SC4501 datasheet(15/21 Pages) SEMTECH

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POWER MANAGEMENT

SC4501

frequency. However it reduces the phase margin. The

values of R

3 and C4 can be determined 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 10(b), 11(c), 12(b) and

12(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. C

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,

Application Information

the size of the loop formed by these components should

be minimized. Since the power switch is integrated inside

the SC4501, grounding the output filter capacitor next to

the SC4501 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 the sizes of the

switching voltage spikes and glitches.

Figure 9 shows how various external components are placed

around the SC4501. 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 SC4501 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.

Typical Application Circuits

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

Lower Trace : Inductor Current, 0.5A/div

µs/div

Figure 10(b). Load Transient Response of the Circuit in Figure

10(a). I

LOAD is switched between 0.1A and 0.4A at

1A/

µs.

2.2

µF

3.3V

VIN

VOUT

SC4501

GND

SHDN

ROSC

COMP

OFF

10BQ015

3.3

µH

µF

47nF

1.5nF

20K

22.1K

174K

12V, 0.4A

9.31K

Figure 10(a). 1.35 MHz All Ceramic Capacitor 3.3V to 12V Boost

L1: Cooper-Bussmann SD25-3R3

Converter. Pinout Shown is for MSOP-8

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