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I am using the BD9C401EFJ for 9V to 5V conversion (with enable pin pulled high). So - input = 9V and output = 5V.

I was just doing some experiments and saw that when I give no input, and I give 5V at the output side pin, strangely I am getting 5V on the input section. I thought that reverse flow of the current is not possible in a DC-DC regulator.

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Generally reverse current can flow in this sort of circuit. Consider the block diagram from the datasheet:

The p-channel MOSFET will have 0V on the gate when input voltage is applied, so VIN and OUTPUT will be connected directly together. If it was a discrete MOSFET there would also be a body diode in parallel.

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