FREQ datasheet(37/56 Pages) FUJITSU

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MB39A214A

Limitation : development tool vendor use only

FUJITSU SEMICONDUCTOR CONFIDENTIAL

DS405-00007-1v0-E

Selection of input capacitor

Select the input capacitor whose ESR is as small as possible. The ceramic capacitor is an ideal. Use the

tantalum capacitor and the polymer capacitor of the low ESR when a mass capacitor is needed as the

ceramic capacitor can not support.

The ripple voltage is generated in the power supply voltage by the switching operation of DC/DC. Calculate

the lower bound of input capacitor according to an allowable ripple voltage. Calculate the ripple voltage of

the power supply from the following formula.

IOUT_MAX

VOUT

ΔIL

ΔVIN =

CIN

VIN

fOSC

+ ESR

(IOUT_MAX +

)

ΔVIN

: Power supply ripple voltage peak-to-peak value [V]

IOUT_MAX : Maximum load current value [A]

CIN

: Input capacitor value [F]

VIN

: Power supply voltage [V]

VOUT

: Output setting voltage [V]

fOSC

: Switching frequency [Hz]

ESR

: Series resistance component of input capacitor [Ω]

ΔIL

: Ripple current peak-to-peak value of inductor [A]

Capacitor has frequency characteristic, the temperature characteristic, and the bias voltage characteristic, etc.

The effective capacitor value might become extremely small depending on the use conditions. Note the

effective capacitor value in the use conditions.

Calculate ratings of the input capacitor by the following formula:

VCIN > VIN

VCIN : Withstand voltage of the input capacitor [V]

VIN : Power supply voltage [V]

√ VOUT (VIN – VOUT)

Irms ≥ IOMAX

VIN

Irms

: Allowable ripple current of input capacitor (effective value) [A]

IOMAX : Maximum load current value [A]

VIN

: Power supply voltage [V]

VOUT : Output setting voltage [V]

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