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UCC21750: UCC21750 Input Abnormal when Connecting to TI TXS0108E PWM

Francis Jiang
Intellectual 2510 points
Part Number: UCC21750
Other Parts Discussed in Thread: TXS0108E

Tool/software:

Hi Team,

Currently, customer found that the UCC21750 PWM input will abnormal when connecting to TI TXS0108E.

  • When disconnecting the UCC21750, the UCC21750 PWM input is normal 5V.
  • When connecting the UCC21750, the PWM is abnormal 2.5V.
  • When directly 3.3/5V PWM to UCC21750, the voltage is normal 3.3/5V.

All the situation the VCC is normal as 5V.

I think this is an impedance matching problem. But the internal pull-up resistance of TXS0108E is 4kohm, and the pull-down resistance of UCC21750 is 55kohm, line resistance is 100ohm. It should not be wrong.

Could you please help provide your professional advice here? Thanks in advance.

BRs,

Francis

  • +1
    TI__Expert 6220 points

    Hi Francis, 

    Only reason I can think about: IN+ has a 55kOhm pull-down to COM and IN- has a 55kOhm pull-up to VCC. If these two pins are somehow connected with both inputs floating, then with the internal resistors' voltage division, the IN+/IN- could be 2.5V. 

    Like mentioned in this E2E, there are internal PU/PD resistors, but should not impact normal device operation. You also mentioned directly input 3.3V/5V to UCC21750 does not cause an issue. 

    Do you have any customer waveforms? 

    Vivian

  • 0 in reply to Vivian Qi
    TI__Intellectual 2510 points

    Hi Vivian,

    Please let me find your support offline due to confidential information here.

    BRs,

    Francis