SC4502 datasheet(14/19 Pages) SEMTECH

14 / 19 page

 2005 Semtech Corp.

POWER MANAGEMENT

SC4502/SC4502H

www.semtech.com

empirically by observing the inductor current and the

output voltage during load transient. Compensation is

optimized when the largest R

3 and the smallest C4without

producing ringing or excessive overshoot in its inductor

current and output voltage are found. Figures 9(b), 10(c),

11(b) and 11(c) show load transient responses of

empirically optimized DC-DC converters. In a battery-

operated system, compensating for the minimum V

IN and

the maximum load step will ensure stable operation over

the entire input voltage range.

5 adds a feedforward zero to the loop response. In some

cases, it improves the transient speed of the converter.

6 rolls off the gain at high frequency. This helps to

stabilize the loop. C

5 and C6 are often not needed.

Board Layout Considerations

In a step-up switching regulator, the output filter

capacitor, the main power switch and the rectifying diode

carry switched 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 SC4502/SC4502H, grounding

the output filter capacitor next to the SC4502/SC4502H

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 the EMI but also decreases the sizes of the

switching voltage spikes and glitches.

Figure 8 shows how various external components are

placed around the SC4502/SC4502H. The frequency-

setting resistor should be placed near the ROSC pin with

a short ground trace on the PC board. These precautions

reduce switching noise pickup at the ROSC pin.

To achieve a junction to ambient thermal resistance (

JA)

of 40°C/W, the exposed pad of the SC4502/SC4502H

should be properly soldered to a large ground plane. Use

only 12mil diameter vias in the ground plane if necessary.

Avoid using larger vias under the device. Molten solder

may seep through large vias during reflow, resulting in

poor adhesion, poor thermal conductivity and low

reliability.

2.2

µF

3.3V

VIN

VOUT

OFF

10BQ015

5.6

µH

µF

47nF

1.8nF

20K

40.2K

174K

12V, 0.3A

9.31K

SC4502

6,7

4,5

GND

SHDN

ROSC

COMP

Figure 9(a). 1.35 MHz All Ceramic Capacitor 3.3V to 12V Boost

L1: Sumida CR43

Converter.

Upper Trace : Output Voltage, AC Coupled, 1V/div

Lower Trace : Inductor Current, 0.5A/div

µs/div

Figure 9(b). Load Transient Response of the Circuit in Figure

9(a). I

LOAD is switched between 0.1A and 0.3A

at 1A/

µs.

Applications Information

Typical Application Circuits

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