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Disable DRV 8842 Pulse Width Modulation

Chihiro Sasaki
Prodigy 80 points
Other Parts Discussed in Thread: DRV8842, DRV8844

I am using the DRV8842 H-Bridge Motor Driver to latch a solenoid on and off. 

Upon scoping the output signal from the DRV 8842, the signal seems to be a pulse-width modulated signal while switching.  The current regulation feature of the DRV 8842 is disabled by connecting the ISENSE pins to ground and the VREF pins to V3P3OUT, which should disable the PWM according to the data sheet. However, the output is still PWMing. Is there a way to disable the PWM completely to get a clean latching signal on the output, and instead get a clean latching signal?

Your help is greatly appreciated. 

  • 0
    TI__Guru 55615 points

    Hello,

    Is the PWM switching at 50kHz?  How are the I0-I4 pins configured?  Try pulling those all HIGH.

    You might also look at the DRV8844 if you need multi-channel and don't care about current control.

  • 0 in reply to Ryan Kehr
    Prodigy 80 points

    Hello Ryan, 

    Thank you for your reply.  The PWM is switching at around 4 kHz.  The I4 pin is tied HIGH (4.5 V), and pins I0-I3 are tied to Ground.  Would all of these pins need to be high in order for the PWM to stop?  

    The PWM signal at the time of the solenoid switching can be seen here.  The blue signal is OUT1 from the DRV8842, the yellow signal is OUT2 also from the DRV 8842, and the pink signal is the IN1 signal into the DRV 8842 triggered by a microcontroller.  

    Thank you.

  • 0 in reply to Chihiro Sasaki
    TI__Guru 55615 points

    Your scope capture was very helpful.

    If it is current regulation, the frequency would be 50kHz.  This device works on the principle of "fixed frequency" current regulation.  Every 50kHz cycle, the driver is enabled and then IF the trip point is exceeded, it is disabled until the next cycle.  If the trip point is never exceeded, the output just retains past state. 

    Is your power supply voltage dipping?  If OUT2 is supposed to follow the power supply, it looks like the supply might be dipping below the UVLO level and the device is tripping UVLO and then reenabling.  Please check that your power supply is not current limiting and folding back the voltage. 

     

  • 0 in reply to Ryan Kehr
    Prodigy 80 points

    Hello Ryan, 

    That was exactly the problem.  I changed out the power supply to one that limits the voltage dip, and the waveform looks nice and square like intended.  Thank you so much for your help.