SC4503 datasheet(14/22 Pages) SEMTECH

14 / 22 page

 2007 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC4503

Applications Information (Cont.)

Figures 7(a)-7(c) show the effects of different values of

inductance and feed-forward capacitance on transient re-

sponses. In a battery-operated system if C

4 is optimized for

the minimum V

IN and the maximum load step, the converter

will be stable over the entire input voltage range.

Different inductances and feed-forward capaci-

tances affect the load transient responses of the

3.3V to 12V step-up converter in Figure 10(a).

OUT is switched between 90mA and 280mA.

Figure 7.

0.5A/div

OUT

0.5V/div

µs/div

(a) L

1 = 5.6µH and C4 = 2.2pF

µs/div

0.5A/div

OUT

0.5V/div

(b) L

1 = 5.6µH and C4 = 3.3pF

µs/div

0.5A/div

OUT

0.5V/div

1 = 3.3µH and C4 = 2.7pF

Board Layout Considerations

In a step-up switching regulator, the output ﬁlter capacitor,

the main power switch and the rectifying diode carry pulse

currents with high di/dt. For jitter-free operation, the size of

the loop formed by these components should be minimized.

Since the power switch is integrated inside the SC4503,

grounding the output ﬁlter capacitor next to the SC4503

ground pin minimizes size of the high di/dt current loop.

The input bypass capacitors should also be placed close to

the input pins. Shortening the trace at the SW node reduces

the parasitic trace inductance. This not only reduces EMI

but also decreases switching voltage spikes.

Figure 8 shows how various external components are

placed around the SC4503.

The large surrounding ground plane acts as a heat sink

for the device.

SHDN/SS

GND

VIN

VOUT

Figure 8. Suggested PCB Layout for the SC4503.

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