I am trying to design a PoE solution using MP8007. It has an integrated PD interface and isolated/non-isolated DC-DC Converter.
The DC-DC converter uses fixed peak current and variable frequency Discontinuous Conduction Mode (DCM) to regulate constant output voltage.
I am using it in Buck mode and following the reference circuit given in the datasheet.
The datasheet has input capacitors (
2.2uF) of voltage rating 100V. These input capacitors are very expensive and large. However, on page 23, the vendor has given a formula to calculate
Input Capacitors. For my application,
Ilim = 2.68 A,
L = 22uH,
Vin (min) = 37V and
Vinp_p = 0.3V (This is what I have assumed). By using these values, I have calculated the required Input capacitors to be
However, the reference design has used much higher input capacitor. I am confused over using calculated capacitor (with some margin) or reference design capacitor values.
The recommended minimum output capacitor is 44 µF and much higher output capacitors are being used in the reference design.
The datasheet says:
Normally, a 44μF or higher ceramic capacitor is recommended as the output capacitor. This allows a small Vo ripple and stable operation.
I am planning to use two 44 µF capacitors (25V). But the values in the reference design are troubling me. Please advise.