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DRV8432 cbc current limit behavior

Marco Pellizzaro
Prodigy 10 points

Hi,
I'm using a DRV8432 to drive a dc brush motor. I have some doubts about the OC protection.
DRV8432 is used in dual full bridge mode (one PWM input each full bridge...) with cycle-by-cycle current limit, so the selection mode pins are M1=1, M2=1, M3=0.
The Rocp resistor is 33K, corresponding to 8A cbc threshold.
When I apply a duty cycle step greater than 30% to PWM input of DRV8432 the FAULT signal is asserted low but the motor continues to run. The Fault remains asserted until the reset signals will be asserted low.
I measured by TA the motor current while varying the size of step: when the step applied was 100% the motor current measured had a peak of 16A. I gradually reduced the size of step and I verified that FAULT was asserted low as long as the step size produced a peak current over about 8A.
Is this behavior correct? Why does the CBC current limit not seem to work when the current rises up to 16A ? Why, when FAULT is asserted low, doesn't the DVR8432 go into shut off state and continues to drive the motor? Shouldn't the FAULT assertion only with overcurrent or overtemperature or undervoltage events? When CBC current limit is used, the device is also protected by OC latching shut down but, what is the current threshold of this protection?

Thanks in advance.
Marco Pellizzaro

  • 0
    TI__Guru 55605 points

    Marco,

    Are you using one of our EVMs for this test or your own board?  This is certainly confusing.

    If /FAULT remains low and does not reset to HIGH again until a toggling of the /RESET lines, then this is certainly a latched a fault and the outputs should be disabled.  CBC protection will NOT report on /FAULT, so only a true latched /FAULT should keep the signl low. 

    GVDD under-voltage protection will also report on /FAULT, but it should auto-recover when GVDD is back in a normal range.  Is your GVDD voltage stable at ~12V over your test cases?

    Finally, thermal protection will also report on /FAULT.  Is /OTW low under any of the conditions above?

    To answer your other question, level 2 current protection is ~6A above the CBC limit set with external resistor.  Again, this should be a latched fault and outputs should be disabled (HIZ).  Is the motor really being driven with measureable current when /FAULT = low?