P-MOSFET not switching off

[ringo888]

Hi,

I'm having trouble with switching off a P-Channel MOSFET which supplies power to an ADC (please see attached schematic). The idea was a "belt and braces" measure to ensure we could cycle the power of the ADC as this particular part has know to hang-up.

Although the p-mosfet is switching power to the ADC when the gate Vg is tied low (i.e. Vgs=-5V) by the digital isolator (NetLabel ADC_EN3), when the output of the isolator is high i.e. 5V the p-mosfet does not turn off and you can see ~3.8V on the output.

Any ideas as to why as I would have expected it to as Vgs=0V?

I have isolated the output of the isolator and you can tie this to 5V or 0V and the output of the p-mosfet switches, would this suggest there isn't sufficient drive coming from the digital isolator?

Any suggestions would be welcome, thanks

Regards,
Ringo

 

[FvM]

Off course all digital ADC input signals must be driven low, otherwise you get "parasitic" power through clamping (substrate) diodes. Did you?

 

[ringo888]

Off course all digital ADC input signals must be driven low, otherwise you get "parasitic" power through clamping (substrate) diodes. Did you?

Thanks for the quick response FvM.

Yes thats exactaly what the issue was, the conductance of the internal protection diodes on the SPI lines when the power was switched off by the P-MOSFET caused the 3.8V on the ADC supply. I had spoken to a colleague and he suggested the same thing. I've now disabled the SPI bus and reverted it to digital outputs and set them all low before turning the mosfet off.

The moral of the story is Murphy will get you every time! Thanks

 

[crutschow]

Murphy is the unpaid engineer on every job I've worked. :wink: