© 2006 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5631/FAN5632 Rev. 1.0.2

10

Applications Information

The FAN5631/FAN5632 requires one ceramic bucket

capacitor in the 0.1µF to 1µF range, one 10µF output

bypass capacitor, and one 10µF input bypass capacitor.

To

obtain

optimum

output

ripple

and

noise

performance, low-ESR (<0.05

Ω) ceramic input and

output bypass capacitors are recommended. X5R- and

X7R-rated capacitors provide adequate performance

over the -40°C to 85°C temperature range.

The bucket capacitor’s value is dependent on load

current requirements. A 1µF bucket capacitor works

well in all applications at all load currents, while a 0.1µF

capacitor supports most applications under 100mA of

load current. The choice of bucket capacitor values

should be verified in the actual application at the lowest

input voltage and highest load current. A 30% margin of

safety is recommended to account for the tolerance of

the bucket capacitor and the variations in the on-

resistance of the internal switches.

One of the key benefits of the ScalarPump architecture

is that the dynamically scaled on resistance of the

switches effectively reduces the peak current in the

bucket capacitor and therefore input and output ripple

currents are also reduced. Nevertheless, due to the

ESR of the input and output bypass capacitors, these

current spikes generate voltage spikes at the input and

output pins. These ESR spikes can be filtered because

their frequencies lie at up to 12 times the clock

frequencies. In applications where conductive and

radiated EMI/RFI interference must be low as possible,

consider additional input and output filtering.

Layout Considerations

While

evaluating

any

switched

capacitor

DC-DC

converter, be careful to keep the power supply source

impedance low; use of long wires causing high lead

inductances and resistive losses should be avoided. A

carefully laid-out ground plane is essential because

current spikes are generated as the bucket capacitor is

charged and discharged. The input and output bypass

capacitors should be placed as close to the device pins

as possible.