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UCC21755-Q1: FLT signal

Shishir Trivedi
Prodigy 80 points
Part Number: UCC21755-Q1

I have designed a gate driver card wherein I have tied the FLT signal together for HS and LS driver ICs. I aim to drive an inverter using three such driver cards (one driver card with 2 driver ICs, HS and LS drive one H-bridge of the inverter). I observe that during the operation, When VCC is sourced at 3.3V, the FLT always shows a reading of 2.3V (whereas the RDY, PWMs, and Vcc read as 3.3V). Even if I remove either the LS or HS driver ICs in order to take a reading of FLT signal, it still shows 2.3V. I have tied up the FLT with a 5k ohm resistor. When I replace the pull up with 10k ohms, it falls to 2.06V. Is it the pull up resistor causing such a large voltage drop or is the IC itself?

  • +1
    TI__Expert 4980 points

    Hi Shishir, 

    nFLT pin is an open-drain output, so there should be no path from nFLT to GND inside the gate driver unless nFLT is pulled low. Therefore, when nFLT is high, there should be minimal current flowing through the pull-up resistor, and it certainly would not cause 1V of voltage drop. 

    I wonder if there's another current path from VCC to the nFLT net to GND - since you reported a lower nFLT voltage even after removing one of the gate drivers. Can you try removing both the gate driver ICs and probe nFLT net voltage? If it still shows below 3.3V, then I think there's another current path. 

    Please let me know of any findings. Thanks! 

    Vivian

  • 0 in reply to Vivian Qi
    Prodigy 80 points

    Hi Vivian,

    You are correct, there is another current path that causes this 1V drop (LED indication). The operation of the IC is fine.

  • 0 in reply to Shishir Trivedi
    TI__Expert 4980 points

    Hi Shishir, 

    Great! Thanks for confirming. Hopefully this can be fixed on the board and the system can work normally. 

    Vivian