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DRV8833: Current Control hard to predict

Part Number: DRV8833

Hi,

We wanted to use the current control of DRV8833 to limit the maximum current applied to DC motors.

The problem is that as soon as the maximum current (200mV via the resistor) is reached, the bridge goes into an unexpected current chopping mode.

For example, lets say we want to limit the current to 1A. Then one would choose the sense resistor of 200mOhm. As long as the current is below 1A, everything works fine. The voltage across the resistor can be used to measure the current through the bridge and is below 200mV, so the "current control comparator" does not kick in.

As soon as the current is above 1A, the voltage reaches above the 200mV and the current control comparator turns of the output. After a period coming from the 50kHz the output is enabled again and the cycle repeats: Voltage rises with current above 200mV, gets shut off until the next cycle.

This leads to an effective PWM duty cycle and average current depending on the voltage/current rise characteristics. In the image above 200mV is reached after approximately half the time and an effective current of 500mA.

Here another scope shot:

Here the threshold is arrived much sooner, leading to a chopping of appr. 30% and therefore an effective duty cycle of 300mA.

The thing is, as far as we understand, with for example 200mOhm, you limit the maximum output current to 1A. But as soon as the 1A is reached, it drops to something between 30% (due to deadtime) and 100% of 1A, depending on the rise time of the voltage.

This is very unexpected, because let's say one drives the DRV8833 with a PWM  signal. As you increase the duty cycle of the PWM, the current through the motor also increases. At one point of PWM duty cycle you reach 1A (right below the 200mV threshold). As you go a little above that, the motor cuts back to around 500mA.

Is that how you are supposed to use that feature? Is there a way to limit it to e.q. 1A, without the "fallback" to a much lower current, depending on the rise time?

Thank you for any help,
Dominik

  • Dominik,

    1A current limitation is a peak current limitation. DRV8833 is a motor driver. The output load should be a motor or have very high inductance. In this case, the ripple current doesn't affect average current too much. In your application, the output inductance is small, the output current can quickly reach 1A peak. So, the average current is much lower than 1A.

    BTW, I didn't see any DRV8833 abnormal behave from your description.

    Regards,

    Wang

  • Hi Wang,

    Thank you for your fast answer.

    Yes, the DRV8833 acts exactly like described in the data sheet.

    The output load from the scope shots above is a motor, but apparently the inductance is much smaller than needed for the current control to work.

    As far as I understand now there needs to be a motor with an inductance that leads to a time constant much greater than 20us (50kHz). Large enough so the current only drops a small percentage in those 20us.

    Thank you for your help, we will have to figure out a different way of limiting the current.

    Regards,
    Dominik