MB39C011APFT-G-BND-ERE1 Datasheet, PDF (39/53 Pages) Cypress Semiconductor – 2 ch DC/DC Converter IC with Synchronous Rectification

MB39C011A
19.4 Output capacitor selection
Because the ripple voltage increases if the ESR is large, a low ESR capacitor needs to be used in order to reduce the ripple voltage.
However, using a capacitor with a low ESR has a large effect on the phase characteristics of the loop, and care needs to be taken
to prevent the system from losing stability. Furthermore, the capacitor that is used should have sufficient tolerance for the ripple
current.
If taking into account the switching ripple voltage, the minimum necessary capacitance is given by the following formula.
C_O ≥
1
2fosc (V_O/IL  ESR)
ESR
V_O
IL
C_O
fosc
: Series resistance element of output capacitance []
: Switching ripple voltage [V]
: Coil ripple current peak to peak value [A]
: Output capacitance [F]
: Oscillation frequency [Hz]
When a capacitive load is connected, it is recommended that the DC/DC converter output capacitor have the same capacitance
as the load capacitance.
The allowable ripple current of the output capacitor is given by the following formula.
IL
Irms ≥ _23
Irms : Allowable ripple current (Root-mean-square value) [A]
IL : Coil ripple current peak to peak value [A]
19.5 Input capacitor selection
Select an input capacitor that has as small an ESR as possible. Ceramic capacitors are ideal. If a large capacitance is required
that cannot be provided by a ceramic capacitor, use a polymer capacitor or a tantalum capacitor with a low ESR. Furthermore, the
capacitor that is used should have sufficient tolerance for the ripple current.
The allowable ripple current is given by the following formula.
Irms ≥ I_OMAX
V_O (V_IN  V_O)
V_IN
Irms
I_OMAX
V_IN
V_O
: Allowable ripple current (Root-mean-square value) [A]
: Maximum load current [A]
: Switching system power supply voltage [V]
: Output voltage [V]
Document Number: 002-08369 Rev. *A
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