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DRV8871: Using currnet limit on half of device

Will Jarrett
Expert 3395 points
Part Number: DRV8871
Other Parts Discussed in Thread: DRV8873

 

I have a customer that wants to use the DRV8871 to drive a solenoid, and wants to leverage the current limiting feature to keep things safe in case of Short Circuit even though they are only planning to use Out1, but not Out2. 

 

Is the current detection on one or the other input or does it somehow measure the current on both legs?  Is there a way to leverage the current limit and only use ½ of the device.  Thanks.

  • 0
    TI__Guru** 114905 points
    Hi Jarrett,

    Driving a solenoid is possible, but can you answer a few questions.

    1) When the customer says current limiting, do they mean Itrip as defined in Equation 1 or overcurrent protection as defined in section 7.3.5.2?
    2) How is the load intended to be connected to the device (VM to solenoid to OUT1, or OUT1 to solenoid to GND)?
    3) Does the customer have any specific concerns about using both outputs (OUT1 to solenoid to OUT2)?
    4) What is the VM voltage and solenoid current?
  • 0 in reply to Rick Duncan
    TI__Expert 3395 points

    Hi Rick,

    Thanks so much for the response.  Let me try to help.

    1.  It would have helped if I would have stated "Over Current Protection".  We want the device to be protected in case something bad happens outside. In general, he plans to use PWM to control the current to the solenoid

    2.  Yes, I beleive VM through DRV8871 OUT1 (or OUT2) to Solenoid to GND

    3.  Yes, I beleive there is an issue with connecting the GND to the other output (Say out2).  The application is agricultural equipment and I beleive that there is a chassis ground that would be difficult to change.

    4.  12V typical.  Not sure of current, but below the 3.6A peak capabilities of the DRV8871.

    FYI, he hooked this up in the lab and the OCP does not work when he only hooks a load to OUT1 or OUT2.  If he connects the load in-between OUT1 and OUT2 the OCP works as expected.  We're not sure where we measure current in the device to take advantage of the OCP feature if only using 1/2 of the device.

    Thanks Rick!

    Will Jarrettt

  • 0 in reply to Will Jarrett
    TI__Guru** 114905 points
    Hi Will,

    There are two circuits. OCP (section 7.3.5.2) is designed to protect against an output short to power or a short to ground; and current limiting as described in section 7.3.3 and Equation 1 which limits the current to a desired setting.


    The device will always protect against short circuits that exceed a minimum of 3.7A. Each FET has this protection built in.

    The current limit function described in section 7.3.3 and Equation 1 limits current through the load. This limit can be set below the overcurrent protection of 3.7A to lower the initial stall current of a motor, or in the case of a solenoid lower the holding current once the solenoid is activated without requiring a PWM at the inputs.

    The current limit function monitor current through the low side FETs. There are two problems when using one output and attempting to limit the current:
    1) When using just the high side to the load, the current is not monitored.
    2) When using just the low side to the load, the output will remain on. This is because the device enters slow decay, which sets both outputs low.

    An alternative device may be the DRV8873 which monitors the current through the high side FET and provides the information via a IPROPI signal. Also, both outputs can be used because there is an independent mode. The automotive version is available now, and the industrial version is scheduled to release very soon.

    The device is a little more complicated and expensive, and the overcurrent protection is set to 6.5A in the hardware version. Both outputs can be used.