copyright ? 2013 microsemi page 1 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet description lx7157b is a step - down pwm regulator ic with i ntegrated high side p - ch mosfet and low side n - ch mosfet. the 2.2mhz switching frequency facilitat es small output filter components. the operational input voltage range of lx7157b is from 3v to 5.5v. lx7157b uses current mode operation with internal compensation allowing for fast transient response with minimum external components . lx7157b employs a pulse skip method at light load to improve the light load efficiency; as a result, the battery life is extended. the internal soft - start limits the inrush current. cycle -by- cycle current limit protects the regulator against over - current conditions. the lx7 157b operates in hiccup mode to further enhance the robustness of the converter for heavy over - load or short circuit fault, and it recovers automatically once the fault is cleared. the thermal protection shuts down the regulator under an over - temperature c ondition. features ? 3a step - down regulator ? input supply voltage range: 3v to 5.5v ? integrated pmos and nmos ? internal compensation ? internal slope compensation ? 2.2mhz switching frequency ? skip p ulse to improve light l oad efficiency ? input uvlo and ov protection ? enable & power good ? internal soft - start ? cycle -by- cycle over current protection ? hiccup mode operation under ocp ? rohs compliant, dfn 3.5mm x 3mm 12l applications ? set - top box and lcd tvs ? notebook/netbook ? server and workstations ? routers ? video cards ? pc peripherals ? poe powered devices pvin en gnd fb pgood sw lx7157b 1.2v @ 3a 100k 3 10f x5r 0.47 h 22f vin = 3v C 5.5v vout off on vcc mode 1f 110k 54.9k 9 11 10 12 3,4,13 1 2 7,8 5,6 0.01 f 10 figure 1: lx7157b typical application downloaded from: http:///
copyright ? 2013 microsemi page 2 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet pin configuration and pinout sw en vout gnd pvin fb gnd 1 2 34 5 7 8 9 10 11 pgood mode vcc 6 12 sw pvin epad (pin 13) (gnd) lx7157b dfn 3.5mm x 3mm 12l top view part marking : 7157b xxxx msc xxxx = date/lot code figure 2: lx7157b pinout ordering information ambient temperature type package part number packag ing type - 10 to 85c rohs compliant, pb - free dfn 3.5mm x 3mm 12l LX7157BCLD bulk LX7157BCLD - tr tape and reel downloaded from: http:///
copyright ? 2013 microsemi page 3 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet pin description pin number pin designator description 1 fb voltage feedback pin. connect to the output terminal through a resistor divider network to set the output voltage of the regulator to the desired voltage. the upper resistor of the divider is part of the closed loop stabilit y and must be selected properly to insure the stability of the regulator. table 1 shows the proper values of this network for selected output voltages. 2 vout output sense pin . 3, 4, 13 gnd ground pin. 5,6 sw switch- node pin. connect the output inductor between this pin and output capacitor. 7,8 pvin input voltage pin of the regulator. a minimum of 10f, x5r type ceramic capacitor must be connected as close as possible from this pin to gnd plane to insure proper operation. 9 vcc internal chip supply . decouple this pin to gnd with a 1f, x5r type (or better) ceramic capacitor and connect it via a 10 resistor to the input rail . 10 en pulling this pin higher than 1.7v will enable the regulator. when pulled below 0.4v, the regulator will turn off. 11 pgood power - good pin. this is an open - drain output and should be connected to a voltage rail (for example, vin) with an external pull- up resistor. during the power on, this pin switches from l ow to h igh state when fb voltage reaches above the power good threshold and the internal soft start has finished its operation. it will be pulled low , when the fb falls below the power good threshold minus the hysteresis. it will turn back on , when the pull fb rises above the threshold. 12 mode this pin should be connected to gnd; skip mode is enabled to improve the light load efficiency. downloaded from: http:///
copyright ? 2013 microsemi page 4 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet bloc k diagram shut down en mode clock pwm logic driver swgnd + - + - peak current over current protection soft start v ref fb g m1 + - dcm comparator pvin pgood pgood bias/ reference vcc pulse generation + - rs latch r s q internal compensation 9 10 11 12 1 3,4 5,6 7,8 uvlo, ovp, thermal shutdown epad vout 2 13 figure 3 : block diagram downloaded from: http:///
copyright ? 2013 microsemi page 5 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production 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 impact long - term operating reliability. min max units p vin , vcc, en, fb, pgood, vout , mode to gnd - 0.3 7 v sw to gnd - 0.3 7 v sw to gnd (shorter than 50ns) -2 7 v maximum operating junction temperature 150 c storage temperature range - 65 150 c lead soldering temperature (40s, reflow) 260 (+0, - 5) c esd protection at all i/o pins (hbm , note 1 ) -2 2 kv note 1: pvin & sw pins are esd sensitive. operating ratings performance is generally guaranteed over this range as further detailed below under electrical characteristics . min max units v cc, pvin (note 2) 3 5.5 v vout 0.8 1.8 v ambient temperature -10 85 c output current 0 3 a note 2 : minimum input voltage 3.5v is required in order to have 3a output current thermal properties thermal resistance min typ max units ja 46 c /w note: the ja numbers assume no forced airflow. junction temperature is calculated using t j = t a + (p d x ja ). in particular, ja is a function of the pcb construction . the stated number above is for a four - layer board in accordance with jesd - 51 (jedec). downloaded from: http:///
copyright ? 2013 microsemi page 6 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet electrical characteristics unless otherwise specified under conditions, the min and max ratings stated below apply over the entire specified operating range 0c t a 85c of the device . typ ical parameter refers to t j = 25 c . vcc = pvin = 5v, mode = gnd. symbol parameter conditions min typ max units operating c urrent i q input current i load = 0, v fb =0.8 v 700 a i in input current at shut down en = gnd 0.1 3 a vcc i nput uvlo v cc under voltage lockout vcc rising 2.6 v v hys uvlo hysteresis 80 mv feedback v ref feedback voltage internal reference t a = 25c 0.792 0.800 0.808 v -1 0 < t a < 8 5c 0.78 8 0.812 v i fb fb pin input current 100 na line regulation vin from 3 v to 5.5 v, i out = 1.5 a 0.5 % /v load regulation i load = 0 to 3 a, - 0.3 % /a output device r dson_h r dson of high side 60 m ? r dson_l r dson of low side 40 m ? i l current limit 5.3 a t sh thermal shut down threshold 150 c t h hysteresis 20 c pvin ovp ovp r rising threshold 6.5 v ovp f falling threshold 6.1 v fb uvlo v fbulvo fb uvlo threshold 70% v ref downloaded from: http:///
copyright ? 2013 microsemi page 7 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet symbol parameter conditions min typ max units oscillator frequency f sw pwm switching frequency range 2.2 m hz f fb switching frequency at f old back v fb < 0.3v 0.75 m hz soft start t ss soft start time from en high to vout reach regulation. 465 s t hiccup hiccup time v fb = 0.2v 11 ms en input en vih input high 1. 7 v en vil input low 0.4 v en h hysteresis 0. 1 v en ii input bias 0.01 1 a power - good v pg power - good high threshold v fb rising, i n percentage of output voltage set -point. 85 % v pghy hysteresis either v fb rising or falling 52 mv pgd filt power good filter 6 clock cycles pg rdson power - good internal fet r dson vcc = 5v 18 i pgd pgood fet leakage current 0.01 1 a downloaded from: http:///
copyright ? 2013 microsemi page 8 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet typical efficiency curve figure 4: efficiency curve 40% 50% 60% 70% 80% 90% 100% 0 500 1000 1500 2000 2500 3000 efficiency load current (ma) lx7157b efficiency / vin = 5v, vout = 1.2v downloaded from: http:///
copyright ? 2013 microsemi page 9 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet application information output voltage calculation output voltage is set by reference voltage and external voltage divider. the reference voltag e is fixed at 0.8v. the divider consists of the ratio of two resistors so that the output voltage applied at the fb pin is 0.8 v when the output voltage is at the desired value. the following equation and picture show the relationship between output voltage and voltage divider. v ref + - v out r 1 r 2 lx7157b figure 5: voltage divider ???? = ? ??? ? 1 + ? 1 ? 2 ? .. (1) the value of upper feedback resistor r 1 has to be set properly in order to have stable syst em. the recommended value of r 1 is shown in the table 1, r 2 can be derived from equation 1. output cap acitor r 1 ( ?) 2 x 10f (x5r, 6.3v) 80.6k 3 x 10f (x5r, 6.3v) 54.9k 2 x 22f (x5r, 6.3v) 49.9k 3 x 22f (x5r, 6.3v) 33.2k table 1: recommended upper resistor value of feedback resistor divider for typical application (l = 0.47h) downloaded from: http:///
copyright ? 2013 microsemi page 10 rev. 1.1, 01/10/2013 analog mixed signal group one enterprise aliso viejo, ca 92656 usa 1-800-713-4113, 1-949-380-6100, fax 1-949-215-4996 lx7157b 3v input, high frequency, 3a step- down converter production datasheet package dimension s 12 - pin plastic dfn 3 .5 mm x 3mm with exposed pad production data C 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 w ithout the express written consent of microsemi . product processing does not necessarily include testing of all parameters. microsemi reserves the right to change the configur ation and performance of the product and to discontinue product at any time. downloaded from: http:///
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