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DRV8830: setting current limit less than 200mA

David Freon
Prodigy 40 points
Part Number: DRV8830


Hi,

We need to drive a DC motor through an I2C interface. The DRV8830 fits very well, except that max current in our DC motor must be limited to 92mA.

In DRV8830 datasheet, Current limit sense voltage VILIM is 200mV typical and Current limit sense resistance RISEN must be less than 1 ohm (§7.3.3). As a consequence, it seams to be impossible to set a current limit less than 200mV/1ohm = 200mA.

Could you explain why sense resistance must be limited to 1 ohm ? What is the issue if a 2.2ohms is used to limit current to 200mV/2.2ohms = 91mA ?

If not possible, do you have an application circuit using a resistance less than 1ohm and still limiting current below 200mA ?

Thanks in advance

David FREON

  • 0
    TI__Guru** 114905 points

    Hi David,

    Why is the current limit of the motor 92mA?

    What is the stall current of the motor?

    The Risense resistor is intended to help detect a stall condition.

    Using a 2.2Ohm resistor should work, but please keep in mind the 200mV varies from 160mV to 240mV. This will result in less than 92mA in some cases and more than 92mA in others.

  • 0 in reply to Rick Duncan
    Prodigy 40 points

    Hi Rick,

    Thanks for yours answer. Our motor is a 615 series micromotor from Faulhaber. Here is the datasheet : https://www.faulhaber.com/fileadmin/Import/Media/EN_0615_S_FMM.pdf.

    Exact part number is 0615N4.5S. As yo can see on the 1st page of the datasheet, 92mA is the current for thermal limit. This is given at 22°C ambient temperature and, as our ambient temperature can be up to 70°C, we will have to choose an even lower current limit.

    The stall current of the motor is not given in datasheet. We can estimate it using Nominal voltage (4.5V), Terminal resistance (37.7ohms) and, as the motor is driven in PWM, Rotor inductance impedance at DRV8830 PWM frequency (95µH => 26.6ohms @ 44.5kHz) : 4.5V/(sqrt(37.7²+26.6²)) = 97mA, witch is the same order as thermal limit current.

    As you mention, Risense value must be chosen 20% higher to take into account the +/-20% Isense threshold tolerance. The lowest tolerance case (160mV) should not be an issue since we need very little current (the motor load is well balanced, so nominal current will be a little bit higher than Motor's No load current, something about 20mA).

    Its a good news that a 2.2ohms resistor should work. But, if it is the case, why Risense resistance is limited to 1ohm in DRV8830 datasheet ? Specifying threshold voltage seams to be sufficient, and we wonder why Risense resistance range is also specified.

    Best regards

    David FREON

  • 0 in reply to David Freon
    TI__Guru** 114905 points

    Hi David,

    I found that the resistor can be increased but there may be some loss of accuracy.

    The 1 Ohm limit was set to provide boundaries for the electrical characteristics.

  • 0 in reply to Rick Duncan
    Prodigy 40 points

    Rick,

    Thanks for this clarification. Could you please give more details ?

    What order of magnitude of accuracy loss are you talking about ? Using either 2, 3 or 4 ohms resistor, we must expect a loss of accuracy in the range 1~2%, 5%, 10% or 20% ?

    1~2%, or even 5% of accuracy loss is small compare to the Sense input +/-20% tolerance (160 to 240mV). But 10 or 20% of accuracy loss will be an issue to use the DRV8830.

    Best Regards

    David FREON

  • 0 in reply to David Freon
    TI__Guru** 114905 points

    Hi David,

    The accuracy loss is <5%.