BD8966FVM

Technical Note

10/14

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2009.05 - Rev.A

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As the output rise time must be designed to fall within the soft-start time, the capacitance of output capacitor should be

determined with consideration on the requirements of equation (5):

Inappropriate capacitance may cause problem in startup. A 10 μF to 100 μF ceramic capacitor is recommended.

3. Selection of input capacitor (Cin)

A low ESR 10μF/10V ceramic capacitor is recommended to reduce ESR dissipation of input capacitor for better efficiency.

Input capacitor to select must be a low ESR capacitor of the capacitance

sufficient to cope with high ripple current to prevent high transient voltage. The

ripple current IRMS is given by the equation (5):

[A]・・・(5)

√

2

Fig.27 Input capacitor

L

Co

Cin

Output capacitor should be selected with the consideration on the stability region

and the equivalent series resistance required to smooth ripple voltage.

Output ripple voltage is determined by the equation (4)：

*Rating of the capacitor should be determined allowing sufficient margin against

output voltage. Less ESR allows reduction in output ripple voltage.

Fig.26 Output capacitor

Co≦

・・・

(5)

Tss: Soft-start time

L

Co

ESR

Co≦

1m×(2-0.8)

1.5

≒

800[μF]

5(5-1.5)

5

√