MB39A214APFT-G-JNERE1 Datasheet, PDF (37/56 Pages) Fujitsu Component Limited. – 2ch DC/DC converter IC with PFM/ PWM synchronous rectification

MB39A214A
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.
ΔV_IN = ^IOUT_MAX ¯
C_IN
V_OUT
V_IN ¯ f_OSC
+ ESR ¯ (I_{OUT_MAX} +
ΔI_{L )}
2
ΔV_IN : Power supply ripple voltage peak-to-peak value [V]
I_{OUT_MAX} : Maximum load current value [A]
C_IN
: Input capacitor value [F]
V_IN
: Power supply voltage [V]
V_OUT : Output setting voltage [V]
f_OSC : Switching frequency [Hz]
ESR : Series resistance component of input capacitor [Ω]
ΔI_L : 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:
V_CIN > V_IN
V_CIN : Withstand voltage of the input capacitor [V]
V_IN : Power supply voltage [V]
Irms ≥ I_OMAX ¯
√ V_OUT¯ (V_IN – V_OUT)
V_IN
Irms : Allowable ripple current of input capacitor (effective value) [A]
I_OMAX : Maximum load current value [A]
V_IN : Power supply voltage [V]
V_OUT : Output setting voltage [V]
DS405-00007-1v0-E
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