FREQ datasheet(38/56 Pages) FUJITSU

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MB39A214A

Limitation : development tool vendor use only

FUJITSU SEMICONDUCTOR CONFIDENTIAL

DS405-00007-1v0-E

Selection of output capacitor

A certain level of ESR is required for stable operation of this IC. Use a tantalum capacitor or polymer

capacitor as the output capacitor. If using a ceramic capacitor with low ESR, a resistor should be connected

in series with it to increase ESR equivalently.

Calculate the output capacitor value by the following formula as a guide.

COUT ≥

fOSC

ESR

COUT

: Output capacitor value [F]

fOSC

: Switching frequency [Hz]

ESR

: Series resistance of output capacitor [Ω]

Moreover, the output capacitor values are also derived from the allowable amount of overshoot and

undershoot. The following formula is represented as the worst condition in which the shift time for a sudden

load change is 0s. The output capacitor value allow a smaller amount than the value calculated by the

following formula when a longer shift time.

ΔIOUT2

COUT ≥

VOUT

ΔVOUT_OVER

…Overshoot condition

ΔIOUT2

(VOUT + VIN

fOSC

tOFF_MIN)

COUT ≥

VOUT

ΔVOUT_UNDER

(VIN – VOUT – VIN

fOSC

tOFF_MIN)

… Undershoot condition

COUT

: Output capacitor value [F]

ΔVOUT_OVER : Allowable amount of output voltage overshoot [V]

ΔVOUT_UNDER : Allowable amount of output voltage undershoot [V]

ΔIOUT

: Current difference in sudden load change [A]

: Inductor value [H]

VIN

: Power supply voltage [V]

VOUT

: Output setting voltage [V]

fOSC

: Switching frequency [Hz]

tOFF_MIN

: Minimum off time

When changing to no load suddenly, the output voltage is overshoot, however, the current sink is not

executed in the mode other than PWM fix. As a result, the decrement of the output voltage might take a

long time. This sometimes results in the stop mode because of the over voltage detection. In the mode other

than PWM fix, select the capacitor value so that the overshoot value is set to the over voltage detection

voltage value or less (115% of the output setting voltage or less).

The capacitor has frequency, operating temperature, and bias voltage characteristics, etc. Therefore, it must

be noted that its effective capacitor value may be significantly smaller, depending on the use conditions.

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