Issue controlling MOSFET with PCF8575

Question

I'm currently trying to control an array of MOSFETS so I can drive several solenoids. I'm using the Boron from particle.io as my main board, the PCF8575 as my I/O expander, the MAX3004 as a level shifter, and the PSMN5R4-25YLDX as my MOSFET (see attached image).

The issue I'm running into is that the output from the PCF8575 is below the 3.3V mark. It's around 2V. The MAX3004 outputs just under 3V. So, the MOSFET never turns on. I read that the PCF8575 provides a "soft" high state, but does that mean that, in this situation, it won't even reach 3.3V?

Experimenting, I found out that if I bypass 180 Ohm resistor leading to the MOSFET, I can get the MOSFET to turn on with no load. With load, it doesn't switch.

I'm not quite sure how I should fix this problem if anyone has suggestions.

Answer 1

The PCF8574 has what the data sheet calls a "quasi-bidirectional" output.

It basically means, it's an open-drain output with weak pull-up (and stronger momentary pull up for rise time acceleration, but that can be ignored for now).

So since it is not a push-pull output like on MCUs or other GPIO expanders, the 10k pull-down as DC load resistance is too much for it to drive.

What you could do is to change the IO expander to something that suits the purpose for best results. Or as a temporary solution, remove all the pull-downs, but the PCF857 will still initialize at power-up to all pins high.

And I just realized the MAX3004 is there between doing level conversion.

It is also intended for data application, instead of driving a capacitive FET gate and DC pull-down resistor. It too has strong momentary rise/fall time accelerator, which then turn off and the state is held very weakly. It is also not a suitable component for driving FETs and the pull-down resistor.

You would need a simpler level shifter, any logic IC that can see 3.3V inputs and drive 5V from supply will work.