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Original question:

UCC21750: pwm input is clamped to 1.3V leading to no waveform output

UCC21750: Interface between UCC21750 and TXS0108E-Q1

Cheng Wang
Prodigy 10 points
Part Number: UCC21750
Other Parts Discussed in Thread: TXS0108E-Q1

Tool/software:

Hello,

We are now using UCC21750 for driving. Primary side we use 5V power supply, and all the signal pins (IN+, IN-, RDY, FLT, RST) are driven by voltage level chip TXS0108E-Q1. TXS0108E-Q1 works for voltage shifting from 3.3V to 5V.

However, when giving high signal on IN+, IN-, or RST, the output signal of TXS0108E-Q1 (or said input signal of UCC21750) were clamped to 2.5V.

We tried testing UCC21750 and TXS0108E-Q1 separately, and both of them works well.

So, is TXS0108 not compatible with UCC21750? I cannot find too much evidence from the datasheet. We did find that TXS0108E-Q1 has a 4kohm pull-up, and UCC21750 IN+ has a 55kohm pull-down resistor. But it seems should not clamp the voltage to 2.5V, which is quite confusing.

  • 0
    TI__Expert 6220 points

    Hi Cheng, 

    UCC21750 does have internal pull-downs for IN+ and nRST/EN pins. IN+ PD has a typical value of 55kOhm but can vary across temperature or device. nRST/EN has a 50kOhm typical pull-down resistor. IN- has a pull-up resistor of 55kOhm typical. However there are no internal structure that would "clamp" the voltage at a certain value. 

    I see you also posted a question to the TXS0108 team - let's see what information the other team can provide. 

    Thanks, 

    Vivian