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p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 1 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator d e s c r i p t i o n NX7101 is a 340khz fixed frequency, current mode, pwm synchronous buck (step-down) dc- dc converter, capable of driving a 2a load with high efficiency, 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 package. k e y f e a t u r e s ? 2a synchronous step-down regulator ? operational input supply voltage range: 4.5v- 18v ? integrated upper nmos and lower nmos ? 340 khz switching frequency ? input uvlo ? enable ? programmable external soft- start ? cycle-by-cycle over-current protection ? over voltage protection ? frequency fold back under short condition a p p l i c a t i o n s ? set-top box ? lcd tvs ? notebook/netbook ? poe powered devices p r o d u c t h i g h l i g h t fb(5) sw(3) bst(1) NX7101 comp(6) gnd(4) in(2) vin out en(7) ss(8) r3 10k c3 3.9nf cout 2*22uf(6.3v,x5r) cin 2*10uf(25v,x5r) c4 0.1u c5 10nf l1 10uh r1 26.1k r2 10k r4 100k 3.3v 12v figure 1 C NX7101 typical application p a c k a g e o r d e r i n f o t a ( ? c) plastic soic 8 p in dm rohs compliant / pb-free -40 to 85 NX7101idm note: available in tape & reel. append the letters t r to the part number. (i.e. NX7101idm-tr) downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 2 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator a b s o l u t e m a x i m u m r a t i n g s supply input voltage (vin) ........................................................................... -0.3v to 20v switch voltage (sw) ..................................................................................................... 21v en .................................................................................................................. -0.3v to vin bst ................................................................................................... -0.3v to (vsw + 6v) comp, fb, ss ................................................................................................ . -0.3v to 6v maximum operating junction temperature .............................................................. 150 c storage temperature range ......................................................................... -65 ? c to 150c package peak temp. for solder reflow (40 seconds maximum exposure) ............... 2 60 c note: exceeding these ratings could cause damage to the d evice. all voltages are with respect to ground. currents are positive into, negative out of specified terminal . p a c k a g e p i n o u t 4 3 2 1 5 6 7 8 bst gnd sw vin fb comp en ss m s c 7 1 0 1 i d m x x x x dm p ackage (top view) dm p art m arking xxxx denote date code and lot identification rohs / pb-free 100% matte tin pin finish msl3 t h e r m a l d a t a dm plastic soic 8-pin thermal resistance - junction to a mbient , ? ja 105 ? c/w junction temperature calculation: t j = t a + (p d x ? ja ). the ? ja numbers are guidelines for the thermal performance of the d evice/pc-board system. all of the above assume no ambient airflow. f u n c t i o n a l p i n d e s c r i p t i o n pin name description 1 bst bootstrap pin. a minimum 10nf bootstrap capacitor is connected between the bs pin and sw pin. the voltage across the bootstrap capacitor drives the internal hi gh side nmos. 2 vin supply input pin. a capacitor should be connected between the in p in and gnd pin to keep the input voltage constant. 3 sw power switch output pin. this pin is connected to the inductor an d bootstrap capacitor. 4 gnd ground. 5 fb feedback pin. this pin is connected to an external resistor divi der to program the system output voltage. when the fb pin voltage exceeds 20% of the nominal regu lation value of 0.925v, the over voltage protection is triggered. when the fb pin voltage is bel ow 0.3v, the oscillator frequency is lowered to realize short circuit protection. 6 comp compensation pin. this pin is the output of the transconduct ance error amplifier and the input to the current comparator. it is used to compensate the control loop. conne ct a series rc network from this pin to gnd. in some cases, an additional capacitor from this pin to gn d pin is required. 7 en control input pin. forcing this pin above 2v enables the ic. forci ng this pin below 0. 75 v shuts down the ic. wh en the ic is in shutdown mode, all functions are disabled to dec rease the supply current below 1a. 8 ss soft-start control input pin. ss controls the soft start period. conne ct a capacitor from ss to gnd to set the soft-start period. a 0.1f capacitor sets the soft-start period to 15 ms. to disable the soft-start feature, leave ss unconnected. downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 3 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator r e c o m m e n d e d o p e r a t i n g c o n d i t i o n s parameter symbol NX7101 units min typ max input operating voltage v in 4. 5 18 v maximum output current i outmax 2 a operating ambient temperature t a - 40 85 c e l e c t r i c a l c h a r a c t e r i s t i c s unless otherwise specified, the following specifications a pply for v in = v en =12v, v out = 3.3v , t a = 25c. parameter symbol test conditions NX7101 units min typ max operating current quiescent current i q v fb = 1v , v en = 3v 0.86 1. 25 1. 4 ma shutdown current i shdn v en = 0v 0. 1 10 a uvlo v in uvlo threshold v uvlo v in rising 3.65 4.0 4.25 v hysteresis v hys 0. 2 v feedback feedback voltage v fb t a = - 40 c to 85c 0.907 0.925 0.9 43 v feedback bias current i fb v fb = 1v -0.1 0.1 a oscillator internal oscillator frequency f osc1 280 340 400 k hz short circuit oscillator frequency f osc2 100 k hz maximum duty cycle d max v fb = 0.85v 90 % minimum duty cycle d min v fb = 1v 0 % error amplifier error amplifier transconductance g ea 800 a /v voltage gain (1) a ea 400 v/v current sensing gain current sensing gain g cs 3.5 a/v soft-start soft-start current v ss = 0v 6 a soft-start time (1) t ss c ss = 0.1f 15 ms output stage high-side switch on resistance r dsonh i sw = 0.2a/ 0.7a 85 115 145 m? low -side switch on resistance r dsonl i sw = -0.2a/ -0.7a 75 105 135 m? high-side switch leakage current i leakh v in = 18v, v en = 0v, v sw = 0v 0.1 10 ua high-side switch current limit i limh 2.7 3.5 a low -side switch current limit i liml from drain to source 1.4 a en en shutdown threshold voltage v en 1.1 1.5 2 v en shutdown threshold voltage hysteresis (1) v enh 350 mv en lockout threshold voltage 2.2 2.5 2.7 v en lockout hysteresis 210 mv downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 4 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator e l e c t r i c a l c h a r a c t e r i s t i c s ( c o n t ) unless otherwise specified, the following specifications apply for v in = v en =12v, v out = 5v, t a = 25 c. parameter symbol test conditions NX7101 units min typ max protection over voltage protection threshold v fbov 1.1 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. s i m p l i f i e d b l o c k d i a g r a m vin sw gnd driver current sensing en osc 340k/90k pwm logic bias 1.5v sd soft start fb comp low side current limit fb uvlo 0.3v 0.925v slope compensation uvlo sd thermal shutdown vcc uvlo regulator ss bst 1.1v en 2.5v figure 2 C simplified block diagram downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 5 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator a p p l i c a t i o n c i r c u i t fb(5) sw(3) bst(1) NX7101 comp(6) gnd(4) in(2) vin out en(7) ss(8) c6330pf cout 1000uf,170mohm cin 10uf/25v c4 0.1u c510nf l122uh r1 41.2k r2 9.3k r4 100k figure 3 C 12v input, 5v output with electrolytic cap fb(5) sw(3) bst(1) NX7101 comp(6) gnd(4) in(2) vin out en(7) ss(8) r3 18k c3 3.9nf cout 2*22uf(10v,x5r) cin 10uf/25v c4 0.1u c5 10nf l1 15uh r1 41.2k r2 9.3k r4 100k figure 4 C 12v input, 5v output with ceramic cap downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 6 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator t y p i c a l w a v e f o r m s @ 2 5 c ( r e f e r t o f i g u r e 1 ) figure 5. dc operation at 2a figure 7. input power recycling figure 6. transient response figure 8. output short operation downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 7 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator t y p i c a l w a v e f o r m s @ 2 5 c ( r e f e r t o f i g u r e 3 ) figure 9. en and soft start 82.00% 84.00% 86.00% 88.00% 90.00% 92.00% 94.00% 0 500 1000 1500 2000 2500 iout(ma) efficiency(%) figure 10 . efficiency vs. iout(vin=12v,vout=5v) downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 8 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator t h e o r y o f o p e r a t i o n d etail d escription the NX7101 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 2a load current. the NX7101 has two internal n-mosfets to step down the voltage. the inductor current is determined by sensing the internal high-side mosfet current. 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 not only fast control loop response but also cycle- by -cycle current limit protection. when load current reaches its maximum output level when the inductor peak current triggers the high-side nmofet current limit. if fb pin voltage drops below 0.3v, the working frequency will be fold back 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. downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 9 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator a p p l i c a t i o n i n f o r m a t i o n 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 NX7101, the schematic is figure 1. v in = 12v v out =3.3v i out =2a o utput i nductor s election the selection of inductor value is based on inductor ripple current, power rating, working 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 efficiency. 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 i v f i k i ... (1) where k is between 0.2 to 0.4. in this design, k is set at 0.35 and 10h inductor value is chosen. in order to avoid output oscillation at light load, a minimum 8.2h inductor is required for all NX7101 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 conditions. the amount of voltage ripple during the dc load condition is determined by equation (2 ). ? ? ? ? ? ? ?? ripple ripple ripple s out i v esr i 8 f c .. . (2) where esr is the output capacitor s equivalent series resistance, c out is the value of output capacitor. typically when large value capacitors are selected such as aluminum electrolytic, poscap and oscon types ar e 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 th is type of capacitor is selected, the amount of capacitance per single unit is not sufficient to meet the transient specification, which results in the need for parallel configuration of multiple capacitors. in this design two 22f 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 noise. the bulk input capacitors are determined by the voltage rating and rms current rating. the rms current in the input capacitors can be calculated as: ? ? ? ? rms out out in i i d 1- d v d v ... (3) in this design two 10f 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. 925 v. 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. downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 10 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator a p p l i c a t i o n i n f o r m a t i o n ? 1 out ref 2 r v =v (1+ ) r ... (4) in this design r1 is 26.1k ? , r2 is 10k ? . compensator design the NX7101 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 3.5a/v, r load is the load resistor. the system itself has one pole 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 l out 1 f 2 r c ... (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= 2 r c ... (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= 2 a c ... (8) where g ea is error amplifier transconductance 800ua/v. the zero z2 set by r3 and c3 is given by the equation (9). ? ? ? ? z2 33 1 f= 2 r c ... (9) when aluminum electrolytic capacitors are chosen as output capacitors, the esr zero is much lower and extra capacitor c6 from comp pin to ground is needed to stabilize the system. NX7101 vout vref fb r1 r2 comp figure 11 voltage divider the pole p3 set by r3 and c6 is given by the equation (10). ? ? ? ? p3 36 1 f= 2 r c ... ( 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 10 h 22 fx2 6k 3.9nf none 2.5v 10 h 22 fx2 8k 3.9nf none 3.3v 10 h 22 fx2 10k 3.9nf none 5v 15 h 22 fx2 18k 3. 9nf none 2.5v 10 h 470 f al. 30m ? esr 85 k 250p f 150pf 5v 15 - 22 h 470 f al. 30m ? esr 150 k 220 nf 82pf downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 11 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator p a c k a g e d i m e n s i o n s dm plastic soic 8 pin a p g b k l f c d j m im m illimeters i nches min max min max a 4.700 5.100 0.185 0.201 b 3.800 4.000 0.150 0.157 g 1.270 bsc 0.050 bsc p 5.800 6.200 0.228 0.244 c 1.350 1.750 0.053 0.069 d 0.330 0.510 0.013 0.020 k 0.100 0.300 0.004 0.012 l 0.320 bsc 0.013 bsc j 0.190 0.250 0.007 0.010 f 0.450 0.800 0.017 0.031 m - 8 - 8 downloaded from: http:///
p roduction d ata s heet NX7101 microsemi analog mixed signal group one enterprise, aliso viejo, ca 92656 949- 380 -6100, fax 949- 215 -4996 page 12 w w w. microsemi . co m n n x x 7 7 1 1 0 0 1 1 copyright ? 2010 rev. 1.7 , 2014 - 06 - 24 2a, high voltage synchronous buck regulator n o t e s production data C information contained in this document is proprietary to microsemi and is current as of publication date. this document m ay not be modified in any way without the express written consent of microsemi. produ ct processing does not necessarily include testing of all parameters. microsemi reserve s the right to change the configuration and performance of the product and to discontinue produc t at any time. downloaded from: http:///

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