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DRV8308: Reduce loss through sense resistor

john sullivan
Prodigy 130 points
Part Number: DRV8308
Other Parts Discussed in Thread: DRV8320

Hello, I really love the DRV8308 because of the commutation logic feature that it has. However I am worried about the amount of power lost through the sense resistor. The device starts limiting current at 0.25V across the sense resistor. This means the the power lost is equal to your limiting current x 0.25V. We want to limit current at 3A, meaning we will use ~70mOhm resistor and burn 750mW accross it. Is there any way to use a smaller resistor without losing the current limiting functionality of the chip? I am worried about applying an amplifier to sense resistor as the output voltage still needs to be very small.

Thanks,

  • 0
    TI__Expert 4435 points
    Hi John,
    Can you please explain what you mean by losing the current limiting functionality of the chip?
    If you use a smaller sense resistor value then the current limit will be less since the Vlimiter is set typically to 0.25V but current still regulate for a lower value.
  • 0 in reply to Luis Riveros
    Prodigy 130 points

    Hi Luis,

    Could you explain what you mean when you say "current [will] still regulate for a lower value"? From the datasheet it seems that the chip will only regulate current if the Vlimiter is 0.25V. This means that for a 3.5A current limit I need a 70mOhm resistor and if I lower that to 50mOhm it will regulate at 5 Amps.

  • 0 in reply to john sullivan
    TI__Expert 4435 points
    Hi John,
    That is correct, the current will regulate according to the resistor selected since Vlimiter is fixed at 0.25V so a 50mOhms will regulate at 5 amps. so you don't lose current limiting functionality, I guess is what I was trying to say, you just will not get what your current needs are unfortunately.
    Have you consider the DRV8230 device? that device is more flexible as the current also depends on the gain of the internal current sense amplifier.
    www.ti.com/.../drv8320.pdf
    You can use 1xPWM mode if there is a limitation on your inputs. Thank you.
  • 0 in reply to Luis Riveros
    Prodigy 130 points

    Hi Luis thanks for the response.

    I like the 8308 because of the single sense resitor, commutation logic, and hall effect inputs. Is there another chip that has these features as well as a variable current limit?

  • 0 in reply to john sullivan
    TI__Expert 4435 points
    Hi John,
    I apologize for the delayed response. the DRV8320 might be a candidate when using 1xPWM(Please check figure 19 for Hall effect sensor connections). Take a look at the datasheet here: www.ti.com/.../drv8320.pdf
    Thank you.