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PCF8575: Output is lower than expected

David Cosio
Prodigy 20 points
Part Number: PCF8575
Other Parts Discussed in Thread: TCA9535, SN74LXC8T245, TCA9539

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.

  • +1
    Guru 317680 points

    The PCF has quasi-bidirectional I/Os, i.e., the output high level is driven weakly so that other devices can override the voltage.
    Better use a device with configurable I/Os like the TCA9535.

    And I have doubts that the MAX3004 is capable of driving the MOSFET gates (its datasheet is not very clear); the SN74LXC8T245 has buffered outputs that are strong enough.

  • 0
    TI__Guru 50028 points

    Hi David,

    Like Clemens has stated, this device has soft high state that allows the high level output to be pulled LOW when driven externally. 

    We can see that the pull-LOW capability is much more effective. 

    The TCA9535 IO expander has stronger IOH capability. IOH for 9535 can pull 10mA of current which is 10 x stronger than the PCF8575. 

    To fix I suggest moving to a different IO expander from TI. Preferably the TCA series, TCA9535, TCA9539, etc. 

    Regards,

    Tyler