[SOLVED] Input decoupling capacitance change vs bias voltage

[ridgemao1983]

The LM27342(buck convertor) datasheet says:
"The recommended input capacitance is 10µF, Make sure to verify if there is any significant change in capacitance at the operating input voltage"

My input voltage is 24v, so I chose an input capacitor rated 10uF/50V. But the capacitor's dc-bias graph shows that its capacitance will reduce 37%(reduce to 6v) at 24v dc bias. Much less than the 10uF input capacitance.

Does it mean I have to choose another capacitor to make sure its capacitance is 10uF at 24v dc-bias(24v input voltage)?

The 10uF/50v capacitor is TDK C5750X7R1H106K(230KB), its capacitance-voltage graph is shown below

 

[pplus]

Am I wrong, or the LM27342 datasheet says: "Vin (Max) (V) 20 ". Your input voltage is 24v. I would put in parallel aluminum
electrolytic capacitor.
https://www.niccomp.com/help/VoltageCoefficientofCapacitors-032012-R1.pdf

 

[emontllo]

Yes, the maximum input voltage is 20V.
The 10uF capacitor will work as 4.7uF is still ok according to the datasheet.

I would put in parallel aluminum
electrolytic capacitor.

The datasheet strongly recommends MLCC capacitors, aluminum electrolytic may not be approppriate for this high frequency buck due to its high ESL.

 

[pplus]

Yeah, sure, I understand that. Therefore, in parallel with MLCC.

- - - Updated - - -

What output current and voltage you want. Would you be satisfied SD45215 dc/dc converter.
https://www.datasheetarchive.com/SD45215-datasheet.html
VIN Supply Voltage +32 V
Up to 1.5A output current
No need for external pulse generator
as a variant 2 * C5750X7R1H685K250KA 5.7x5.0 C=6.8uF

 

[ridgemao1983]

Thank you all. My input voltage is 10v, I mistyped the 24v.