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NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 1 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 description NX7102 is a 340khz fixed frequency, current mode, pwm synchronous buck (step-down) dc- dc converter, capable of driving a 3a load with high ef ficiency, excellent line and load regulation. the device integrates n-channel power mosfet switches with low on-resistance. current mode control provides fast transient response and cycle-by-cycle current limit. the controller is equipped with output over-voltage protection which protects the ic under a open load condition. additional safety features include under voltage lock-out (uvlo), programmable soft-start and over-temperature protection (otp) to protect the circuit. this ic is available in soic-8 with exposed pad package. key features ? 3a synchronous step-down regulator ? operational input supply voltage range: 4.75v-18v ? integrated upper nmos and lower nmos ? 340khz switching frequency ? input uvlo ? enable ? programmable external soft- start ? cycle-by-cycle over-current protection ? over voltage protection ? frequency fold back under short condition applications ? set-top box ? lcd tv?s ? notebook/netbook ? poe powered devices product highlight fb(5) sw(3) bst(1) NX7102 comp(6) gnd(4) in(2) vin out en(7) ss(8) r3 10k c3 5.6nf cout 2*22uf(10v,x5r) cin 2x10uf(25v,x5r) c4 0.1u c5 10nf l1 10uh r1 26.1k r2 10k r4 100k 3.3v 12v pad r5 20k figure 1 ? 12v input, 3.3v output with ceramic cap package order info t a ( c) plastic soic 8 pin with exposed pad de rohs compliant / pb-free -40 to 85 NX7102ide note: available in tape & reel. append the letters ?tr? to the part number. (i.e. NX7102ide-tr)
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 2 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 absolute maximum ratings supply input voltage (v in) ............................................................................ -0.3 v to 20v switch voltage (sw) ............................................................................. -1v to (vin + 1v) en ................................................................................................... -0.3v to (vin + 0.3v) bst ................................................................................................... -0.3v to (vsw + 6v) comp, fb, ss ................................................................................................. -0.3v to 6v maximum operating junc tion temper ature ............................................................... 150 c storage temperat ure rang e ........................................................................ -65 c to 150 c package peak temp. for solder reflow (40 seconds maximum exposure) ................ 260c note: exceeding these ratings could cause damage to the device. all voltages are with respect to ground. currents are positive into, negative out of specified terminal . package pin out 4 3 2 1 5 6 7 8 bst gnd sw vin fb comp en ss m s c 7 1 0 2 i d e x x x x de p ackage (top view) de p art m arking ?xxxx? denote date code and lot identification rohs / pb-free 100% matte tin pin finish thermal data de plastic soic 8-pin with exposed pad thermal resistance - junction to a mbient , ja 75 c/w junction temperature calculation: t j = t a + (p d x ja ). the ja numbers are guidelines for the thermal performan ce of the device/pc-board system. all of the above assume no ambient airflow. functional pin description pin name description 1 bst bootstrap pin. a minimum 10nf boot strap capacitor is connected be tween the bs pin and sw pin. the voltage across the bootstrap capacitor drives the internal high side nmos. 2 vin supply input pin. a capacitor should be connec ted between the in pin and gnd pin to keep the input voltage constant. 3 sw power switch output pin. this pin is co nnected to the inductor and bootstrap capacitor. 4 gnd ground. 5 fb feedback pin. this pin is connected to an extern al resistor divider to program the system output voltage. when the fb pin voltage exceeds 20% of the nominal regulation va lue of 0.925v, the over voltage protection is triggered. when the fb pin vo ltage is below 0.3v, the oscillator frequency is lowered to realize short circuit protection. 6 comp compensation pin. this pin is t he output of the transconductance er ror amplifier and the input to the current comparator. it is used to compensate the control loop. connect a series rc network from this pin to gnd. in some cases, an additional capacitor from this pin to gnd pin is required. 7 en control input pin. forcing this pin above 1.5v enables the ic. forcing this pin below 0.4v shuts down the ic. when the ic is in shutdown mode, al l functions are disabled to decrease the supply current below 1 a. 8 ss soft-start control input pin. ss controls the soft start period. connect a capacitor from ss to gnd to set the soft-start period. a 0.1 f capacitor sets the soft-start per iod to 9ms. to disable the soft- start feature, l eave ss unconnected.
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 3 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 recommended operating conditions parameter symbol NX7102 units min typ max input operating voltage v in 4.75 18 v maximum output current i outmax 3 a operating ambient temperature t a -40 85 c electrical characteristics unless otherwise specified, the following specifications apply for v in = v en =12v, v out = 5v, t a = 25 c. parameter symbol test conditions NX7102 units min typ max ` operating current quiescent current i q v fb =1v 0.8 1.0 1.2 ma shutdown current i shdn v en =0v 1 10 a ` uvlo v in uvlo threshold v uvlo v in rising 3.65 4.05 4.45 v hysteresis v hys 0.2 v ` feedback feedback voltage v fb t a = -40 c to 85 c 0.900 0.925 0.950 v feedback bias current i fb v fb =1v -0.1 0.1 a ` oscillato r internal oscillator frequency f osc1 300 340 380 khz short circuit oscillator frequency f osc2 65 90 115 khz maximum duty cycle d max v fb =0.8v 83 90 97 % minimum duty cycle d min v fb =1v 0 % minimum on time (1) t on 180 ns ` error amplifie r error amplifier transconductance g ea 560 800 970 a/v voltage gain (1) a ea 560 v/v ` current sensing gain current sensing gain g cs 4 5.2 6.4 a/v ` soft-start soft-start current 6 a soft-start time t ss c ss =0.1uf 9 ms ` output stage high-side switch on resistance r dsonh 70 100 130 mohm low-side switch on resistance r dsonl 60 90 120 mohm high-side switch leakage current i leakh v in =18v, v en =0v, v sw =0v 0.1 10 a high-side switch current limit i limh 4.3 5.5 6.7 a low-side switch current limit i liml 0.85 1.45 2.05 a ` en high threshold v enh 1.5 v low threshold v enl 0.4 v
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 4 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 electrical characteristics (cont) unless otherwise specified, the following specifications apply for v in = v en =12v, v out = 5v, t a = 25 c. parameter symbol test conditions NX7102 units min typ max ` protection over voltage protection threshold v fbov 1 1.1 1.2 v fb short circuit protection 0.23 0.3 0.41 v thermal shutdown threshold t otsd 160 c thermal shutdown hysteresis t hys 30 c notes: 1) guaranteed by design, not tested. simplified block diagram vin sw gnd driver current sensing en osc 340k/90k pwm logic bias 1.1v sd soft start fb comp low side current limit fb uvlo 0.3v 0.925v slope compensation uvlo sd therrnal shutdown vcc uvlo regualtor ss bst 1.3v figure 2 ? simplified block diagram
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 5 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 application circuit fb(5) sw(3) bst(1) NX7102 comp(6) gnd(4) in(2) vin out en(7) ss(8) c6 2200pf cout 1000uf,170mohm cin 2x(10uf,25v) c4 0.1u c5 10nf l1 22uh r1 42.2k r2 9.53k pad r4 100k r5 20k figure 3 ? 12v input, 5v output with electrolytic cap fb(5) sw(3) bst(1) NX7102 comp(6) gnd(4) in(2) vin out en(7) ss(8) r3 10k c3 5.6nf cout 2*22uf(10v,x5r) cin 2*10uf(25v,x5r) c4 0.1u c5 10nf l1 10uh r1 42.2k r2 9.53k pad r4 100k r5 20k figure 4 ? 12v input, 5v output with ceramic cap
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 6 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 typical waveforms @ 25c (refer to figure 3) figure 5. dc operation at 3a figure 7. start up with no load figure 6. transient response figure 8. input power recycling
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 7 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 typical waveforms @ 25c (refer to figure 3) figure 9. start into 2a resistive load figure 10. output short operation
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 8 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 theory of operation d etail d escription the NX7102 is a current-mode, pwm synchronous step- down dc-dc converter with 340khz fixed working frequency, it can convert input voltages from 4.75v to 18v down to an output voltage as low as 0.925v, and supply up to 3a load current. the NX7102 has two internal n-mosfet to step down the voltage. the inductor current is determined by sensing the internal high-side mosfet curr ent. the output of current sense amplifier is summed with the slope compensation signal to avoid subharmonic oscillation at duty cycles greater than 50%. the combined signal is then compared with the error amplifier output to generate the pwm signal. current mode control provides no only fast control loop response but also cycle-by-cyc le current limit protection. when load current reaches its maximum output level when the inductor peak current triggers high-side nmofet current limit. if fb pin voltage drops below 0.3v, the working frequency will be fold back to typically 90khz to protect chip from run-away. when fb pin voltage exceeds 1.1v, the over voltage protection is triggered. the high side mosfet is turned off. once the ovp condition is gone, the chip will resume the operation following soft-start. the soft-start time is programmable through the ss pin. in order to have desired soft-start time, however when vin rises very fast, it is necessary to add a resistor divider connected between the vin and en pin, so that system will not start until vin reaches 4v. the typical resistor value on en pin is shown in figure 4 for vin 8v application.
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 9 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 application information s ymbol u sed i n a pplication i nformation : 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 d esign e xample the following is typical application for NX7102, the schematic is figure 1. v in = 12v v out =3.3v i out =3a o utput i nductor s election the selection of inductor value is based on inductor ripple current, power rating, workin g frequency and efficiency. a larger inductor value normally means smaller ripple current. however if the inductance is chosen too large, it results in slow response and lower effi ciency. usually the ripple current ranges from 20% to 40 % of the output current. this is a design freedom which can be determined by the design engineer according to various application requirements. the inductor value can be calculated by using the following equations: = = in out out out ripple in s ripple output v-v v 1 l ivf iki ... (1) where k is between 0.2 to 0.4. in this design, k is set at 0.23 and 10uh inductor value is chosen. in order to avoid output oscillation at light load, a minimum 8.2uh inductor is required for all NX7102 application. o utput capacitor s election output capacitor is basically decided by the 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 satisfy both condition. the amount of voltage ripple during the dc load condition is determined by equation (2). =+ ripple ripple ripple sout i v esr i 8f c ... (2) where esr is the output capacitor?s equivalent series resistance, c out is the value of output capacitor. typically when large value capac itors are selected such as aluminum electrolytic, poscap and oscon types are used, the amount of the output voltage ripple is dominated by the first term in equation(2) and the second term can be neglected. if ceramic capacitors are chosen as output capacitors, both terms in equation (2) need to be evaluated to determine the overall ripple. usually when this type of capacitor is selected, the amount of capacitance per single unit is not sufficient to meet the transient specification, which results in parallel configuration of multiple capacitors. in this design two 22uf 6.3v x5r ceramic capacitors are chosen as output capacitors. input capacitor s election input capacitors are usually a mix of high frequency ceramic capacitors and bulk capacitors. ceramic capacitors bypass the high frequency noise, and bulk capacitors supply current to the mosfets. usually 1uf ceramic capacitor is chosen to decouple the high frequency noi se. the bulk input capacitors are determined by voltage rating and rms current rating. the rms current in the input capacitors can be calculated as: = = rms out out in ii d1-d v d v ... (3) in this design two 10uf 25v x5r ceramic capacitors are chosen. o utput v oltage c alculation output voltage is set by reference voltage and external voltage divider. the reference voltage is fixed at 0.925v. the divider consists of two ratioed resistors so that the output voltage applied at the fb pin is 0.925v when the output voltage is at the desired value. the following equation and picture show the relationship between and voltage divider. NX7102 vout vref fb r1 r2 comp figure 5 voltage divider
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 10 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 application information 1 out ref 2 r v=v (1+) r ... (4) in this design choose r1 26.1k , choose r2 10k . compensator design the NX7102 uses peak current mode control to provide fast transient and simple compensation. the dc gain of close loop can be estimated by the equation (5). fb ea cs load out v gain=a g r v ... (5) where a ea is error amplifier voltage gain 560v/v, g cs is current sensing gain 5.2a/v, r load is the load resistor. the system itself has one pol e p1, one zero z1 and double pole p double at half of switching frequency f s . the system pole p1 is set by output capacitor and output load resistor. the calculation of this pole is given by the equation (6). = p1 lout 1 f 2rc ... (6) the system zero z1 is set by output capacitor and esr of output capacitor. the calculation of this zero is given by the equation (7). z1 esr out 1 f= 2rc ... (7) the crossover frequency is recommended to be set at 1/10 th of switching frequency. in order to achieve this desired crossover frequency and make system stable, the resistor r3 and the capacitor c3 is needed in typical applications which use ceramic capacitors as output capacitors. the pole p2 set by output resistance of error amplifier and c3 is given by the equation (8). ea p2 ea 3 g f= 2ac ... (8) where g ea is error amplifier tr ansconductan ce 800ua/v. the zero z2 set by r3 and c3 is given by the equation (9). z2 33 1 f= 2rc ... (9) when aluminum electrolytic cap acitors are chosen as output capacitors, the esr zero is mu ch lower and extra capacitor c6 from comp pin to ground is needed to stabilize the system. the pole p3 set by r3 and c6 is given by the equation (10). p3 36 1 f= 2rc ... (10) the compensation values for typical output voltage application are given in the table below. v out l c out r3 c3 c6 1.8v 8.2uh 22ufx2 4.02k 5.6nf none 2.5v 10uh 22ufx2 5.11k 5.6nf none 3.3v 10uh 22ufx2 6.49k 5.6nf none 5v 10uh 22ufx2 10k 5.6nf none 2.5v 10uh 470uf al. 30m esr 40.2k 390pf 220pf 5v 10- 15uh 470uf al. 30m esr 150k 220pf 120pf
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 11 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 package dimensions de plastic soic 8 pin with exposed pad
NX7102 p roduction d ata s heet microsemi analog mixed signal group 11861 western avenue, garden grove, ca. 92841, 714-898-8121, fax: 714-893-2570 page 12 www. microsemi . com 3a high voltage synchroous buck converter tm ? copyright ? 2010 rev.1.3, 2011-04-22 NX7102 notes production data ? information contained in this document is proprietary to microsemi and is current as of publication date. this document may not be modified in any way without the express written consent of microsemi. product processing does not necessarily include testing of all parameters. microsemi reserves th e right to change the configuration and performance of the product and to discontinue product at any time.

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