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DRV8814 only works in fast decay mode

bang1840
Prodigy 20 points
Other Parts Discussed in Thread: DRV8814, DRV8840

Hi,

We are a group of students trying to make a servo-lab for our university. We needed a motor-driver capable of driving both brushed DC-motors and stepper motors in chopper mode, and chose the DRV8814 for the Job.

Both the stepper and DC-motor are working fine while in fast decay mode(decay pin pulled high). However in slow decay, the stepper driver goes into fault-mode almost instant, and the DC-motor driver goes into fault sometimes, typically when the motor changes direction.

The setup for the motor driver is:
 12 V supply(VCP pin measured to 23V), 0.15 Ohm sense resistors, the current chopping treshold is set to 2A(1A for the stepper motor). we are controling the chip with a digital signal on the phase-pin and PWM(32kHz) on the enable-pin.

motor resistanse is 2.37Ohm and motor inductance = 1,8 mH.

As far as I understand the datasheet the chip will only go into fault mode during overtemperature or overcurrent. As the chip barely gets warm, the problem must be some sort of current spike, right? Why wount this happend in fast decay? I tried putting a 10 Ohm, 1000pF snubber across the motor terminals, but it didn't seem to help much.

For the stepper-motor there is no problem driving it with fast decay, but for the dc-motor we would like to use slow decay.


Is there some way to fix this problem, or do we need a bigger driver?

  • 0
    TI__Mastermind 26065 points

    Hi Olsen

    The nFault should be OCP. For this motor, the DC current will be 12V/2.37ohm = 5A. There are different scenarios of the OCP issue.

    At Startup (SLOW decay):

    The current is rising very fast because the motor is still not moving and no BEMF. When the blanking time ended, the current is very high but maybe still < OCP value because of the inductance. But the SLOW mode will continually inject more current at the following blanking time than the current decayed at the PWM off time. So the current will keep on rising until the OCP threshold reached and nfault claimed.

    At braking or direction change (SLOW decay):

    The BEMF will generate very big current at the braking state in the SLOW decay current path. This will cause the current can't be regulated and reach the OCP threshold.

    At FAST decay mode, the current usually can be regulated to the setting value at all start, stop or reverse states. So there will be no issue. But the motor can only do coast at stop.

    To fix this problem, you can use FAST decay mode for startup and reverse. If a braking stop is needed, you can try lower the input PWM duty gradually, when the motor's speed is lower enough, then apply the braking control without generating too big current on the SLOW decay path.

    A bigger driver like DRV8840 will be helpful because the OCP threshold is higher.

    Thanks.