• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 23 subscribers
  • Views 620 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Loss of PWM signal detection: Product Selection

Ray Vincent Aranzaso
Mastermind 18180 points
Other Parts Discussed in Thread: SN74LVC1G17, SN74LVC2G17

Hi,

Good Day. Could you please suggest an IC which would trigger a fault if the PWM signal is high continuously for 3ms? If the signal is low continuously it should be considered fine. Please advise. Thank you very much.

Best Regards,

Ray Vincent

  • 0
    TI__Guru* 97066 points

    Hi Ray,

    It sounds like you want a circuit that counts to about 3 ms any time a signal is high, and resets any time the signal is low. If it reaches 3 ms, then the output goes high, otherwise the output remains low.

    The easiest method for this is to use a rising edge delay circuit. It's very simple with a Schmitt-trigger input logic device (please forgive my poor artwork):

    The resistor and capacitor values will depend on the desired timing and threshold for the given device. Assuming a 0.7*Vcc threshold value and approximately 3 ms delay, I would select R = 2.49kohm and C = 1 uF. Any Schmitt-trigger buffer would work. One example is the SN74LVC1G17.

    Just as fair warning, this circuit isn't extremely precise -- if all you need is "about 3 ms" then it should be fine. For precision, use a microcontroller + crystal oscillator.

  • 0 in reply to Emrys Maier
    TI__Expert 7015 points

    Hi Emrys

    Thanks so much for your help!

    This customer has a follow up problem:

    "Will this circuit for 99% duty cycle? I have concerns over 99% duty cycle."

    Thank you very much.

    Best regards,
    Yuki

  • 0 in reply to Qi Liu
    Guru 318740 points

    When the input goes low, it discharges the capacitor immediately through the diode. How fast this happens depends on the drive strength of the signal source. If in doubt, add a strong buffer (i.e., use SN74LVC2G17) in front of the RC circuit.