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Capacitive load driving with TCA6424ARGJR I/O expander.

Christos Harissis
Prodigy 250 points

Hi.

I intend to connect the TCA6424ARGJR expander I/O ports DIRECTLY to high input capacitance mosfets (400 pF), i.e., WITHOUT series resistances between them.(The only reason for doing that is the small available area on the p. c. board.)

Can I do that safely, considering that the outputs logic levels WILL NOT change at high repetition rates? In other words, can this expander drive capacitive loads directly?

Christos

  • 0
    TI__Genius 16510 points
    Christos,

    I would consider putting a series resistor on the line (even if just 0 ohms for now) to limit some of the current spikes donw the road. I do not anticipate an issue with driving this large of a load. Since it will only be transient currents while switching, you should be fine.
  • 0 in reply to Jonathan Valdez
    Prodigy 250 points
    Thank for your prompt return!

    Placing zero ohms resistor is not a good option for me due to the reduced space on the circuit board.

    I faced a problem with capacitive loads when I tried to drive them directly by a Microchip microcontroller... To garantee its ports state changing I had to add NOP instructions after each output setting command... (Don't ask me why!)

    Although this expander is NOT a microcontroller, I`m afraid that some unexpected behavior can occur, whereas it has "latched" integrated logics.

    What do you say?
  • 0 in reply to Christos Harissis
    TI__Genius 16510 points
    Cristos,

    That is indeed an interesting issue.

    I have never experienced such an issue with our microcontrollers. I have seen some extreme loading conditions (technically well past the recommended limits) which did not cause issues. You should be fine with this. Are you planning to change a large number of MOSFETs at the same time? Or would it be 1 at a time?

    I don't think there is a real concern either way.
  • 0 in reply to Jonathan Valdez
    Prodigy 250 points
    Thank you again!

    Although it would be better to change two or three outputs state at the same time, if necessary I can do that individually.

    A better descrition about the problem with the microcontroller (if I remember well) is that, with loading capacitance greater than the specified one, I was obligated to execute at least three any instructions between two pin state changing command to the same port. If I didn't do that, the firts change didn't occur (or, else, the state changed temporarily and returned to the previous one)...

    Best regards!