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Small values of R_isense in DRV88xx drivers

Mikolaj Filar
Expert 1550 points
Other Parts Discussed in Thread: DRV8811EVM

Hi,

the current regulation is calculating by V_REF / (5 * R_ISENSE)  (in some cases there is "8" in the dominator). We can suppose that in most uC systems  V_REF=3,3 V (or less). If we want to gain higher current (2A for example) we need to use a resistor much lower than 1 Ohm. I think that it is a big disatvantage of these drivers.

Is there any "trick" that allow to use higher resistance? I would like to construct a system with digital pot(s) which will regulate the current, but it is impossible now.

Regards,

Mikolaj

  • TI__Expert 7455 points

    Hi Mikolaj,

    I may be wrong on my current impression so please bear with me. It seems to me you want to use a digital resistor as your SENSE resistor in order to configure ICHOP according to the stated equation?

    Unfortunately you cannot use a digital resistor as the SENSE resistor. The SENSE resistor is not just a passive component used to set the current chopping. It is actually an "active" member in the H Bridge and is in series with the motor. Hence, whatever current the motor sees, so does the SENSE resistor. And this could be amps, so most digital resistors would be immediately fried under such conditions.

    Taking in consideration how many amps we are made to push through these power components, we do not want the SENSE resistor to be big in value. Because if it were so, we would then be wasting lots of energy (I^2*R). Ideally we want it to be as small as possible in order to reduce this loss. The only disadvantage of this venue is that reading the voltage across this resistor (which is directly proportional to the current) becomes harder. Our devices, however, have been meticulously crafted to ensure accuracy while regulating current, even when the resistor is smaller than 1 Ohm.

    What you want to do is use the digital resistor to ser the VREF analog input. How you implement the VREF voltage divider is up to you and it can be an actual voltage divider as with an external potentiometer or a digital potentiometer. It can also be a DAC or even a PWM output with an RC filter. I suggest you take a look at one of our EVM's in order to see how we have implemented the VREF analog input around a set SENSE power resistor.

    Hope the info helps. Best regards,

    Jose Quinones

  • in reply to Jose Quinones
    Expert 1550 points

    Hi Jose,

    Of course, I have seen functional diagrams in datasheets and I know that the resistor is an active element. This is a main problem :)

    I haven't thought that VREF controll will be a much easier. What a shame.  In DRV8811EVM you use DAC method. It is simple and cool.

    Now I' m thinking about my problem and I arrive at the conclusion that my question wasn't nimble.

    Thank you very much for your help,

    regards,

    Mikolaj