an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 1 block diagram application note for sc1101/03 introduction description the sc1101/03 is a versatile, LOW-COST, voltage-mode pwm controller designed for use in single ended dc/dc power supply ap- plications. a simple, fixed-voltage buck regu- lator can be implemented using the sc1101/03 with a minimum of external com- ponents. internal level shift and drive circuitry eliminates the need for an expensive p- channel, high-side switch. the small device footprint allows for compact circuit design. sc1101/03 features include a temperature compensated voltage reference, triangle wave oscillator, current limit comparator, fre- quency shift over-current protection, and an internally compensated error amplifier. pulse by pulse current limiting is implemented by sensing the differential voltage across an ex- ternal resistor, or an appropriately sized pc board trace. the sc1101/03 operates at a fixed frequency of 200khz, providing an optimum compro- mise between efficiency, external component size, and cost. the sc1103?s manufacturing process has been optimized for 12v operation and is capable of handling up to 26v at it?s bst pin. applications ? pentium? p55 core supply ? low cost microprocessor supplies ? peripheral card supplies ? industrial power supplies ? high density dc/dc conversion features ? low cost / small size ? switch mode efficiency (90%) ? 1% reference voltage accuracy ? over current protection ? 500ma output drive ? 5v to 12v input power source
an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 2 layout guidelines careful attention to layout requirements are necessary for successful implementation of the sc1101/03 pwm controller. high currents switching at 200khz are present in the application and their effect on ground plane volt- age differentials must be understood and minimized. 1). the high power parts of the circuit should be laid out first. a ground plane should be used, the number and position of ground plane interruptions should be such as to not unnecessarily compromise ground plane integrity. isolated or semi-isolated areas of the ground plane may be deliberately introduced to constrain ground currents to particular areas, for example the input capacitor and bottom schottky ground. 2). the loop formed by the input capacitor(s) (cin), the top fet (q1) and the schottky (d1) must be kept as small as possible. this loop contains all the high current, fast transition switching. connections should be as wide and as short as possible to minimize loop inductance. minimizing this loop area will reduce emi, lower ground injection currents, resulting in electrically ?cleaner? grounds for the rest of the system and minimize source ringing, resulting in more reliable gate switching signals. 3). the connection between the junction of q1, d1 and the output inductor should be a wide trace or copper re- gion. it should be as short as practical. since this con- nection has fast voltage transitions, keeping this connec- tion short will minimize emi. the connection between the output inductor and the sense resistor should be a wide trace or copper area, there are no fast voltage or current transitions in this connection and length is not so impor- tant, however adding unnecessary impedance will re- duce efficiency. 4) the output capacitor(s) (cout) should be located as close to the load as possible, fast transient load currents are supplied by cout only, and connections between cout and the load must be short, wide copper areas to minimize inductance and resistance. 5) the sc1101/03 is best placed over an isolated ground plane area. gnd and pgnd should be returned to this isolated ground. this isolated ground area should be connected to the main ground by a trace that runs from the gnd pin to the ground side of (one of) the out- put capacitor(s). if this is not possible, the gnd pin may be connected to the ground path between the output capacitor(s) and the cin, q1, d1 loop. under no circum- stances should gnd be returned to a ground inside the cin, q1, d1 loop. 6) vcc for the sc1101/03 should be supplied from the vin supply through a 10 ? resistor, the vcc pin should be decoupled directly to gnd by a 0.1mf ceramic capaci- tor, trace lengths should be as short as possible. 7) the current sense resistor and the divider across it should form as small a loop as possible, the traces run- ning back to cs(+) and cs(-) on the sc1101/03 should run parallel and close to each other. the 0.1f capacitor should be mounted as close to the cs(+) and cs(-) pins as possible. 8) to minimize noise pickup at the sensitive fb pin, the feedback resistors should both be close to the sc1101/03 with the bottom resistor (rb) returned to ground at the gnd pin. vout 12v 4uh 5mohm + cout + cin 10 0.1uf 2.32k 1.00k q1 0.1uf 24v in heavy lines indicate high current paths. d1 sc1103cs gnd 8 vcc 1 cs(-) 2 cs(+) 3 pgnd 4 dh 5 bst 6 fb 7 rb ra
an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 3 typical applications q1 si4420dy r4 2.7 r2 1k d1 b520c r1 10 l1 6.8uh c7 1500/6.3v c2 1500/6.3v c4 0.01 r7 124 r6 124 c1 0.1 c5 0.01 +2.5v +5v gnd c8 0.1 r5 0.02 r3 1k gnd vcc 1 cs(-) 2 cs(+) 3 pgnd 4 dh 5 bst 6 fb 7 gnd 8 u1 sc1103 c3 0.1 d2 ll42 c6 0.1 1. small footprint - low power this circuit utilizes so-8 mosfet and smc schottky diode. the area is 1 sq. in. top - component view top - copper bottom - component view bottom - copper 5v to 2.5v @ 4a with ?flying capacitor? boost voltage.
an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 4 typical applications (cont.) 2. small footprint - medium power this circuit utilizes dpak mosfet and schottky diode. the area is 2.2 sq. in. top - component view bottom - component view top - copper bottom - copper q1 irlr3103 r4 2.7 r2 1k d1 mbrd835ll r1 10 l1 5.6uh c9 1500/6.3 c10 1500/6.3 c11 1500/6.3v c3 1500/6.3v c2 1500/6.3v c6 0.01 r7 124 r6 205 c1 0.1 c7 0.01 +3.3v +5v gnd c8 0.1 c12 0.1 r5 0.01 r3 1k gnd vcc 1 cs(-) 2 cs(+) 3 pgnd 4 dh 5 bst 6 fb 7 gnd 8 u1 sc1101 c5 0.1 +12v 5v to 3.3v @ 8a
an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 5 typical applications (cont.) 3. medium footprint - high power this circuit utilizes d2pak mosfet and schottky diode. the area is 4.1 sq. in. q1 irl3103s r4 2.7 r2 1k d1 mbrb1530ct r1 10 l1 5.6uh c9 1500/6.3 c10 1500/6.3 c11 1500/6.3v c3 820/16v c2 820/16v c4 820/16v c6 0.01 r7 124 r6 205 c1 0.1 c7 0.01 vout(+) +12v gnd c8 0.1 c12 0.1 r5 0.01 r3 1k vout(-) vcc 1 cs(-) 2 cs(+) 3 pgnd 4 dh 5 bst 6 fb 7 gnd 8 u1 sc1103 c5 1.0 d2 ll42 3.3v 12v to 3.3v @ 10a with ?flying capacitor? boost voltage. top bottom
an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 6 fig. 3: v ripple @ v in = 12v, v o = 3.3v, i o = 10a fig. 2: load regulation @ v o = 3.3v, v in = 12v -10 -5 0 5 10 15 20 25 30 35 40 100.0e+0 1.0e+3 10.0e+3 100.0e+3 1.0e+6 10.0e+6 frequency (hz) gain (db) -45 0 45 90 135 180 phase (deg) gain phase fig.1: error amplifier, gain and phase fig. 6: line regulation @ v o = 3.3v, i o = 10a fig. 5: efficiency @ v in = 12v fig. 4: load regulation @ v in = 12v -0.020 -0.010 0.000 0.010 0.020 0.030 0.040 0.050 0246810 current (amps) voltage change (v) normalized to 0 at io=2a. -1.0% -0.8% -0.6% -0.4% -0.2% 0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 0 2 4 6 8 10 12 14 output current, (a) load regulation 1.8v 2.5v 3.3v 5.0v vo= 40% 50% 60% 70% 80% 90% 100% 02468101214 output current, (a) efficiency 1.8v 2.5v 3.3v 5v -0.5% -0.4% -0.3% -0.2% -0.1% 0.0% 0.1% 0.2% 0.3% 0.4% 0.5% 11.4 11.6 11.8 12.0 12.2 12.4 12.6 input voltage, (v) line regulation typical plots & waveforms
an99-5 low cost pwm controller ? 2000 semtech corp. 652 mitchell road newbury park ca 91320 january 13, 2000 7 outline drawing jedec ref: ms-012aa land pattern so-8
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