Issue in Disabling the Power Supply MP1479GTF-Z

[skumar]

Hi, I’m using MP1479GTF-Z DC-DC Power module to step down +12VDC Input to +3.3VDC output. The Enable pin is pulled up to VIN via 100K ohm as mentioned in Datasheet & reference design. It works fine & I measure 3.3VDC output at the Output.
But when I try to disable the Module, Pull down Enable pin to Zero, the output still remains at 2.4VDC & doesn’t turn off completely.

I have removed the 100K ohm resistor connected between VIN & EN pin, Pulled down the EN pin to Ground via MOSFET switching, Directly pulling it down via 10K ohm resistor, Directly connecting the EN pin to Ground.
The EN pin goes 0V but I see the Output volt being 2.4V. I don’t understand the Power Module is still outputting 0V even when the EN pin pulled to 0V.

[jshannon]

Leakage current, after you turn it off, put an ammeter on the output and see how much current it reads. Also the 3.3V rail could be being supported by leakage through I/O pins on the 3.3V rail that are at 3.3V So for instance the 3.3V rail powers some chip, and that chip has 3.3V on one of its pins from some external source, then what can happen is the unpowered 3.3V rail can get powered by current flowing from the external device through the I/O pin to the unpowered 3.3V rail. I find demo boards so helpful in resolving these kinds of questions