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DRV8840: DRV8840 drive circuit current does not reach the set maximum

Annie Liu
Genius 10295 points
Part Number: DRV8840

Hi Team,

The demo board schematic and layout were made according to the TI official document. Customer set VREF with the DAC of the microcontroller. Risen=0.1R. Vref is 2A. OUT1 and OUT2 are connected to a 5 ohm resistor. But when he measured the current through the resistor, he found it was only 0.54A. I0-I5 has been fully raised.
He modified Vref to 1.5V, but the current is still 0.54A. He modified the I0-I5 current ratio, but still can't adjust normally. Two boards were experiencing the same issue. Customer would like to know how to resolve this issue？

Thanks.
Annie

  • 0
    TI__Mastermind 37683 points

    Hi Annie,

    Using resistive loads across the outputs of the H-bridge will have a different behavior than typical inductive loads. To start, the current measured through resistor will depend on the VM value, the resistive load, Risen, RDS(on), and any other parasitic resistance. Adjusting Vref or [I0-I4], will not change the value of the current through resistance but only adjust the Ichop threshold. That is the reason why the customer is seeing the same current despite adjusting Vref and [I0-I4].

    When the Ichop value is set higher than the current through resistive load, the H-bridge will drive load continuously. However, if the Ichop value is set below the current through resistive load, the H-bridge will go into decay after TBLANK time (~3.75us) for TOFF time of ~16.25us. During the decay time, the current would suddenly drop to zero or go negative for a brief amount of time (if set to fast/mixed decay mode) but the device will detect the negative current and disable the H-bridge for the remainder of the decay time which will make the current through resistor equal to zero.

    How did the customer measure the current?

    -If DMM is used, then the actual current measured is the RMS average and could explain the 0.54A value. I suggest using an oscilloscope to measure the current.

    What is the supply voltage value?

    -If the resistance values through H-bridge don't change, then Vm will set the current through the resistive load (I~=Vm/(5ohm+0.1ohm+RDS(on))). 

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hi Annie,

    Has your issue been resolved?

  • 0 in reply to Pablo Armet
    TI__Genius 10295 points

    Hi Pablo,

    Thanks for your support.

    The customer did not give me any response. You are able to close this thread now. Once there is feedback, I will let you know.

    Best Regards,

    Annie