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power management dual channel 2.5mhz, 1.8a synchronous step-down regulator SC284 features v in range 2.9 C 5.5v v out selectable 0.8 - 3.3v up to .8a output current for each channel ultra-small footprint, < mm height solution switching frequency 2.5mhz efciency up to 94% excellent light load efciency low output noise across load range excellent transient response start up into pre-bias output 00% duty-cycle low dropout operation shutdown current <a internal soft start input under-voltage lockout output over-voltage, current limit protection over-temperature protection adjustable output voltage package 3 x 3 x 0.6(mm) ut20 temperature range -40 to +85c lead-free, halogen-free, and rohs/weee compliant applications desktop computing set-top box lcd tv network cards printer ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? description the SC284 is a dual channel .8a synchronous step- down regulator designed to operate with an input voltage range of 2.9 to 5.5 volts. each channel ofers ffteen pre-determined output voltages via four control pins programmable from 0.8 to 3.3 volts. the control pins allow for on-the-fy voltage changes, enabling system designers to implement dynamic power savings. the SC284 is also capable of adjusting the output voltage via an external resistor divider. the device operates with a fxed 2.5mhz oscillator frequency, allowing the use of small surface mount external components. connecting ctl0 ctl3 to logic low forces the device into shutdown mode reducing the supply current to less than a. connecting any of the control pins to logic high enables the converter and sets the output voltage according to table . other features include under- voltage lockout, soft-start to limit inrush current, and over-temperature protection. the SC284 is available in a 3 x 3 x 0.6 (mm) mlpq-ut20 package and has a rated temperature range of -40 to +85c. typical application circuit revision 2.0 sc 284 ctl 0 a ctl 1 a ctl 2 a ctl 3 a ctl 0 a ctl 1 a ctl 2 a ctl 3 a ctl 0 b ctl 1 b ctl 2 b ctl 3 b ctl 0 b ctl 1 b ctl 2 b ctl 3 b avina v ina c avina 10 nf r avina 1 pvina avinb v inb c avinb 10 nf r avinb 1 pvinb pgndb agndb pgnda agnda c ina 10 f c inb 10 f c outa 22 f lxa vouta v outa l a 2 . 2 h l b 2 . 2 h c outb 22 f lxb voutb v outb www.datasheet.co.kr datasheet pdf - http://www..net/
2 SC284 pin confguration ordering information device package SC284ultrt ()(2) 3 x 3 x 0.6(mm) mlpq-ut20 SC284evb (3) evaluation board notes: () available in tape and reel only. a reel contains 3,000 devices. (2) available in lead-free package only. device is fully weee and rohs compliant and halogen-free. (3) please specify the default vouta & voutb when ordering. tablef1fCfoutputfvoltagefsettings ctl3_ ctl2_ ctl1_ ctl0_ output voltage 0 0 0 0 shutdown 0 0 0 0.80 0 0 0 .00 0 0 .025 0 0 0 .05 0 0 .20 0 0 .25 0 .30 0 0 0 .50 0 0 .80 0 0 2.20 0 2.50 0 0 2.60 0 2.80 0 3.00 3.30 3fxf3fxf0.6(mm)fmlpq-ut20 ja f40c/w marking information yywwf=fdatecode xxxxf=fsemtechflotfnumber top view 1 2 3 4 t pvina ctl 0 a agnda avina 5 6 7 8 9 10 p g n d b pvinb ctl 0 b agndb avinb ctl 1 b 15 14 13 12 11 16 17 18 19 20 ctl 1 a l x b v o u t b c t l 3 a c t l 2 a c t l 2 b p g n d a l x a c t l 3 b v o u t a 284 yyww xxxx www.datasheet.co.kr datasheet pdf - http://www..net/
3 SC284 electrical characteristics exceeding the absolute maximum ratings may result in permanent damage to the device and/or device malfunction. operation outside of the parameters specifed in the electrical characteristics section is not recommended. notes: () 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 jesd5 standards. (2) tested according to jedec standard jesd22-a 4-b. unless specifed: vina= vinb= 5.0v, vouta= voutb= .50v, c ina =c inb = 0f, c oa =c ob = 22f, l= 2.2h, -40c t j + 25 c. unless otherwise noted typical values are t a = +25 c. parameter symbol conditions min typ max units under-voltage lockout uvlo rising vina, vinb 2.65 2.75 2.85 v hysteresis 240 300 mv output voltage tolerance () v out channel a & b; vin= 2.9 C 5.5v; i out =0a -2.0 +2.0 % current limit i limit channel a & b; peak lx current 2.25 3.0 3.75 a supply current i q channel a & b; no load, per channel 0 ma shutdown current i shdn ctl 0-3 = gnd, per channel 0 a high side switch resistance (2) r dson_p channel a & b; i lx = 00ma, t j = 25 c 95 m low side switch resistance (2) r dson_n channel a & b; i lx = - 00ma, t j = 25 c 65 l x leakage current (2) i lk(lx) channel a & b; vin= 5.5v; lx= 0v; ctl 0-3 = gnd 0 a channel a & b; vin= 5.5v; lx= 5.0v; ctl 0-3 = gnd -0 - load regulation v load-reg channel a & b; vin= 5.0v; i out =ma C .8a 0.5 % oscillator frequency f osc channel a & b 2.0 2.5 3.0 mhz soft-start time t ss channel a & b; i out = .8a 850 s absolute maximum ratings vina and vinb supply (v) . . . . . . . . . . . . . . . . . . -0.3 to +6.0 lxa, lxb (v) . . . . . . . . . . . . . - to vin+ , -3 (20ns max), 6 max vouta, voutb (v) . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to vin+0.3 ctlxa/b pins (v) . . . . . . . . . . . . . . . . . . . . -0.3 to (v out + 0.3) peak ir reflow temperature (c) . . . . . . . . . . . . . . . . . . . . . . . . 260 esd protection level (2) (kv) . . . . . . . . . . . . . . . . . . . . . . . . . 3kv recommended operating conditions vina and vinb supply (v) . . . . . . . . . . . . . . . . . . . . 2.9 to 5.5 maximum output current each channel (a) . . . . . . . . . . .8 temperature range (c) . . . . . . . . . . . . . . . . . . . . . . . . . -40 to +85 thermal information thermal resistance, junction to ambient () (c/w) . . . . . . . . 40 maximum junction temperature (c) . . . . . . . . . . . . . . . . . + 50 storage temperature range ( c ) . . . . . . . . . . . . . . . . . -65 to + 50 www.datasheet.co.kr datasheet pdf - http://www..net/
4 SC284 parameter symbol conditions min typ max units foldback holding current i cl_hold average lx current, vout= .5v 240 ma average lx current, vout=3.3v 30 ma ctlx input current (2) i ctl_ channel a & b; ctl 0-3 =vin or gnd -2.0 2.0 a ctlx input high threshold v ctlx_hi channel a & b .2 v ctlx input low threshold v ctlx_lo channel a & b 0.4 v v out over voltage protection v ovp channel a & b 5 % thermal shutdown temperature t sd channel a & b (3) 60 c thermal shutdown hysteresis t sd_hys channel a & b (3) 0 c electrical characteristics (continued) notes: ( ) the output voltage tolerance includes output voltage accuracy, voltage drift over temperature and the line regulation. (2) the negative current means the current fows into the pin and the positive current means the current fows out from the pin. (3) the thermal shutdown for both channel a and b is independent from each other. www.datasheet.co.kr datasheet pdf - http://www..net/
5 SC284 typical characteristics efciencyfvs.floadfcurrent totalflossf(perfchannel)fvs.floadfcurrent loadfregulation circuit conditions: c in = 0uf/6.3v; c out = 22uf/6.3v, unless otherwise noted, l= 2.2uh (toko: 27as-2r2m). dropoutfvoltagefinf100%fdutyfcyclefoperation dropout voltage of 100% duty cycle operation 0 50 100 150 200 250 300 350 400 450 500 0.0 0.3 0.6 0.9 1.2 1.5 1.8 output current (a) dropout voltage (mv) t a = 25c l= 1071as-2r2m (dcr= 60m ? _max) l= 1127as-2r2m (dcr=48m ? _max) uvlofrisingfthresholdfvariation uvlo rising threshold variation -1.0% -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% -40 -15 10 35 60 85 ambient temperature (c) variation i out = 0a uvlo hysteresis variation -5% -4% -3% -2% -1% 0% 1% 2% 3% 4% 5% -40 -15 10 35 60 85 ambient temperature (c) variation i out = 0a uvlofhysteresisfvariation efficiency 60% 65% 70% 75% 80% 85% 90% 95% 100% 0.0 0.3 0.6 0.9 1.2 1.5 1.8 output current (a) efficiency (%) v in =5.0v;v out =3.3v t a =25c v in =5.0v;v out =1.5v v in =3.3v;v out =1.5v total loss 0 200 400 600 800 1000 0.0 0.3 0.6 0.9 1.2 1.5 1.8 output current (a) loss (mw) t a =25c v in =5.0v;v out =3.3v v in =5.0v;v out =1.5v v in =3.3v;v out =1.5v load regulation -1.0% -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 0.0 0.3 0.6 0.9 1.2 1.5 1.8 output current (a) load regulation t a =25c v in =5.0v;v out =3.3v v in =5.0v;v out =1.5v v in =3.3v;v out =1.5v www.datasheet.co.kr datasheet pdf - http://www..net/
6 SC284 linefregulation linefregulationfvs.ftemperature r ds(on) fvariationfvs.finputfvoltage r ds(on) fvariationfvs.ftemperature ffffffffffffff switchingffrequencyfvariationfvs.finputfvoltage ffffffffffff switchingffrequencyfvariationfvs.ftemperature rdson (p & n) variation over line -10% -5% 0% 5% 10% 15% 20% 25% 30% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) variation i lx = 100ma t a = 25c n-channel p-channel rdson (p & n) variation over temperature -20% -15% -10% -5% 0% 5% 10% 15% 20% -40 -15 10 35 60 85 ambient temperature (c) variation v in = 5.0v i lx = 100ma n-channel p-channel switching frequency variation over line -5% -4% -3% -2% -1% 0% 1% 2% 3% 4% 5% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) variation i out = 0a t a = 25c v out = 3.3v v out = 1.5v line regulation ove line -1.0% -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 2.5 3.0 3.5 4.0 4.5 5.0 5.5 input voltage (v) regulation i out = 0a t a = 25c v out = 3.3v v out = 1.5v line regulation over temperature -1.0% -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% -40 -15 10 35 60 85 ambient temperature (c) regulation v out = 1.5v i out = 0a switching frequency variation -1.0% -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% -40 -15 10 35 60 85 ambient temperature (c) variation v in = 5.0v i out = 0a typical characteristics (continued) circuit conditions: c in = 0uf/6.3v; c out = 22uf/6.3v, unless otherwise noted, l= 2.2uh (toko: 27as-2r2m). www.datasheet.co.kr datasheet pdf - http://www..net/
7 SC284 outputfvoltagefripplef(v out =1.5v) outputfvoltagefripple f(v out =3.3v) outputfvoltagefripple f(v out =1.5v) outputfvoltagefripple f(v out =3.3v) v out 10mv/div v lx 2v/div i lx 1a/div 500ns/div output voltage ripple (v out =1.5v) v in =3.3v i out =1.8a typical waveforms circuit conditions: c in = 0uf/6.3v; c out = 22uf/6.3v, l= 2.2uh (toko: 27as-2r2m). v out 10mv/div v lx 2v/div 500ns/div output voltage ripple (v out =1.5v) v in =5.0v i out =1.8a i lx 1a/div v out 10mv/div v lx 2v/div 500ns/div output voltage ripple (v out =3.3v) v in =5.0v i out =0a i lx 0.5a/div v out 10mv/div v lx 2v/div 500ns/div output voltage ripple (v out =3.3v) v in =5.0v i out =1.8a i lx 1a/div v out 100mv/div i out 1a/div 50s/div transient response (v out =1.5v) v in =5.0v i out =0a to 1a v out 100mv/div i out 500ma/div 50s/div transient response (v out =3.3v) v in =5.0v i out =0a to 1a transientfresponsef( v out =1.5v;f 0aftof1aftof0a) transientfresponsef( v out =3.3v;f 0aftof1aftof0a) www.datasheet.co.kr datasheet pdf - http://www..net/
8 SC284 v out 0.5v/div 50s/div start up (v out =1.5v) v in =5.0v r out =1k ? v ctlx 2v/div v in 2v/div typical waveforms (continued) circuit conditions: c in = 0uf/6.3v; c out = 22uf/6.3v, l= 2.2uh (toko: 27as-2r2m). v out 0.5v/div 200s/div start up (v out =1.5v) v in =5.0v r out =0.83 ? (1.8a) v ctlx 2v/div v in 2v/div v out 0.5v/div 200s/div start up (v out =1.5v), en=vin v in 2v/div v in =5.0v r out =1k ? v out 0.5v/div 200s/div start up (v out =1.5v), en=vin v in 2v/div v in =5.0v r out =0.83 ? (1.8a) v out 1v/div 100s/div start up (v out =3.3v) v in =5.0v r out =1k ? v ctlx 2v/div v in 2v/div v out 1v/div 200s/div start up (v out =3.3v) v in =5.0v r out =1.83 ? (1.8a) v ctlx 2v/div v in 2v/div startfupf(enable)( v out =1.5v) startfupf(powerfupfv in =v ctlx )f ( v out =1.5v) startfupf(enable)( v out =1.5v) startfupf( powerfupfv in =v ctlx )f ( v out =1.5v) startfupf(enable)( v out =3.3v) startfupf(enable)( v out =3.3v) www.datasheet.co.kr datasheet pdf - http://www..net/
9 SC284 typical waveforms (continued) circuit conditions: c in = 0uf/6.3v; c out = 22uf/6.3v, l= 2.2uh (toko: 27as-2r2m). shutdownf(disable)( v out =1.5v) shutdownf(disable)( v out =3.3v) 200s/div shutdown-disable (1.5v) v out 1v/div v ctlx 2v/div v in 2v/div v in =5.0v r out =1.5 ? 200s/div shutdown-disable (3.3v) v out 1.5v/div v ctlx 2v/div v in 2v/div v in =5.0v r out =3.3 ? v out 1.5v/div 200s/div start up (v out =3.3v), en=vin v in 2v/div v in =5.0v r out =1k ? v out 1.5v/div 200s/div start up (v out =3.3v), en=vin v in 2v/div v in =5.0v r out =1.83 ? (1.8a) startfupf( powerfupfv in =v ctlx )f ( v out =3.3v) startfupf( powerfupfv in =v ctlx )f ( v out =3.3v) www.datasheet.co.kr datasheet pdf - http://www..net/
0 SC284 pin descriptions pin # pin name pin function p vin a channel a input supply voltage for the converter power stage and internal circuitry. 2 agnda ground connection for internal circuitry connect directly to pgnda. 3 avina power supply for internal circuitry must be connected to pvina using an r-c flter of and 0nf. 4 ctl0a channel a control bit 0, see table for decoding. this pin has a m internal pull-down resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 5 ctl a channel a control bit , see table for decoding. this pin has a m internal pull-down resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 6 ctl2a channel a control bit 2, see table for decoding. this pin has a m internal pulldown resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 7 ctl3a channel a control bit 3, see table for decoding. this pin has a m internal pull-down resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 8 voutb output voltage sense pin of channel b 9 pgndb channel b ground connection for converter power stage and internal circuitry. 0 lxb switching node of channel b connect an inductor between this pin and the output capacitor. p vin b channel b input supply voltage for the converter power stage and internal circuitry. 2 agndb ground connection for internal circuitry connect directly to pgndb. 3 avinb power supply for internal circuitry must be connected to pvinb using an r-c flter of and 0nf. 4 ctl0b channel b control bit 0, see table for decoding. this pin has a m internal pulld-own resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 5 ctl b channel b control bit - see table for decoding. this pin has a m internal pull-down resistor. this resistor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 6 ctl2b channel b control bit 2, see table for decoding. this pin has a m internal pull-down resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 7 ctl3b channel b control bit 3, see table for decoding. this pin has a m internal pull-down resistor. this resis - tor is switched in circuit whenever the pin voltage is below the input high threshold, or when the part is in under-voltage lockout. 8 vouta output voltage sense pin of channel a 9 pgnda channel a ground connection for converter power stage and internal circuitry. 20 lxa switching node of channel a connect an inductor between this pin and the output capacitor. www.datasheet.co.kr datasheet pdf - http://www..net/
SC284 block diagram control logic plimit amp current amp voltage select oscillator and slope generator ctl 1 a 5 ctl 2 a 6 ctl 3 a 7 pwm comp error amp 500 mv ref a v ina 3 vout a 18 ctl 0 a 4 a gnd a 2 p gnd a 19 l xa 20 p v ina 1 control logic plimit amp current amp voltage select oscillator and slope generator ctl 1 b 15 ctl 2 b 16 ctl 3 b 17 pwm comp error amp 500 mv ref a v inb 13 vout b 8 ctl 0 b 14 a gnd b 12 p gnd b 9 l xb 10 p v inb 11 www.datasheet.co.kr datasheet pdf - http://www..net/
2 SC284 applications information detailed description the SC284 is a two channel synchronous step-down converter. both channels on this device are designed to operate in fxed-frequency pwm mode at 2.5mhz and provide the same current capacity of up to .8a. the switching frequency is chosen to minimize the size of the external inductor and capacitors while maintaining high efciency. both channels of SC284 are independent. operation during normal operation, the pmos mosfet is activated on each rising edge of the internal oscillator. the voltage feedback loop uses an internal feedback resistor divider. the period is set by the internal oscillator. the device has an internal synchronous nmos rectifer and does not require a schottky diode on the lx pin. the device operates as a buck converter in pwm mode with a fxed frequency of 2.5mhz. programmable output voltage both channels on SC284 have ffteen pre-determined output voltage values which can be individually selected by programming the ctl input pins (see table output voltage settings). each ctl pin has an active m internal pull-down resistor. the m resistor is switched in circuit whenever the ctl input voltage is below the input threshold, or when the part is in under-voltage lockout. it is recommended to tie all high ctl pins together and use an external pull-up resistor to vin if there is no enable signal, or if the enable input is an open drain/collector signal. the ctl pins may be driven by a microprocessor to allow dynamic voltage adjustment for systems that reduce the supply voltage when entering sleep states. avoid all zeros being present on the ctl pins when changing programmable output voltages as this would disable the device. SC284 is also capable of regulating a diferent (higher) output voltage, which is not shown in the table , via an external resistor divider. there will be a typical 2a cur - rent fowing into the vout pin. the typical schematic for an adjustable output voltage option from the standard .0v with ctlx=[00 0], is shown in figure . rfba/b and rfb2a/b are used to adjust the desired output volt - age. if the rfb2a/b current is such that the 2a vout pin current can be ignored, then rfb a/b can be found by the next equation. rfb2a/b needs to be low enough in value for the current through the resistor chain to be at least 20a in order to ignore the vout pin current. 2 fb ostd ostd out 1 fb r v v v r u where v ostd is the pre-determined output voltage via the ctl pins. c ff is needed to maintain good transient response performance. the correct value of c ff can be found using the following equation. ) 5 . 0 v v ( v v ] k [ r 5 . 0 v 5 . 2 ] nf [ c ostd ostd ostd out 1 fb 2 out ff u u : u to simplify the design, it is recommended to program the desired output voltage from a standard .0v as shown in figure with the correct c ff calculated from equation 2. for programming the output voltage from other standard voltages, r fb , r fb2 and c ff need to be adjusted to meet equations and 2. sc 284 ctl 0 b ctl 1 b ctl 2 b ctl 3 b enable b ctl 0 a ctl 1 a ctl 2 a ctl 3 a enable a pgnda pgndb r fb 1 b = ( v outb - 1 ) x r fb 2 b for ctlb x = 0010 ( 1 . 0 v ) avinb v inb c avinb 0 . 1 f r avinb 1 pvinb c inb 10 f agndb avina v ina c avina 0 . 1 f r avina 1 pvina c ina 10 f agnda v outb c outb lxb voutb l r fb 2 b 10 k c ffb r fb 1 b v outa c outa lxa vouta l r fb 2 a 10 k c ffa r fb 1 a r fb 1 a = ( v outa - 1 ) x r fb 2 a for ctla x = 0010 ( 1 . 0 v ) figure 1 output voltage programming www.datasheet.co.kr datasheet pdf - http://www..net/
3 SC284 applications information (continued) maximum power dissipation each channel of SC284 has its own ja of 40c/w when only one channel is in operation. since both channels are within same package, there is about 50% heat which will be transferred to the adjacent channel. the equivalent total thermal impedance will be higher when the neighboring channel is also in operation. to guarantee an operating junction temperature of less than 25c, figure 2 shows the maximum allowable power loss of each channel. the curve is based upon the junction temperature of either channel reaching a maximum of 25c. each channel of SC284 can support up to .8a load current. figures 3a and 3b show the maximum allowable load current based upon the limit of maximum loss for v in =3.3v and v in =5.0v, respectively. the curves are drawn for high duty-cycle operation. if the operating duty-cycle is lower, the loss is lower allowing higher load current. 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 2 . 2 2 . 4 2 . 6 2 . 8 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 2 . 2 2 . 4 2 . 6 2 . 8 loss of channel a ( w ) l o s s o f c h a n n e l b ( w ) t a = 25 c t a = 55 c t a = 85 c figure 2 maximum allowable loss for eac cannel for a maximum unction temperature of 125c protection features the SC284 provides the following protection features: current limit over-voltage protection soft-start operation thermal shutdown ? ? ? ? 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 load current channel a ( a ) l o a d c u r r e n t c h a n n e l b ( a ) t a = 85 c t a = 68 c t a = 75 c (a) v in = 3.3v v out =2.5v 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 0 0 . 2 0 . 4 0 . 6 0 . 8 1 1 . 2 1 . 4 1 . 6 1 . 8 2 load current a ( a ) l o a d c u r r e n t b ( a ) t a < 76 c t a < 85 c (b) v in = 5.0v v out =3.3v figure 3 maximum allowable load current for eac cannel for a maximum unction temperature of 125c current limit and protection the internal pmos power device in the switching stage is protected by a current limit feature. if the inductor current is above the pmos current limit for 6 consecutive cycles, the part enters foldback current limit mode and the output current is limited to the current limit holding current (i cl_hold ) of a few hundred milliampere. under this condition, the output voltage will be the product of i cl_hold and the load resistance. the current limit holding current will decrease when the output voltage increases. the load presented must fall below the current limit holding current for the part to exit foldback current www.datasheet.co.kr datasheet pdf - http://www..net/
4 SC284 applications information (continued) limit mode. figure 4 shows how the typical current limit holding current varies with output voltage. the SC284 is capable of sustaining an indefnite short circuit without damage and will resume normal operation when the fault is removed. the foldback current limit mode is disabled during soft-start. current limit functionality is shown in figure 5 at the end of this section. current limit holding current over vout 0 50 100 150 200 250 300 1.0 1.5 2.0 2.5 3.0 3.5 output voltage (v) current limit holding current (ma) t a = 25c v in = 5.0v v in = 3.6v v in = 3.3v figuref4fftypicalfcurrentflimitfholdingfcurrentf vs.foutputfvoltage over-voltagefprotection in the event of a 5% over-voltage on the output, the pwm drive is disabled leaving the lx pin foating. soft-start the soft-start mode is activated after vin reaches its uvlo and one or more ctl pins are set high to enable the part. a thermal shutdown event will also activate the soft start sequence. soft-start mode controls the maximum current during startup thus limiting inrush current. the pmos current limit is stepped through four soft start levels of approximately 20%, 25%, 40%, & 00%. each step is main - tained for 200s following an internal reference start up duration of 50s giving a total nominal startup period of 850s. during startup, the chip operates by controlling the inductor current swings between 0a and current limit. if at any time v out reaches 86% of the target or at the end of the soft-start period, the SC284 will switch to pwm mode operation. figure 6 at the end of this section shows the typical diagram of soft start operation. the SC284 is capable of starting up into a pre-biased output. when the output is precharged by another supply rail, the SC284 will not discharge the output during the soft start interval. shut down when all ctl pins of each channel are low, the channel will run in shutdown mode, drawing less than a from the input power supply. the internal switches and bandgap voltage will be immediately turned of. thermal shutdown the device has a thermal shutdown feature to protect the SC284 if the junction temperature exceeds 60c. during thermal shutdown, the on-chip power devices are disabled, tri-stating the lx output. when the temperature drops by 0c, it will initiate a soft start cycle to resume normal operation. inductor selection the SC284 converter has internal loop compensation. the compensation is designed to work with an output flter corner frequency of less than 40khz for a v in of 5v and 50khz for a v in of 3.3v over any operating condition. the corner frequency of the output flter is shown in the following equation. out c c l 2 1 f u s values outside this range may lead to instability, malfunction, or out-of-specifcation performance. in general, the inductance is chosen by making the inductor ripple current to be less than 30% of maximum load current. when choosing an inductor, it is important to consider the change in inductance with dc bias current. the inductor saturation current is specifed as the current at which the inductance drops a specifc percentage from the nominal value. this is approximately 30%. except for short-circuit or other fault conditions, the peak current must always be less than the saturation current specifed by the manufacturer. the peak current is the maximum load current plus one half of the inductor ripple current at the maximum input voltage. load and/or line transients can cause the peak current to exceed this level for short durations. maintaining the peak current www.datasheet.co.kr datasheet pdf - http://www..net/
5 SC284 applications information (continued) below the inductor saturation specifcation keeps the inductor ripple current and the output voltage ripple at acceptable levels. manufacturers often provide graphs of actual inductance and saturation characteristics versus applied inductor current. the saturation characteristics of the inductor can vary signifcantly with core temperature. core and ambient temperatures should be considered when examining the core saturation characteristics. when the inductance has been determined, the dc resistance (dcr) must be examined. the efciency that can be achieved is dependent on the dcr of the inductor. the lower values give higher efciency. the rms dc current rating of the inductor is associated with losses in the copper windings and the resulting temperature rise of the inductor. this is usually specifed as the current which produces a 40?c temperature rise. most copper windings are rated to accommodate this temperature rise above maximum ambient. magnetic felds associated with the output inductor can interfere with nearby circuitry. this can be minimized by the use of low noise shielded inductors which use the minimum gap possible to limit the distance that magnetic felds can radiate from the inductor. however shielded inductors typically have a higher dcr and are thus less efcient than a similarly sized non-shielded inductor. final inductor selection depends on various design considerations such as efciency, emi, size, and cost. table 2 lists the manufacturers of recommended inductor options. the saturation characteristics and dc current ratings are also shown. manufacturer part number l (h) dcr max (?) rated current (a) l at rated current (h) dimen - sions lxwxh (mm) toko 1071as-2r2m 2.2020% 0.060 .80 .54 2.8x3.0x.5 toko 1071as-1r0n .0030% 0.040 2.70 0.70 2.8x3.0x.5 toko 1127as-2r2m 2.2020% 0.048 2.50 .54 3.5x3.7x.8 panasonic ellvgg1r0n .0023% 0.062 2.20 0.70 3.2x3.2x.5 table 2 C recommended inductors c out selection the internal voltage loop compensation in the SC284 limits the minimum output capacitor value to 22f if using a 2.2h inductor or 44f if using a h inductor. this is due to its infuence on the the loop crossover frequency, phase margin, and gain margin. increasing the output capacitor above this minimum value will reduce the crossover frequency and provide greater phase margin. the total output capacitance should not exceed 50f to avoid any start-up problems. for most typical applications it is recommended to use an output capacitance of 22f to 44f. when choosing the output capacitors capacitance, verify the voltage derating efect from the capacitor vendors data sheet. capacitors with x7r or x5r ceramic dielectric are recommended for their low esr and superior temperature and voltage characteristics. y5v capacitors should not be used as their temperature coefcients make them unsuitable for this application. the output voltage droop due to a load transient is determined by the capacitance of the ceramic output capacitor. the ceramic capacitor supplies the load current initially until the loop responds. within a few switching cycles the loop will respond and the inductor current will increase to match the required load. the output voltage droop during the period prior to the loop responding can be related to the choice of output capacitor by the relationship from the following equation. 26 & '5223 /2$' 28 7 i 9 , & u ' u the output capacitor rms ripple current may be calculated from the following equation. ? ? 1 ? u u u ,1 26 & 28 7 0$; ,1 28 7 50 6 &287 9 i / 9 9 9 , www.datasheet.co.kr datasheet pdf - http://www..net/
6 SC284 applications information (continued) table 3 lists the manufacturers of recommended output capacitor options. manufacturer part nunber value (f) type rated voltage (vdc) value at 3.3v (f) dimensions lxwxh (mm) murata grm21br60j106k 00% x5r 6.3 4.74 2.0x.25x.25 (eia:0805) murata grm219r60j106k 00% x5r 6.3 4.05 2.0x.25x0.85 (eia:0805) murata grm21br60j226m 2220% x5r 6.3 6.57 2.0x.25x.25 (eia:0805) murata grm31cr60j476m 4720% x5r 6.3 20.3 3.2x.6x.6 (eia:206) table 3 C recommended capacitors c in selection the SC284 source input current is a dc supply current with a triangular ripple imposed on it. to prevent large input voltage ripple, a low esr ceramic capacitor is required. a minimum value of 0f should be used. it is important to consider the dc voltage coefcient charac - teristics when determining the actual required value. it should be noted a 0f, 6.3v, x5r ceramic capacitor with 5v dc applied may exhibit a capacitance as low as 4.5f. to estimate the required input capacitor, determine the acceptable input ripple voltage and calculate the minimum value required for c in as shown by the following equation. 26 & 28 7 ,1 28 7 ,1 28 7 ,1 i (65 , 9 9 9 9 9 & u ? ? 1 ? ' ? ? 1 ? the input capacitor rms ripple current varies with the input and output voltage. the maximum input capacitor rms current is found from the next equation . ? ? 1 ? ,1 28 7 ,1 28 7 50 6 &, 1 9 9 9 9 , the input voltage ripple and rms current ripple are at a maximum when the input voltage is twice the output voltage or 50% duty cycle. the input capacitor provides a low impedance loop for the edges of pulsed current drawn by the pmos switch. low esr/esl x5r ceramic capacitors are recommended for this function. to minimise stray inductance ,the capacitor should be placed as closely as possible to the vin and gnd pins of the SC284. www.datasheet.co.kr datasheet pdf - http://www..net/
7 SC284 applications information (continued) figuref6ffsoftfstartfoperation figuref5ffcurrentflimitfprotection 6wdjh 6wdjh 6wdjh 6wdjh 6wdjh 6wdjh 6wdjh $ % ) & * ' + ( , 6w d j hv 2shudwlrqghvfulswlr q 6r iw v wduwhqgv 1rup do3: 0rshudwlrq 2y huordgsurwhfwlrqlvhqdeohgdqgshdn fxuuhqwolp lwdwo hyho &\ fo he\ f \f ohshd n fxuuhqwolp lw 2& 3su rwhfwlrqlvdfwlydwhg ir ogedf n shdn fxuuhqwolp lw 3: 02 1zkhqlqgxfwrufxuuhqwr i $ 3: 02 ))z khqlqgxfwrufxuuhqwklwvshdn fxuuhqwolp lwr i ir og edf n p rgh &rqglwlrqv 2shudwlrqghvfulswlrq - ,qgxfwrufxuuhqwklwvshd n fxuuhqwolp lw . 3hdn fxuuhqwolp lwi rufrqvhfxwlyhf\ fo hv / 9rxw? wdujhw 0 ,qgxfwrufxuuhqwgrhvq
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8 SC284 pcb layout considerations the layout diagram in figure 7 shows a recommended top-layer pcb for the SC284 and supporting components. figure 8 shows the bottom layer for this pcb. fundamental layout rules must be followed since the layout is critical for achieving the performance specifed in the electrical characteristics table. 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 guidelines are recommended when developing a pcb layout: the input capacitor, c in , should be placed as close to the vin and gnd pins as possible. this capacitor provides a low impedance loop for the pulsed currents present at the buck converters input. use short wide traces to connect as closely to the ic as possible. this will minimize emi and input voltage ripple by localizing the high frequency current pulses. keep the lx pin traces as short as possible to minimize pickup of high frequency switching edges to other parts of the circuit. c out and l should be connected as close as possible between the lx and gnd pins, with a direct return to the gnd pin from c out . route the output voltage feedback/sense path away from the inductor and lx node to minimize noise and magnetic interference. use a ground plane referenced to the SC284 gnd pin. use several vias to connect to the component side ground to further reduce noise and interference on sensitive circuit nodes. if possible, minimize the resistance from the output and gnd pin to the load. this will reduce the voltage drop on the ground plane and improve the load regulation. it will also improve the overall efciency by reducing the copper losses on the output and ground planes. . 2. 3. 4. 5. figure 7 recommended pcb layout (top layer) figure 8 bottom layer detail applications information (continued) www.datasheet.co.kr datasheet pdf - http://www..net/
9 SC284 outline drawing C 3x3 mlpq-ut20 land pattern C 3x3 mlpq-ut20 www.datasheet.co.kr datasheet pdf - http://www..net/
semtech corporation power management products division 200 flynn road, camarillo, ca 930 2 phone: (805) 498-2 fax: (805) 498-3804 www.semtech.com contact information 20 SC284 ? semtech 20 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 consequence of its use. publication thereof does not convey nor imply any license under patent or other industrial or intellectual 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 specifed maximum ratings or operation outside the specifed 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 ap - plications 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 ofcers, em - ployees, subsidiaries, afliates, and distributors harmless against all claims, costs damages and attorney fees which could arise. www.datasheet.co.kr datasheet pdf - http://www..net/

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