Re: Problem using a mosfet to switch devices on/off

galapogos wrote:

On Jan 30, 6:54 pm, Paul Taylor <paul_ng_pls_...@xxxxxxxxxxxxx> wrote:


I was referring to the diodes that manufacturers put in their devices to
protect the pins of the device from overvoltage. It is not uncommon for
them to cause a problem in switched power circuits, if the designer has
not paid attention to them. But as you have fixed your problem, you don't
need to concern yourself, at least for this design.


Anyway, I fixed problem 1, but problem 2 is still there.

What was the problem?


Well, one of the devices communicated with my MCU's UART, and when I disconnecting the Vcc, the TX pin was still set to an output pin and pulled high. For some reason this interfered with the switching on/
off. I set the pin to now be an input pin instead everytime I disconnected the device right before pulling the gate voltage to 3.3V and now it works. I don't know why the TX output might cause a problem though. Any idea?

Power can find many ways to at least partially travel.

So a TX line that is left hi, driving into a block you are trying to power off, can hit the substrate diode Paul mentioned, and then power will flow from the uC HI, thru the ESD diodes, and into the 'off' Vcc.

Driving the last uA of power out of systems can take a while, as you
have to find, and resolve, these residual paths.

-jg


.

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