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Unexplained issues with DRV8871

Other Parts Discussed in Thread: DRV8871

So I built the attached circuit from the data sheet reference altering for my scenario and the circuit behaves with a couple interesting aspects that I am not sure I can explain.

First, my requirement is to drive a 9-12 ohm, , 19-20mH coil with 24VDC at 125mA and 250mA (this is a coil not and not a motor).  For testing I chose to test 125mA current which derived to approximately a 225k Ohm resistor.  For testing I used a wire coil of 11.7 ohm and 19.9mH.

What is interesting:

  • Driving in one direction (signal IN1 driven high with 3.3V and signal IN2 unconnected), I can indeed see the current limiting working, limiting current to roughly 125mA for more than 3 minutes at 100% duty.  However, driving the coil in the opposite direction (signal IN1 unconnected, signal IN2 high at 3.3V), the current starts at roughly 150mA and slowly ramps up to 280mA over the period of 3 minutes.
  • I also tried both signal pins, IN1 and IN2, with a pull down resistor of 10K with no change in reported current.
  • I thought it might be related to thermal run-away or some such so measured the package with a thermal probe.  The package never exceeded 28C during testing
  • Strangely, the rise in current was not instantaneous, rather it proceeded in small rises followed by a 'plateau' and then another bump in current.
  • Probably just noise, but It also seems that there is a slight difference in Toff time between driving in the two dirrections, IN1 driven high has a Toff time of roughly 27us while driving IN2 high has a Toff time of roughly 27us 

So did I just destroy the device or is this the expected behavior?   Thank you for your time.