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DRV8870EVM: Issue with max allowable current

Ibram Sahibu
Prodigy 150 points
Part Number: DRV8870EVM
Other Parts Discussed in Thread: DRV8870, DRV8701

Hi Guys,

We are using DRV870 part in our design and observed an issue, So I decided to perform same test on DRV8870EVM. The EVM also had the same issue. Let me try to explain below,

1. Set the motor voltage as 16.8V

2. Motor consumed ~2.4A while it is operating.

3. I was able to feel the temperature goes up gradually as motor is running.

4. After few mins (less than 5 mins), Motor starts to fluctuate (trying off and on) and current drops to half of it. I took the scope image on ISEN pin when this fluctuation occurred, attached it here for your reference.

5. Then I removed the sense resistor on the EVM and connected ISEN directly to ground. Still I observed the same behavior of motor getting fluctuated after few mins. Also, I saw the current went upto 2.6 from 2.4.

Actually, I want to use this part to allow 3A to motor. Could you provide your suggestions on this ASAP?

Regards,

Ibram

  • 0
    TI__Guru** 114905 points

    Hi Ibram,

    This appears to be a thermal shutdown issue.The image shows no current for almost 90ms. The overtemperature circuit would cause this.

    If you are using the DRV8870EVM with a 150mOhm resistor, the current appears to be approaching 3.5A. Can you confirm this by zooming in on the ISEN voltage?

    A scope capture using a current probe would also help.

  • 0 in reply to Rick Duncan
    Prodigy 150 points

    Hi Rick,

    I am trying to give you possible scope images since I don't neither have a current probe nor high end scope. 

    The pictures shown below taken on ISEN pin with 150mOhm mounted on EVM.

    Below picture taken when it is working normally. The peak max is 440mV and min is 340mV.

    The Picture below shows when motor starts to fluctuating.The peak max is 550mV and min is 32mV. This fluctuation starts between 1 to 2mins.

    I also connected the motor directly to the bench power supply (16.8V), It consumed 3.3A while it is running. I still wonder why EVM is not supporting this 3.3A of current.

    Regards,

    Ibram

  • 0 in reply to Ibram Sahibu
    TI__Guru** 114905 points
    Hi Ibram,

    There is a difference between connecting directly to the power supply and connecting to the DRV8870.

    At room temperature, there is typically 700mOhm more resistance using the DRV8870 (HS Rdson = 307mOhm + LS Rdson =258 mOhm + sense resistor = 150mOhm). This can reduce the current through the motor.

    As the device temperature increases, the Rdson increases further.
  • 0 in reply to Rick Duncan
    Prodigy 150 points
    Hi Rick,
    Yes, I understand the difference in connecting to bench power supply and driver.

    DRV8870 is rated for 3.6A of current but thermal protection gets activated when it allows 2.5A of current.

    How do we rectify this?

    Regards,
    Ibram
  • 0 in reply to Ibram Sahibu
    TI__Guru** 114905 points
    Hi Ibram,

    The device is rated for 3.6A peak current. It cannot sustain that current level indefinitely. At some point, there will be thermal issues. Please note section 7.4.2 of the datasheet.

    If you need a device that can provide 3.6A continuously, there are other devices like the DRV8432 with integrated FETs , or gate drivers like the DRV8701 which allows you to select the FETs.
  • 0 in reply to Rick Duncan
    Prodigy 150 points

    Hi Rick,

    Can you confirm the current carrying capability of this device? I mean the normal operating current.

    Regards,

    Ibram

  • 0 in reply to Ibram Sahibu
    TI__Guru** 114905 points
    Hi Ibram,

    Are you referring to the EVM or your board?

    Can you describe the conditions? Ambient temperature, voltage, copper area and thickness?
  • 0 in reply to Rick Duncan
    Prodigy 150 points
    Hi Rick,

    For now, Please consider the EVM. Since I observe the same behavior in both EVM and my board.

    I am testing this at room temperature of 70deg F. Voltage is 16.8V.

    For copper and thickness, please refer your EVM design.

    Regards,
    Ibram
  • 0 in reply to Ibram Sahibu
    TI__Guru** 114905 points
    Hi Ibram,

    I ran a test last night, and found that 2.1 Amp could be run continuously. This will vary some depending on the Rdson of the FETs, the overtemperature shutdown circuit variation, and the amount of solder coverage between the thermal pad and the PCB.

    I would expect ~2A to be the max continuous current on the EVM.
  • 0 in reply to Rick Duncan
    Prodigy 150 points
    It is really disappointing to know that this device is capable of allowing ~2A continuously and it is not mentioned in the datasheet. I selected this device based on the 3.6A current rating mentioned in the datasheet and it seems like I have to start my selection process again.

    Regards,
    Ibram