View SC121ULTRT_5402358.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

? 2010 semtech corporation power management 1 sc121 low voltage synchronous boost regulator features input voltage 0.7v to 4.5v minimum start-up voltage 0.85v output voltage fi xed at 3.3v; adjustable from 1.8v to 5.0v peak input current limit 1.2a output current at 3.3 v out 80ma with v in = 1.0v, 190ma with v in = 1.5v forced pwm operation at all loads effi ciency up to 94% internal synchronous rectifi er no forward conduction path during shutdown switching frequency 1.2mhz soft-start startup current limiting shutdown current 0.1a (typ) ultra-thin 1.5 2.0 0.6 (mm) mlpd-ut-6 package lead-free and halogen-free weee and rohs compliant applications mp3 players smart phones and cellular phones palmtop computers and handheld instruments pcmcia cards and memory cards digital cordless phones personal medical products wireless voip phones small motors ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description the sc121 is a high efficiency, low noise, synchronous step-up dc-dc converter that provides boosted voltage levels in low-voltage handheld applications. the wide input voltage range allows use in systems with single nimh or alkaline battery cells as well as in systems with higher voltage battery supplies. it features an internal 1.2a switch and synchronous rectifi er to achieve up to 94% effi ciency and to eliminate the need for an external schottky diode. the output voltage can be set to 3.3v with internal feedback, or to any voltage within the speci- fi ed range using a standard resistor divider. the sc121 operates exclusively in pulse width modulation (pwm) mode for low ripple and fi xed-frequency switching. output disconnect capability is included to reduce leakage current, improve effi ciency, and eliminate external com- ponents sometimes needed to disconnect the load from the supply during shutdown. low quiescent current is maintained with a high 1.2mhz operating frequency. small external components and the space saving mlpd-ut-6, 1.52.00.6 (mm) package make this device an excellent choice for small handheld applications that require the longest possible battery life. l1 c in 3.3v single cell (1.2v) in out lx en fb sc121 c out gnd typical application circuit april 13, 2010
sc121 2 ordering information device package SC121ULTRT (1)(2) mlpd-ut-6 1.52 sc121evb evaluation board notes: (1) available in tape and reel only. a reel contains 3,000 devices. (2) lead-free packaging, only. device is weee and rohs compliant, and halogen-free. pin confi guration mlpd-ut top view t in en gnd fb out lx 6 5 4 1 2 3 mlpd-ut; 1.5 2, 6 lead ja = 84c/w mlpd-ut; 1.52, 6 lead yw = date code 121 yw marking information mlpd-ut
sc121 3 exceeding the above specifi cations may result in permanent damage to the devic e or device malfunction. operation outside of the parameters specifi ed in the electrical characteristics section is not recommended. notes: (1) tested according to jedec standard jesd22-a114. (2) calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer fr4 pcb with thermal vias under the exposed pad per jesd51 standards. absolute maximum ratings in, out, lx, fb (v) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +6.0 en (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to (v in + 0.3) esd protection level (1) (kv) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 recommended operating conditions ambient temperature range (c) . . . . . . . . . . . . -40 to +85 v in (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.7 to 4.5 v out (v) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 to 5.0 thermal information thermal res. mlpd, junction-ambient (2) (c/w) . . . . . . . 84 maximum junction temperature (c) . . . . . . . . . . . . . . . 150 storage temperature range (c) . . . . . . . . . . . -65 to +150 peak ir refl ow temperature (10s to 30s) (c) . . . . . . +260 unless otherwise noted v in = 2.5v, c in = c out = 22f, l 1 = 4.7h, t a = -40 to +85c. typical values are at t a = 25c. parameter symbol conditions min typ max units input voltage range v in 0.7 4.5 v minimum startup voltage v in-su i out < 1ma, t a = 0c to 85c 0.85 v shutdown current i shdn t a = 25c, v en = 0v 0.1 1 a operating supply current (1) i q i out = 0, v en = v in 3.5 ma internal oscillator frequency f osc 1.2 mhz maximum duty cycle d max 90 % minimum duty cycle d min 20 % output voltage v out v fb = 0v 3.3 v adjustable output voltage range v out_rng for v in such that d min < d < d max 1.8 5.0 v regulation feedback reference volt- age accuracy (internal or external programming) v reg-ref -1.5 1.5 % fb pin input current i fb v fb = 1.2v 0.1 a startup time t su 1ms electrical characteristics
sc121 4 parameter symbol conditions min typ max units p-channel on resistance r dsp v out = 3.3v 0.6 n-channel on resistance r dsn v out = 3.3v 0.5 n-channel current limit i lim(n) v in = 3.0v 0.9 1.2 a p-channel startup current limit i lim(p)-su v in > v out , v en > v ih 150 ma lx leakage current pmos i lxp t a = 25c, v lx = 0v 1 a lx leakage current nmos i lxn t a = 25c, v lx = 3.3v 1 a logic input high v ih v in = 3.0v 0.85 v logic input low v il v in = 3.0v 0.2 v logic input current high i ih v en = v in = 3.0v 1 a logic input current low i il v en = 0v -0.2 a electrical characteristics (continued) notes: (1) quiescent operating current is drawn from out while in regula tion. the quiescent operating current projected to in is app roximately i q (v out /v in ).
sc121 5 0.1 0.2 0.5 1 2 5 10 20 50 100 200 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, t a = 25 c 0.1 0.2 0.5 1 2 5 10 20 50 100 200 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, v in = 1.2v typical characteristics v out = 1.8v v in = 1.6v v in = 1.2v v in = 0.8v t a = 25c t a = 85c t a = C40c 0 50 100 150 200 250 1.76 1.78 1.8 1.82 i out (ma) v out (v) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, t a = 25 c 0 50 100 150 200 250 1.76 1.78 1.8 1.82 i out (ma) v out (v) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, v in = 1.2v v in = 1.6v v in = 1.2v v in = 0.8v t a = 25c t a = C40c 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.76 1.78 1.8 1.82 v in (v) v out (v) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, i out = 1ma 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.76 1.78 1.8 1.82 v in (v) v out (v) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, i out = 50ma t a = 25c t a = 85c t a = C40c t a = 25c t a = 85c t a = C40c t a = 85c effi ciency vs. i out (v out = 1.8v) effi ciency vs. i out (v out = 1.8v) load regulation (v out = 1.8v) load regulation (v out = 1.8v) line regulation low load (v out = 1.8v) line regulation high load (v out = 1.8v) t a = C40c t a = 85c
sc121 6 -50 -25 0 25 50 75 100 1.76 1.78 1.8 1.82 junction temperature ( o c) v out (v) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, i out = 1ma -50 -25 0 25 50 75 100 1.76 1.78 1.8 1.82 junction temperature ( o c) v out (v) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf, i out = 50ma typical characteristics v out = 1.8v (continued) v in = 1.2v v in = 0.8v v in = 1.6v v in = 1.2v v in = 0.8v temperature reg. low load (v out = 1.8v) temperature reg. high load (v out = 1.8v) 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 0 50 100 150 200 250 300 350 v in (v) i out (ma) r 1 = 499k , r 2 = 1m , l = 4.7 h, c fb = 22pf t a = 25c t a = 85c t a = C40c max. i out vs. v in (v out = 1.8v)
sc121 7 typical characteristics v out = 3.3v 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) fb grounded, l = 4.7 h, v in = 2v 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) fb grounded, l = 4.7 h, t a = 25 c v in = 2.0v v in = 1.0v v in = 2.95v t a = 25c t a = 85c t a = C40c 0 50 100 150 200 250 300 350 400 450 500 3.2 3.22 3.24 3.26 3.28 3.3 3.32 3.34 i out (ma) v out (v) fb grounded, l = 4.7 h, v in = 2v 0 50 100 150 200 250 300 350 400 450 500 3.2 3.22 3.24 3.26 3.28 3.3 3.32 3.34 i out (ma) v out (v) fb grounded, l = 4.7 h, t a = 25 c v in = 2.0v v in = 2.95v v in = 1.0v t a = 25c t a = 85c t a = C40c 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.22 3.24 3.26 3.28 3.3 3.32 3.34 v in (v) v out (v) fb grounded, l = 4.7 h, i out = 90ma 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.22 3.24 3.26 3.28 3.3 3.32 3.34 v in (v) v out (v) fb grounded, l = 4.7 h, i out = 1ma t a = 25c t a = 85c t a = C40c t a = 25c t a = 85c t a = C40c effi ciency vs. i out (v out = 3.3v) effi ciency vs. i out (v out = 3.3v) load regulation (v out = 3.3v) load regulation (v out = 3.3v) line regulation high load (v out = 3.3v) line regulation low load (v out = 3.3v)
sc121 8 typical characteristics v out = 3.3v (continued) -50 -25 0 25 50 75 100 3.2 3.22 3.24 3.26 3.28 3.3 3.32 3.34 junction temperature ( o c) v out (v) fb grounded, l = 4.7 h, i out = 90ma -50 -25 0 25 50 75 100 3.2 3.22 3.24 3.26 3.28 3.3 3.32 3.34 junction temperature ( o c) v out (v) fb grounded, l = 4.7 h, i out = 1ma v in = 2.0v v in = 2.95v v in = 1.0v v in = 2.0v v in = 2.95v v in = 1.0v temperature reg. high load (v out = 3.3v) temperature reg. low load (v out = 3.3v) 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 0 50 100 150 200 250 300 350 400 450 500 v in (v) i out (ma) fb grounded, l = 4.7 h max. i out vs. v in (v out = 3.3v) t a = 85c t a = C40c t a = 25c
sc121 9 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, t a = 25 c typical characteristics v out = 4.0v 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, v in = 2.4v v in = 2.4v v in = 1.2v v in = 3.6v t a = 25c t a = 85c t a = C40c 0 50 100 150 200 250 300 350 400 450 500 550 3.85 3.9 3.95 4 4.05 4.1 i out (ma) v out (v) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, t a = 25 i out (ma) v out (v) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, v in = 2.4v v in = 3.6v v in = 2.4v v in = 1.2v t a = 25c t a = 85c t a = C40c 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 3.85 3.9 3.95 4 4.05 4.1 v in (v) v out (v) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, i out = 1ma 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 3.85 3.9 3.95 4 4.05 4.1 v in (v) v out (v) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, i out = 110ma t a = 25c t a = 85c t a = C40c t a = 25c t a = 85c t a = C40c effi ciency vs. i out (v out = 4.0v) effi ciency vs. i out (v out = 4.0v) load regulation (v out = 4.0v) load regulation (v out = 4.0v) line regulation low load (v out = 4.0v) line regulation high load (v out = 4.0v)
sc121 10 typical characteristics v out = 4.0v (continued) -50 -25 0 25 50 75 100 3.85 3.9 3.95 4 4.05 4.1 junction temperature ( o c) v out (v) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, i out = 1ma -50 -25 0 25 50 75 100 3.85 3.9 3.95 4 4.05 4.1 junction temperature ( o c) v out (v) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf, i out = 110ma v in = 3.6v v in = 2.4v v in = 1.2v v in = 3.6v v in = 2.4v v in = 1.2v temperature reg. low load (v out = 4.0v) temperature reg. high load (v out = 4.0v) 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 0 50 100 150 200 250 300 350 400 450 500 v in (v) i out (ma) r 1 = 976k , r 2 = 412k , l = 4.7 h, c fb = 22pf max. i out vs. v in (v out = 4.0v) t a = 25c t a = 85c t a = C40c
sc121 11 typical characteristics v out = 5.0v 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, t a = 25 c 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 0 10 20 30 40 50 60 70 80 90 100 i out (ma) efficiency (%) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, v in = 3.6v v in = 2.2v v in = 3.2v v in = 4.2v t a = 25c t a = 85c t a = C40c 0 50 100 150 200 250 300 350 400 450 500 550 4.8 4.85 4.9 4.95 5 5.05 i out (ma) v out (v) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, t a = 25 c 0 50 100 150 200 250 300 350 400 450 500 550 4.8 4.85 4.9 4.95 5 5.05 i out (ma) v out (v) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, v in = 3.6v v in = 4.2v v in = 2.2v v in = 3.2v t a = 25c t a = 85c t a = C40c 0.5 1 1.5 2 2.5 3 3.5 4 4.5 4.8 4.85 4.9 4.95 5 5.05 v in (v) v out (v) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, i out = 1ma 0.5 1 1.5 2 2.5 3 3.5 4 4.5 4.8 4.85 4.9 4.95 5 5.05 v in (v) v out (v) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, i out = 85ma t a = 25c t a = 85c t a = C40c t a = 25c t a = 85c t a = C40c effi ciency vs. i out (v out = 5.0v) effi ciency vs. i out (v out = 5.0v) load regulation (v out = 5.0v) load regulation (v out = 5.0v) line regulation low load (v out = 5.0v) line regulation high load (v out = 5.0v) v in = 1.2v v in = 1.2v
sc121 12 typical characteristics v out = 5.0v (continued) -50 -25 0 25 50 75 100 4.8 4.85 4.9 4.95 5 5.05 junction temperature ( o c) v out (v) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, i out = 1ma -50 -25 0 25 50 75 100 4.8 4.85 4.9 4.95 5 5.05 junction temperature ( o c) v out (v) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf, i out = 85ma v in = 4.2v v in = 2.2v v in = 3.2v v in = 4.2v v in = 2.2v v in = 3.2v temperature reg. low load (v out = 5.0v) temperature reg. high load (v out = 5.0v) 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 50 100 150 200 250 300 350 400 450 500 v in (v) i out (ma) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf t a = 25c t a = 85c t a = C40c max. i out vs. v in (v out = 5.0v) v in = 1.2v v in = 1.2v
sc121 13 typical characteristics (continued) time = (400ns/div) v out = 3.3v, v in = 1.5v, i out = 50ma v out ripple (10mv/div) i l (100ma/div) v lx (5v/div) time = (100s/div) v out = 3.3v, v in = 1.5v, t a =25c i out = 40ma to 140ma (50ma/div) v out (100mv/div) ac coupled load transient pwm operation 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 20 40 60 80 100 v in (v) i out (ma) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 20 40 60 80 100 120 140 160 v in (v) equivalent r load ( ) r 1 = 931k , r 2 = 294k , l = 4.7 h, c fb = 22pf t a = 25c t a = 85c t a = C40c t a = 25c t a = 85c t a = C40c startup max load current vs. v in (any v out ) startup min load res. vs. v in (any v out ) v out = 3.3v, i out = 1ma 0.6 0.65 0.7 0.75 0.8 0.85 0.9 -40 -20 0204060 80 100 temperature (c) startup voltage (v) min. start-up voltage vs. temperature (any v out )
sc121 14 pin descriptions mlpd pin # pin name pin function 1 lx switching node connect an inductor from the input supply to this pin. 2 gnd signal and power ground. 3in battery or supply input requires an external 10f bypass capacitor (capacitance evaluated while under v in bias) for normal operation. 4 en enable digital control input active high. 5fb feedback input connect to gnd for preset 3.3v output. a voltage divider is connected from out to gnd to adjust output from 1.8v to 5.0v. 6 out output voltage pin requires an external 10f bypass capacitor (capacitance evaluated while under v out bias) for normal operation. t thermal pad thermal pad is for heat sinking purposes connect to ground plane using multiple vias not connected internally.
sc121 15 block diagram - + v ref 1.2 v start-up oscillator gate drive and logic control error amp. - + slope comp. oscillator and slope generator pwm control - + - + - + - + + - current amplifier p lim amp. 1.7 v v out comp. in en fb gnd lx out bulk bias + - n lim amplifier pwm comp. output voltage selection logic
sc121 16 detailed description the sc121 is a synchronous step-up fi xed frequency pulse width modulated (pwm) dc-dc converter utilizing a 1.2mhz fi xed frequency current mode architecture. it is designed to provide output voltages in the range 1.8v to 5.0v from an input voltage as low as 0.7v, with a (output unloaded) start up input voltage of 0.85v. quiescent current consumption is typically 3.5ma, entirely into the out pin during boost regulation. (see footnote 1 of the electrical characteristics table.) the regulator control circuitry is shown in the block diagram. it is comprised of a programmable feedback controller, an internal 1.2mhz oscillator, an n- channel field eff ect transistor (fet) between the lx and gnd pins, and a p-channel fet between the lx and out pins. the current fl owing through both fets is monitored and limited as required for startup and pwm operation. an external inductor must be connected between the in pin and the lx pin. output voltage selection the sc121 output voltage can be programmed to an internally preset value or it can be programmed with external resistors. the output is internally programmed to 3.3v when the fb pin is connected to gnd. any output voltage in the range 1.8v to 5.0v can be programmed with a resistor voltage divider between out and the fb pin as shown in figure 1. the values of the resistors in the voltage divider network are chosen to satisfy the equation v r r 1 191 . 1 v 2 1 out ? ? 1 ? u a large value of r 2 , ideally 590k or larger, is preferred for stability for v in within approximately 400mv of v out . for lower v in , lower resistor values can be used. the values of r 1 and r 2 can be as large as desired to achieve low quies- cent current. c fb = 22pf is recommended to improve transient response. the enable pin the en pin is a high impedance logical input that can be used to enable or disable the sc121 under processor control. v en < 0.2v will disable regulation, set the lx pin in a high-impedance state (turn off both fet switches), and turn on an active discharge device to discharge the output capacitor via the out pin. synchronous rectifi er (p-channel fet) bulk switching prevents pass-through conduction from lx to out while disabled. v en > 0.85v will enable the output. the startup sequence from the en pin is identical to the startup sequence from the appli- cation of input power. applications information l1 c in v out in out lx en fb sc121 c out gnd r 1 r 2 c fb figure 1 output voltage feedback circuit
sc121 17 applications information (continued) pwm operation the pwm cycle runs at a fi xed frequency (f osc = 1.2mhz), with a variable duty cycle (d). pwm operation continually draws current from the input supply, except for low output loads in which current fl ows periodically from, and back into, the input. during the on-state of the pwm cycle, the n-channel fet is turned on, grounding the inductor at the lx pin. this causes the current fl owing from the input supply through the inductor to ground to ramp up. during the off -state, the n-channel fet is turned off and the p-channel fet (synchronous rectifi er) is turned on. this causes the inductor current to flow from the input supply through the inductor into the output capaci- tor and load, boosting the output voltage above the input voltage. the cycle then repeats to re-energize the inductor. ideally, the steady state (constant load) duty cycle is determined by d = 1 C (v in /v out ), but must be greater in practice to overcome dissipative losses. the sc121 pwm controller constrains the value of d such that 0.20 < d < 0.90 (approximately). the average inductor current during the off -state multi- plied by (1-d) is equal to the average load current. the inductor current is alternately ramping up (on-state) and down (off -state) at a rate and amplitude determined by the inductance value, the input voltage, and the on-time (t on = dt, t = 1/f osc ). therefore, the instantaneous induc- tor current will be alternately larger and smaller than the average. if the average output current is sufficiently small, the minimum inductor current can ramp down to zero during the off -state. discontinuous mode operation (where both fets turn off as the inductor current reaches zero) is not supported in the sc121, since this would result in a fi nite positive minimum current from input to output, which would cause an uncontrolled rise in output voltage in this case. instead, the inductor current will reverse for the remainder of the off-state, flowing from the output capacitor into the out pin, through the p-channel fet to the lx pin, and through the inductor to the input capaci- tor. negative inductor current ripple allows regulation even with zero output load. the energy returned to the input capacitor is not wasted, but dissipative conduction losses will inevitably occur. the minimum on-time limitation imposes a minimum boost ratio, so if v in is too close to v out (v in > v out C 400mv, approximately), v out will rise above the programmed value for a suffi ciently small output load. a higher output load requires a higher duty cycle to overcome dissipative losses, such that regulation at programmed v out will eventually be restored. but this regulation-restoration load rises rapidly with v in , so this phenomenon can be benefi cially exploited in only rare circumstances. if opera- tion with high v in and low load is required, please consider using the sc120, a pin compatible dual mode (pwm/ psave) boost converter. the sc120 will support zero load in psave mode for v in up to v out + 150mv. regulator startup, short circuit protection, and current limits the sc121 permits power up at input voltages from 0.85v to 4.5v. soft-start startup current limiting of the internal switching n-channel and p-channel fet power devices protects them from damage in the event of a short between out and gnd. as the output voltage rises, pro- gressively less-restrictive current limits are applied. this protection unavoidably prevents startup into an exces- sive load. upon enable, the p-channel fet between the lx and out pins turns on with its current limited to approximately 150ma, the short-circuit output current. when v out approaches v in (but is still below 1.7v), the n-channel current limit is set to 350ma (the p-channel limit is dis- abled), the internal oscillator turns on (approximately 200khz), and a fixed 75% duty cycle pwm operation begins. (see the section pwm operation.) when the output voltage exceeds 1.7v, fi xed frequency pwm opera- tion begins, with the duty cycle determined by an n- channel fet peak current limit of 350ma. when this n-channel fet startup current limit is exceeded, the on- state ends immediately and the off-state begins. this determines the duty cycle on a cycle-by-cycle basis. when v out is within 2% of the programmed regulation voltage, the n-channel fet current limit is raised to 1.2a, and normal voltage regulation pwm control begins. once normal voltage regulation pwm control is initiated, the output becomes independent of v in and output regu- lation can be maintained for v in as low as 0.7v, subject to the maximum duty cycle and peak current limits. the
sc121 18 duty cycle must remain between 20% and 90% for the device to operate within specifi cation. note that startup with a regulated active load is not the same as startup with a resistive load. the resistive load output current increases proportionately as the output voltage rises until it reaches programmed v out /r load , while a regulated active load presents a constant load as the output voltage rises from 0v to programmed v out . note also that if the load applied to the output exceeds an applicable v out Cdependent startup current limit or duty cycle limit, the criterion to advance to the next startup stage may not be achieved. in this situation startup may pause at a reduced output voltage until the load is reduced further. output overload and recovery the pwm steady state duty cycle is determined by d = 1 C (v in /v out ), but must be somewhat greater in prac- tice to overcome dissipative losses. as the output load increases, the dissipative losses also increase. the pwm controller must increase the duty cycle to compensate. eventually, one of two overload conditions will occur, determined by v in , v out , and the overall dissipative losses due to the output load current. either the maximum duty cycle of 90% will be reached or the n-channel fet 1.2a (nominal) peak current limit will be reached, which eff ec- tively limits the duty cycle to a lower value. above that load, the output voltage will decrease rapidly and in reverse order the startup current limits will be invoked as the output voltage falls through its various voltage thresh- olds. how far the output voltage drops depends on the load voltage vs. current characteristic. a reduction in input voltage, such as a discharging battery, will lower the load current at which overload occurs. lower input voltage increases the duty cycle required to produce a given output voltage. and lower input voltage also increases the input current to maintain the input power, which increases dissipative losses and further increases the required duty cycle. therefore an increase in load current or a decrease in input voltage can result in output overload. please refer to the max. i out vs. v in typical characteristics plots for the condition that best matches the application. once an overload has occurred, the load must be decreased to permit recovery. the conditions required for overload recovery are identical to those required for suc- cessful initial startup. component selection the sc121 provides optimum performance when a 4.7h inductor is used with a 10f output capacitor. diff erent component values can be used to modify input current or output voltage ripple, improve transient response, or to reduce component size or cost. inductor selection the inductance value primarily aff ects the amplitude of inductor peak-to-peak current ripple ( i l ). reducing inductance increases i l and raises the inductor peak current, i l-max = i l-avg + i l /2, where i l-avg is the inductor current averaged over a full on/off cycle. i l-max is subject to the n-channel fet current limit i lim(n) , therefore reducing the inductance may lower the output overload current threshold. increasing i l also lowers the inductor minimum current, i l-min = i l-avg C i l /2, thus raising the load current threshold below which inductor negativeCpeak current becomes zero. equating input power to output power and noting that input current is equal to inductor current, average the inductor current over a full pwm switching cycle to obtain in out out avg l v i v 1 i u u k where is effi ciency. neglecting the n-channel fet r ds-on and the inductor dcr, for duty cycle d, and with t = 1/f osc , l t d v dt v l 1 i in dt 0 in on l u u ' 3 this is the change in i l during the on-state. during the o -state, again neglecting the p-channel fet r ds-on and the inductor dcr, d 1 l t v v dt v v l 1 i out in t dt out in off l u ' 3 applications information (continued)
sc121 19 note that this is a negative quantity, since v out > v in and 0 < d < 1. for a constant load in steady-state, the inductor current must satisfy i l-on + i l-off = 0. substituting the two expressions and solving for d, obtain d = 1 C v in /v out . using this expression, and the positive valued expression i l = i l-on for current ripple amplitude, obtain expanded expression for i l-max and i l-min . in out out in in out out max,min l v v v v l 2 t v i v i u u u r k u u from this result, obtain an alternative expression for i l . in out out in min l max l l v v v v l t i i i u u ' the inductor selection should consider the n-channel fet current limit for the expected range of input voltage and output load current. the largest i l-avg will occur at the expected smallest v in and largest i out . determine the largest expected i l . then for the largest expected i l-avg , ensure that the n-channel fet current limit is not exceed. that is, for the minimum n-channel fet current limit, worst case inductor tolerance, highest expected output current, and lowest expected v in , ensure that i l-max = i l-avg + i l /2 < i lim(n) . many of these equations include the parameter , effi - ciency. effi ciency varies with v in , i out , and temperature. estimate using the plots provided in this datasheet, or from experimental data, at the operating condition of interest. any chosen inductor should have low dcr compared to the r ds-on of the fet switches to maintain efficiency, though for dcr << r ds-on , further reduction in dcr will provide diminishing benefi t. the inductor i sat value should exceed the expected i l-max . the inductor self-resonant fre- quency should exceed 5f osc . any inductor with these properties should provide satisfactory performance. l = 4.7h should perform well for most applications. the following table lists the manufacturers of recom- mended inductor options. the specifi cation values shown are simplifi ed approximations or averages of many device parameters under various test conditions. see manufac- turers documentation for full performance data. manufacturer/ part # value (h) dcr () rated current (ma) tolerance (%) dimensions lxwxh (mm) murata lqm31pn4r7m00 4.7 0.3 700 20 3.2 x 1.6 x 0.95 coilcraft xfl2006-472 4.7 0.7 500 20 2 x 2 x 0.6 capacitor selection input and output capacitors must be chosen carefully to ensure that they are of the correct value and rating. the output capacitor requires a minimum capacitance value of 10f at the programmed output voltage to ensure sta- bility over the full operating range. this must be consid- ered when choosing small package size capacitors as the dc bias must be included in their derating to ensure this required value. for example, a 10f 0805 capacitor may provide suffi cient capacitance at low output voltages but may be too low at higher output voltages. therefore, a higher capacitance value may be required to provide the minimum of 10f at these higher output voltages. low esr capacitors such as x5r or x7r type ceramic capacitors are recommended for input bypassing and output filtering. low-esr tantalum capacitors are not recommended due to possible reduction in capacitance seen at the switching frequency of the sc121. ceramic capacitors of type y5v are not recommended as their tem- perature coeffi cients make them unsuitable for this appli- cation. the following table lists recommended capacitors. for smaller values and smaller packages, it may be neces- sary to use multiples devices in parallel. manufacturer/ part number value (f) rated volt- age (vdc) type case size case height (mm) murata grm21br60j226me39b 22 6.3 x5r 0805 1.25 murata grm31cr71a226ke15l 22 10 x7r 1206 1.6 murata grm185r60g475me15 4.7 4 x5r 0603 0.5 tdk c2012x5r1a226m 22 10 x5r 0805 0.85 taiyo yuden jmk212bj226mg-t 22 20 x5r 0805 1.25 applications information (continued)
sc121 20 gnd v in c in sc121 fb en in lx l x r 2 (2 nd layer) out gnd 7.0mm 5.2mm c fb r 1 v out c out figure 4 layout drawing pcb layout considerations poor layout can degrade the performance of the dc-dc converter and can contribute to emi problems, ground bounce, and resistive voltage losses. poor regulation and instability can result. the following simple design rules can be implemented to ensure good layout: place the inductor and fi lter capacitors as close to the device as possible and use short wide traces between the power components. route the output voltage feedback path away from the inductor and lx node to minimize noise and magnetic interference. ? ? maximize ground metal on the component side to improve the return connection and thermal dissipation. separation between the lx node and gnd should be maintained to avoid cou- pling capacitance between the lx node and the ground plane. use a ground plane with several vias connecting to the component side ground to further reduce noise interference on sensitive circuit nodes. a suggested layout is shown in figure 4. ? ? applications information (continued)
sc121 21 (laser mark) indicator pin 1 1 n 2 min aaa bbb b e l n d a1 a dim millimeters nom dimensions max nom inches min max a a1 d1 e1 .035 .035 .026 .031 0.80 0.65 0.90 0.90 .055 -- 1.40 e .079 2.00 - - bxn a2 (.006) (.152) .055 .063 1.40 1.60 .075 .083 1.90 2.10 a2 lxn d e e1 d1 notes: controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as terminals. 2. 1. .003 .007 6 .010 .059 .000 .020 0.08 0.25 6 .012 0.18 .024 .002 0.00 0.50 0.30 1.50 0.05 0.60 .004 0.10 0.50 bsc .020 bsc 0.30 .012 .016 .014 0.35 0.40 aaa c seating plane a bbb c a b b e c - - outline drawing mlpd-ut-6 1.5x2
sc121 22 land pattern mlpd-ut-6 1.5x2 k h .031 .051 0.80 1.30 .106 .020 .012 .030 2.70 0.30 0.75 0.50 (.077) .047 1.20 (1.95) notes: 2. thermal vias in the land pattern of the exposed pad shall be connected to a system ground plane. functional performance of the device. failure to do so may compromise the thermal and/or inches dimensions g k h x y p z c dim millimeters (c) g z p x r .006 0.15 r 3. y 1. controlling dimensions are in millimeters (angles in degrees). this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met.
semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805) 498-2111 fax: (805) 498-3804 www.semtech.com contact information sc121 23 ? semtech 2010 all rights reserved. reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. the information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. no liability will be accepted by the publisher for any conse- quence of its use. publication thereof does not convey nor imply any license under patent or other industrial or intellec- tual property rights. semtech assumes no responsibility or liability whatsoever for any failure or unexpected operation resulting from misuse, neglect improper installation, repair or improper handling or unusual physical or electrical stress including, but not limited to, exposure to parameters beyond the specifi ed maximum ratings or operation outside the specifi ed range. semtech products are not designed, intended, authorized or warranted to be suitable for use in life- support applications, devices or systems or other critical applications. inclusion of semtech products in such applications is understood to be undertaken solely at the customers own risk. should a customer purchase or use semtech products for any such unauthorized application, the customer shall indemnify and hold semtech and its offi cers, employees, subsidiaries, affi liates, and distributors harmless against all claims, costs damages and attorney fees which could arise. notice: all referenced brands, product names, service names and trademarks are the property of their respective owners.

▲Up To Search▲

Price & Availability of SC121ULTRT

	To Download SC121ULTRT Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .