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SN74LVC541A Pull-down resistor at input

shiwei luan
Intellectual 750 points
Other Parts Discussed in Thread: SN74LVC541A

Hi,

I designed a board with SN74LVC541A as a logic voltage converter for SPI.

In order to get steady idle states, I put pull-down resistors with value of 20K at all the input pins.

After the chip is enabled, I applied a square wave of 500Hz (from a signal generator with 50 Ohm output) at inputs of SN74LVC541A, and measured this input signal with scope, but the input became a flat ground.

Then I removed pull-down resistor, did the same test, now I got a very good square wave output.

Pull-down resistors are forbidden at input for SN74LVC541A?

Thank you,

Shiwei

  • 0
    TI__Guru 62050 points
    Hi Shiwei san,

    I believe your driver is not able to source enough current to overcome the pulldown resistor to produce the high voltage . If you concerned about the signal integrity at the input , you can have a very small cap (0.1nf cap ) close to the device input .
  • 0 in reply to ShreyasRao
    Intellectual 750 points

    Hi Shreyas,

    I use another function generator and I can get waveforms now. But the problem is these waveforms are out of shape with ringings.

    I applied a square wave to SN74LVC541A, as you can see from the attached picture, the green(5V) is measured at input pin of SN74LVC541A, the blue(3.3V) is measured at output pin of SN74LVC541A.

    Any suggestion on how to kill these ringing? The overshoot/undershoot of blue is too large.

    And could you explain your suggestion with a a very small cap (0.1nf cap )?

    Thank you,

    Shiwei

  • 0 in reply to shiwei luan
    TI__Guru 62050 points

    Hi Shiwei ,

    looks like you have got a lot of ringing here which is exceeding the abs max of the part . This is undesirable for long term reliability for the device . It seems that you have a long inductive trace at input which is causing it to ring a lot . Having a small cap at your input can smoothen this .You can also probe your Vcc /gnd pins to ensure no ringing on the rails. sometimes the output will be ringing even if input is clean due to Vcc rail instability .

    The previous issue was a bad function generator or just plain driving issues ?  

  • 0 in reply to ShreyasRao
    Intellectual 750 points

    Hi Shreyas,


    When I first test the N74LVC541A, I use the square wave from the Oscilloscope and get a ground signal measured at input. I realized the probe is not connected to the TSSOP footprint on the board, but I can get wave form at input now. So the 20K resistor could work, but your suggestion is very helpful.

    Then I improved the connection for all my test and ground the probe very close to the pin where I take measurements, and the waveforms are much better now without changing any resistor. See attached picture.

    Thank you for your replies.

  • 0 in reply to shiwei luan
    TI__Guru 62050 points
    Thanks for letting know .