PWM Problem

Discussion in 'General Electronics Chat' started by Dritech, Jan 17, 2014.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi,

    I am using the BTS7960 driver with 20KHz PWM, but when connecting an oscilloscope to the driver OUT pin I am getting the attached waveforms. What can be causing this please?

    Note: Photo 1 is the output when the PWM is connected to the IN pin
    Photo 2 is the output when the PWM is connected to the INH pin
     
  2. eeabe

    Member

    Nov 30, 2013
    59
    9
    If I'm reading the picture correctly, the o-scope is set to 100ms per division on the time scale. That is a pretty long time when you're looking at a 20kHz signal which has a period of 50 microseconds, so you could just be seeing aliasing on the scope. If that's the case, zoom in your time scale and you'll see the signal better.

    Other than that, you may need to provide some more information such as what load is connected to the output and what your power supply is, or maybe just your entire circuit. Is is possible that you're not driving both IN and INH at the same time? If you leave one open, it might oscillate on and off.
     
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the reply.

    When the o-scope was connected to the PWM output (not the H-bridge output), the waveform was fine. This is only happening on the H-bridge output. Even the scope is reading the duty cycle as 98%.

    For this test, no load was connected. I was only testing the PWM feature. The PWM was not applied to the IN pin and the INH pin at the same time.
     
  4. eeabe

    Member

    Nov 30, 2013
    59
    9
    I think I was not clear enough. You say the PWM was not applied to both inputs, but I still don't know if you are driving both inputs with something or if you are leaving one open while you apply the PWM to the other one. If you leave one open, that is a problem. You should drive the INH pin high constantly while you apply the PWM to the IN pin. If you apply the PWM to the IN pin and leave the INH undriven (open), it may oscillate on and off.

    The next thing I would look at is the power supply. The IC has under- and over-voltage protection, and maybe a transient could kick that into effect. If that's happening, it may just need some decoupling capacitance.
     
  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi,

    I tested the PWM as following:

    Test 1 (photo 1): I applied 5V to the IN pin and the PWM to the INH pin. The o-scope probe was connected to the OUT pin of the Driver.

    Test 2 (photo 2): I applied 5V to the INH pin and the PWM to the IN pin. The o-scope probe was connected to the OUT pin of the Driver.

    As a power supply I am using a benchtop power supply.
     
  6. eeabe

    Member

    Nov 30, 2013
    59
    9
    Thanks for clarifying. My last thoughts are that maybe the IC is bad, or maybe it is not fully set up with resistors on the SR and IS pins. I wonder if it may not operate without some resistors on those pins as shown in the application example. Especially the SR pin which has an absolute maximum rating of 1V.

    Other than that, if you see any variation on the supply voltage that correlates with the on/off behavior, certainly post that and maybe it will explain something. Good luck.
     
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