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DRV8837 not driving big motor: OCP problem?

GregoireGentil
Expert 2320 points
Other Parts Discussed in Thread: DRV8837, DRV8837EVM

I'm using DRV8837 to drive a RC toy motor. There are two motors, very similar, from the same Chinese supplier, recommended at 3.7V. The only difference is the size:

- the right one is 7mm diameter.
- the left one is 8.5mm diameter.

http://imgur.com/rkRnRZH

When I use the the small one (7mm) with my design or the DRV8837evm, there is no problem as shown here:
http://tinypic.com/r/2816c91/8

When I use the big one (8.5mm) with my design or the DRV8837evm, it's not working as shown here:
http://tinypic.com/r/mijp94/8

On the scope, I see a spike every 1ms which according to the datasheet seems linked to OCP.

If I use the big one (8.5mm) with my design or the DRV8837evm, AND IF I GO UP VERY SLOWLY BETWEEN 10% AND 20%, it's working as shown here:
http://tinypic.com/r/jhq0ds/8

The small motor consumes around 500mA at 3.7V, the big one probably goes around 1 to 1.5A at 3.7V nothing near the over current protection level.

Is there any explanation? Could I add something to fix the problem? DRV8837 provides the advantage to work at 1.8V which is not the case of 8835 or 8836 so we are reluctant to switch to a dual motor.

  • 0
    TI__Mastermind 26065 points

    Hi,

    What is the DC resistance of the 8.5mm motor? DRV8837 has a MIN OCP threshold as 1.9A. So if the motor's DC resistance is too small and < 3.7/1.9 (about 2ohm), a true OCP event will happen at startup.

    Start the motor with low PWM duty could solve this problem.

    Thanks.

  • 0 in reply to Wilson Zuo
    Expert 2320 points

    This is China! I don't have the datasheet of the motors. :-(

    I have measured the DC resistance of the motors by using a standard voltmeter and it reports 0.6ohm for the big one and 1ohm for the small one. I don't know if it's valid measurement but at least it gives an indication that the resistance of the big motor is smaller than the one of the small motor, which makes sense with the reported problem. If there is a better way to measure the DC resistance value, please let me know. 1ohm for the small motor (which works) would trigger OCP but reality is not so the value is probably higher.

    Doing small throttle at start is what I already do as shown on the third video, but it's not very practical. Is there another way to fix the problem?

  • 0 in reply to GregoireGentil
    TI__Mastermind 26065 points

    Hi

    A simple measurement of the DC resistor is fine.

    Because we only spec the MIN OCP as 1.9A, also considering the slow down of the current rising caused by the phase inductance, it is possible that the small one with 1ohm got no issue.

    So, another possible solution is to add some resistance or inductance in serial with the motor phase.

  • 0 in reply to Wilson Zuo
    Expert 2320 points

    I have put in series a 0.39ohm 0603 resistor (which I have luckily in my personal stock) and it's working. But I guess that I'm consuming "unnecessary" power like R*I^2  = 0.39*1^2 = 0.39W (if I=1Amp).

    Would you have a recommendation of an inductor that could successfully do the trick and consumes less power at high duty than the resistor?

  • 0 in reply to GregoireGentil
    TI__Mastermind 26065 points

    Hi,

    Yes, an inductor will be more efficient.

    If use an inductor, 4.7uH will be an enough value for the worse case. So you probably need a 2uH to 4.7uH inductor with 2A rating (the smaller r and higher current rating, the bigger the inductor and higher cost) .

    Thanks.