MB39A132QN-G-ERE1 Datasheet, PDF (40/56 Pages) Fujitsu Component Limited. – ASSP For Power Management Applications (Rechargeable Battery) Synchronous Rectification DC/DC Converter IC for Charging Li-ion Battery

MB39A132
• Input capacitor selection
Select an input capacitor that has an ESR as small as possible. A ceramic capacitor is ideal. If a high-
capacitance capacitor is needed for which there is no suitable ceramic capacitor use a polymer capacitor or
a tantalum capacitor having a low ESR.
The ripple voltage by the switching operation of the DC/DC converter is generated in the power supply
voltage. Please consider the lower limit value of the input capacitor according to the allowable ripple voltage.
The ripple voltage of the power supply can be easily found by the following formula.
IOMAX
VO
ΔIL
ΔVIN =
×
+ ESR × (IOMAX +
)
CIN
VIN × fOSC
2
ΔVIN : Switching power supply ripple voltage peak-to-peak value [V]
IOMAX : Maximum charge current [A]
CIN : Input capacitance [F]
VIN : Switching power supply voltage [V]
VO : Charge voltage [V]
fOSC : Switching frequency [Hz]
ESR : Series resistance element of input capacitance [Ω]
ΔIL : Inductor ripple current peak-to-peak value [A]
The ripple voltage of the power supply can be decreased by raising the switching frequency besides using
the capacitor.
The capacitor has its own frequency, temperature and bias voltage, therefore its effective capacitance can
be extremely small depending on the application conditions.
Select a capacitor whose rating has a sufficient margin against input voltage.
In addition, when using a capacitor having an allowable ripple current rating, select a capacitor that has a
sufficient margin against ripple current.
The allowable ripple current can be found by the following formula.
Irms ≥ IOMAX ×
√VO × (VIN − VO)
VIN
Irms : Allowable ripple current (Root-mean-square value) [A]
IOMAX : Maximum charge current [A]
VIN : Switching power supply voltage [V]
VO : Charge voltage [V]
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DS04–27265–3E