is chosen. downloaded from: http:///
copyright ? 2013 microsemi page 17 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) i i = v i ? v t l = 0v ? . v 0ns . h ?v i = esr ?i i + ?i i f c current ripple is recalculated as below: = 1.738a output capacitor selection output capacitor value is basically determined by t he amount of the output voltage ripple allowed during steady state(dc) load condition as well as specification for the load transient. the optimum design may require a couple of iterations to satisf y both conditions. based on dc load condition the amount of voltage ripple during the dc load condition is determined by equation(5). where esr is the output capacitors' equivalent serie s resistance, c out is the value of output capacitors. typically poscap is recommended to use in NX9548's applications. the amount of the output voltage ripple is dominated by the first term in equation(5 ) and the second term can be neglected. esr i = ?v i ?i i = 0mv . a = . m n = esr ?i i ?v i n = m . a 0mv ?v < ?v a @step load ?i for this example, one poscap 2r5tpe330mc is chosen as output capacitor, the esr and inductor current typically determines the output voltage ripple. when v in reach maximum voltage, the output voltage ripple is in the worst case. if low esr is required, for most applications, mult iple capacitors in parallel are needed. the number of output capacitor can be calculate as the following: n = 0.70 the number of capacitor has to be round up to a integer. choose n =1. based on transient requirement typically, the output voltage droop during transien t is specified as downloaded from: http:///
copyright ? 2013 microsemi page 18 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) ?? ?? ? = ? ?? ? + ? ? ? ? ? ? ? = 0 ?? ? ? ?? ? ?? ? ? ? ? ? ? ? ?? ? ? ?? l i = esr c v ?i = esr c v ?i 0 the voltage droop during the transient is composed of two sections. one section is dependent on the e sr of capacitor, the other section is a function of th e inductor, output capacitance as well as input, outp ut voltage. for example, for the overshoot when load from high load to light load with a ? i step transient load, if assuming the bandwidth of system is high enough, the overshoot can be estimated as the following equation. where is a function of capacitor, etc. where where esr e and c e represents esr and capacitance of each capacitor if multiple capacitors are used in parallel. the above equation shows that if the selected outpu t inductor is smaller than the critical inductance, t he voltage droop or overshoot is only dependent on the esr of output capacitor. for low frequency capacit or such as electrolytic capacitor, the product of esr and capacitance is high and l ? l crit is true. in that case, the transient spec is mostly like to dependent on t he esr of capacitor. n = esr ?i ?v a + v l c ?v a = 0 if l l i l ?i v ? esr c if l l i l i = esr c v ?i = m 00f . v a = . h n = esr ?i ?v a = m . a 0mv = 0. in most cases, the output capacitor is multiple capacitors in parallel. the number of capacitors c an be calculated by the following: where for example, assume voltage droop during transient is 60mv for 3a load step. if one poscap 2r5tpe330mc(330 f, ? esr) is used, the critical inductance is given as: the selected inductor is 3.3h which is smaller than critical inductance. in that case, the output volt age transient mainly dependent on the esr. number of capacitors are: choose n=1. downloaded from: http:///
copyright ? 2013 microsemi page 19 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) f = esr c f w i = i d ? d d = t f based on stability requirement esr of the output apaito a t be chosen too low which will cause system instability . the zero caused by output capacitor's esr must satisfy the requirement as below: besides that, esr has to be big enough so that the output voltage ripple can provide enough voltage ramp to error amplifier through fb pin. if esr is too small, the error amplifier is unable to correctly d etect the ramp and the high side mosfet will be only turned off for the minimum time of 400ns . double pulsing and bigger output ripple will be observed. in summary, the esr of output capacitor has to be larg e enough to make the system stable, but also has to b e small enough to satisfy the transient and dc ripple requirements. input capacitor selection input capacitors are usually a mix of high frequenc y ceramic capacitors and bulk capacitors. ceramic capacitors bypass the high frequency noise, and bulk capacitors supply switching current to the mosfets. usually a 1 f ceramic capacitor is chosen to decouple the high frequency noise. the bulk input capacitors are determined by voltage rating and rms current rating. the input capacitors rms current can be calculated as: when v in = 22v, v out = 1.5v, i out = 8a, the resulting input rms current calculates to 2.05 a. r = r v v ? v for higher efficiency, low esr capacitors are recommended. one 10 f/x5r/25v and two 4.7 f/x5r /25v ceramic capacitors are chosen as input capacitors. output voltage calculation output voltage is set by reference voltage and external voltage divider. the reference voltage is fixed at 0.75v. the divider consists of two resisto rs so that the output voltage applied at the fb pin is 0.75v when the output voltage is at the desired va lue. the following equation applies to figure 16, which shows the relationship between v out , v ref and voltage divider. figure 16 . voltage divider where r 2 is part of the compensator, and the value of r 1 value can be set by voltage divider. downloaded from: http:///
copyright ? 2013 microsemi page 20 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) mode selection NX9548 can be operated in psm mode, ultrasonic psm mode, ccm mode and shutdown mode by applying different voltages to the en _mode pin. when vcc is applied to en_mode pin the NX9548 is in psm mode. the low side mosfet emulates the function of a diode when discontinuous continuous mode happens, often in light load conditions. in t his condition the inductor current crosses the zero ampere border and becomes negative current. when the inductor current reaches negative territory, th e low side mosfet is turned off and it takes longer f or the output voltage to drop, the high side mosfet waits longer to be turned on. at the same time regardless of the load level the on time of high si de mosfet remains constant. therefore the lighter load, the lower the switching frequency will be. however in ultrasonic psm mode, the lowest frequency is set to be 25khz to avoid audio frequen cy modulation. thus in psm mode this kind of reductio n of frequency maintains high efficiency even at ligh t loads. in ccm mode the inductor current zero-crossing sensing is disabled and the low side mosfet remains on even when inductor current becomes negative. this causes the efficiency to be lower compared with psm mode at light load, but frequency will remain constant. v w = ?i r i r + v w over current protection over current protection for NX9548 is achieved by sensing current through the low side mosfet. a typical internal current source of 24 a flows through an external resistor connected from ocp pin to sw node and sets the over current protection threshold . when the synchronous fet is on, the voltage at node sw is given as the voltage at pin ocp is given as when the voltage is below zero, the over current occurs as shown in figure below. figure 17. over voltage protection downloaded from: http:///
copyright ? 2013 microsemi page 21 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) i = i r r r = i r i = 0a m a = . k the over current limit can be set by the following equation. the typical low side mosfet r dson is ? at the ocp threshold, and the current limit is set at 10a, then: choose r = . k . power good output pgood output is an open drain output, a pull up resistor is needed. typically when softstart is fi nished and fb pin voltage is over 90% of v ref , the pgood pin is pulled to high after a 1.6ms delay. output over voltage protection typically when the fb pin voltag e exceeds 125% of v ref , the high side mosfet will be turned off and the low side mosfet will be latched on to discharge the output voltage. to resume the switching operation, a reset to vcc or en_mode is necessary. output under voltage protection when the fb pin voltage is typically under 70% of v ref , the high side and low side mosfet will be turned off. to resume the switching operation, vcc or en_mode has to be reset. setting switching frequency the NX9548 has a frequency setting resistor rfreq etwee pi 4 ad the iput ail. the current through this resistor sets the theoretical switching frequency of the regulator as shown in fig 18 and fig 19. both of these are the same, but the latter is on a log versus log scale to clarify the extremities. keep in mind that these are valid curves provided the minimum on-time (t onmin ), or the minimum off-time (t offmin ) of the cot regulator does not come into play, causing significant deviation from the frequency programmed as per fig 18 and fig 19. in all cases, if a t onmin or t offmin brickwall is encountered, the switching frequency will trail off negating typical expectations of constant frequency operation (in ccm mode). this veering away of frequency is discussed in more detail in the following section. if however that does not happen, the curves in figures 18 and 19 are nominal, and the typical spread as input voltage varies, is 10% (not including process variation which typically adds another 10% ). the equation to use (for nominal frequency) as verified on the bench is, (using fsw in hz, rfreq in ?): downloaded from: http:///
copyright ? 2013 microsemi page 22 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) figure 18: setting frequency (linear axes) figure 19: setting frequency (log axes) safe operating regions (for constant frequency in ccm mode) as mentioned above, if the t onmin or t = brickwalls are encountered, the typically constant frequency of any adaptive cot regulator is affected. though the egulato woks, all alulated peditios ae off since the frequency can drop abruptly at those brick wall contention points. in other words, the natural duty cycle demand cannot be met at the programmed frequency because t on = d/fsw is less than t onmin . therefore fsw must decrease as a result, otherwise output regulation would suffer. note that this behavior is not the pulse-skipping (power saving) mode, which only occurs at very light loads. this is definitely an avoidable mode, because it occurs even at max load. by lowering the frequency suddenly at max load, we run the risk of a huge increase in output ripple for example. in some cases, this reduced frequency can also cause inductor saturation and consequential field reliability issues. so these regions should be avoided by careful design. downloaded from: http:///
copyright ? 2013 microsemi page 23 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) a simple mathcad file was created, using a t onmin brickwall of 100ns (typically measured on the bench to be 80ns), and a t offmin (guaranteed max value) of 800ns to find this safe operating region. note that although the latter number seems large, it is not a device limitation. it is in fact an important design-in parameter for ensuring proper response of the cot regulator under abnormal operating conditions. it provides enough time for the inductor current to slew down under such strange conditions, rather than causes flux staircasing. the results of the mathcad file are presented in figures 20 to 25 which shows the limitations on input/output voltage combinations vis--vis switching frequency. figure 20: setting switching frequency correctly for v out = 0.75v, and its safe input operating region figure 21: setting switching frequency correctly for v out = 1.0v, and its safe input operating region figure 22: setting switching frequency correctly for v out = 1.2v, and its safe input operating region figure 23: setting switching frequency correctly for v = = 1.5v, and its safe input operating region downloaded from: http:///
copyright ? 2013 microsemi page 24 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet operation theory (continue) figure 24: setting switching frequency correctly for v out = 2.5v, and its safe input operating region figure 25: setting switching frequency correctly for v out = 3.3v, and its safe input operating region downloaded from: http:///
copyright ? 2013 microsemi page 25 rev. 1.1, 04/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa , 1 - 800 - 713 -4113, 949- 380 -6100, fax 949- 215 -4996 NX9548 4.5v to 24v, 8a adaptive constant on time (cot) synchronous buck regulator pr oduction datasheet package dimensions vqfn 5x5mm 32l with 3 exposed pads e d top a e side a1 a3 bottom d2 l e2 b e4 d3 e3 note: 1. dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm (.006) on any side. lead dimension shall not include solder coverage. 2. d imensions are in mm, inches are for reference only. dim m illimeters i nches min max min max a 0.800 1.000 0.035 a1 0 0.05 0 0.002 a3 0.203 ref 0.008 ref e 0.50bsc 0.020 d 4.950 5.050 0.195 0.199 e 4.950 5.050 0.195 0.199 d2 3.400 3.500 0.134 0.138 d3 1.475 1.575 0.058 0.062 l 0.350 0.450 0.014 0.018 e2 1.475 1.575 0.058 0.062 e3 1.475 1.575 0.058 0.062 e4 0.300x45 0.012x45 b 0.200 0.300 0.008 0.012 land pattern recommendation 3.50mm 0.65mm 1.575mm 0.30mm 0.30mmx45 0.50mm 5.45mm 5.45mm 2.725mm 1.575mm 1.575mm 1.575mm disclaimer this pcb land pattern recommendation is based on information available to microsemi by its suppliers. the actual land patte rn to be used could be different depending on the material s an d processes used in the pcb assembly, end user must account for this in th eir final layout. microsemi makes no warranty or representatio n of performance based on this recommended land pattern. production data C information contained in this document is proprietary to microsemi and is current as of publication date. this document may not be m odified in any way without the express written consent of microsemi. product processing does not n ecessarily include te sting of all parameters. microsemi reserves the right to change the confi guration and performance of the product and to discontinue product at any time. downloaded from: http:///

▲Up To Search▲

Price & Availability of NX9548

	To Download NX9548 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .