MB39A214APFT-G-JNERE1 Datasheet, PDF (38/56 Pages) Fujitsu Component Limited. – 2ch DC/DC converter IC with PFM/ PWM synchronous rectification

MB39A214A
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.
1
C_OUT ≥ _{4 ¯fOSC ¯ ESR}
C_OUT
f_OSC
ESR
: Output capacitor value [F]
: Switching frequency [Hz]
: 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.
C_OUT ≥
ΔI_OUT2 ¯ L
…Overshoot condition
2 ¯ V_OUT ¯ ΔV_{OUT_OVER}
C_OUT ≥
ΔI_OUT2 ¯ L ¯ (V_OUT + V_IN ¯ f_OSC ¯ t_{OFF_MIN})
… Undershoot condition
2 ¯ V_OUT ¯ ΔV_{OUT_UNDER} ¯ (V_IN – V_OUT – V_IN ¯ f_OSC ¯ t_{OFF_MIN})
C_OUT
: Output capacitor value [F]
ΔV_{OUT_OVER} : Allowable amount of output voltage overshoot [V]
ΔV_{OUT_UNDER} : Allowable amount of output voltage undershoot [V]
ΔI_OUT
: Current difference in sudden load change [A]
L
: Inductor value [H]
V_IN
: Power supply voltage [V]
V_OUT
: Output setting voltage [V]
f_OSC
: Switching frequency [Hz]
t_{OFF_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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DS405-00007-1v0-E