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UCC21540: UCC21540 DT is connected to VCCI, logic problem

yu Wang
Prodigy 100 points
Part Number: UCC21540

Dears,

As SPEC in the 9.3.2 Input and Output Logic Table on page 21 of the Technical Manual, the DT pin should be directly connected to the VCCI. The INA and INB should be independently controllable. When the input is high, the output should also be high, but in the actual measurement. When both INA and INB are high, the output of both channels is low, that is, the logic of DT connected to VCCI in the truth table cannot be realized.

During the test, the voltage of the DT pin was paid attention to. The measurement was about 0.8V when connected to RC, and about 4.98V when connected to VCCI. In addition, in the circuit design, the DIS pin is grounded to a 10K resistor, and the resistance is always present during the test. And when the input INA and INB are not high, the output logic is the same as the input, in theory the chip can work.

Please help me to determine the reason why the logic can not be achieved.

Thanks!

  • 0
    TI__Expert 3355 points

    Hi Yu,

    Thank you for your question. I work on the applications team in the high power drivers group.

    You are correct that tying DT to VCCI should disable the DT circuit completely and allow outputs to overlap. It sounds like the DT circuit is still activated when you are doing this measurement. Can you tell me how you are measuring the voltage at the DT pin? Is it routed through the PCB, or are you directly probing at the pin? It is possible that the DT pin is floating, which is not recommended. In this mode, the DT circuit will put a minimum ~10ns deadtime, and prevent output overlap. I recommend measuring resistance between DT and VCCI pins directly, to see if DT is floating. 

    If you are still having trouble after checking this, could you please share your schematic, layout, and waveforms? This will help me understand what you are seeing so we can debug the issue together.

    If this answered your question, could you please press the green button? If not, feel free to ask more questions.

    Thanks and best regards,

    John

  • 0 in reply to John Geiger
    Prodigy 100 points

    Dear John,

    Tks for your reply;

    The schematic is a schematic diagram of the circuit. In the figure, for example, the DT pin is soldered to ground RC, and then the pad is reserved for the 5V power supply. During the test, R71 and C55 are removed, and the R70 pads are short-circuited at both ends. That is, the DT pin is directly connected to the VCCI. Since the chip output is powered by an isolated power supply and our oscilloscope lacks an isolated probe, the measurement uses a multimeter voltage file to test the level of the chip's output pin to the isolated power supply.
    Thanks!
    ply.
  • 0 in reply to yu Wang
    TI__Expert 3355 points

    Hi,

    Thank you for your question. I work on the applications team in the high power drivers group.

    What kind of circuit are you switching? Are you able to use two single ended probes to measure OUTA and VSSA in reference to GND? Then you can use the oscilloscope math function to subtract the two signals to take a pseudo-differential measurement.

    I’m just worried that the multimeter does not allow you to see the output transition according to inputs.

    Thanks and best regards,

    John

  • 0 in reply to John Geiger
    TI__Expert 3355 points

    Hi,

    I haven’t heard from you in a while, so I assume you have resolved your issue. I am going to mark this as ‘TI Thinks Resolved’. Feel free to share how you resolved your issue so that others on the forum can learn as well.

    If you still have more questions, feel free to ask.

    Thanks and best regards,

    John