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SN74ACT244: SN74ACT244PWR

Shritesh
Prodigy 235 points
Part Number: SN74ACT244

Dear TI team,

I am using SN74ACT244PWR as buffer for PWM signal which is up to 200 kHz. I see that there is slight overshoot from control board PWM signal (30% overshoot on 3.3 V PWM signal) which is input to buffer. I am observing that similar overshoot (25% on 5 V PWM signal) at the output of buffer.

Kindly let me know if there is a possibility to avoid this noise transmission.

Thanks,

Shritesh

  • 0
    TI__Mastermind 38225 points

    Hi Shritesh,

    do you have a schematic or any scope shots to show what you are seeing?

  • 0 in reply to Albert Xu1
    Prodigy 235 points

    Above waveform is input to Buffer.

    Above waveform is output of Buffer.

  • +1 in reply to Shritesh
    TI__Mastermind 38225 points

    Hi Sritesh,

    This doesn't show too much, are you able to zoom into 1 cycle of this? In addition do you have a picture of your test setup?

    One of the most common reasons in these situations is from the oscilloscope probes. They have a large ground loop which can interfere. 

    If the oscilloscope is not the issue, another issue can be a "light load". You can try adding extra capacitance or resistance to the output to clean this up.

  • 0 in reply to Albert Xu1
    Prodigy 235 points

    Hi Albert,

    I am using ground spring as shown is below image to capture PWM waveform to minimize ground loop.

    When I zoomed input PWM waveform of buffer, I found that input of buffer (from micro-controller) still has positive overshoot (around 34%) and negative overshoot (around 25%) as shown in below images:-

      

    While output of buffer has very minimal value of positive overshoot (around 9%) and negative overshoot (around 9%) as shown in below images:-

      

    As you correctly pointed out, I am carrying out above test at no load.

    As seen from above input waveform, input PWM to buffer consist of negative peak of -1 V, Is it okay for buffer operation or this may damage buffer?

  • +1 in reply to Shritesh
    Guru 317250 points

    Undershoots are harmless as long as the clamping current is less than 20 mA. According to the IBIS model, −1 V results in less than 3 mA, so you should be fine.

    To reduce such reflections, properly terminate the trace. See, for example, p. 11 of SCAA034. The simplest is source termination, but if you have no control over the device that drives the input, use AC termination.

  • 0 in reply to Clemens Ladisch
    Prodigy 235 points

    Thanks a lot Clemens for suggestions. Document shared by you is very helpful.