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Original question:

DRV8842EVM: Regarding Current Regulation

DRV8842EVM: Regarding Current Regulation(disagree expected value and Actual value)

Sho Kobori
Intellectual 640 points
Part Number: DRV8842EVM
Other Parts Discussed in Thread: DRV8842-EP, DRV8842,

Hello,

This is a question regarding current regulation of DRV8842-EP.

We are now testing the current regulation function of DRV8842 on the evaluation board.

The test evaluation is below;

Target : DRV8842EVM

Rsense : 5.1ohm

Vref : 2.5v

Moter(TM) : L=43mH, R=24.9ohm

VM : 24v

We would like to check current regulation with adjustable Ichop.

IN1 and IN2 is constant. IN1 is 14%, IN2 is 0%.

In this case, Ichop is adjusted between 0% to 100%, and expected current value is 100mA.

Results of each test case is as a following;

In these results, Act current and expected current is disagree when Ichop is lower than 80%.

We would like to operate current regulation when Ichop is between 0% to 100%.

Could you please tell me about configuration of correctly operated current regulation?

Thanks,
Sho

  • 0
    Intellectual 640 points

    this is load connection image.

  • 0 in reply to Sho Kobori
    TI__Guru 54645 points

    Kobori-san,

    This seems similar to your other post, but you are fixing input duty cycle and adjusting chopping current using the I0-4 pins?  Again, I have to ask if it is possible to consider a lower current device.

    Regards,

    Ryan

  • 0 in reply to Ryan Kehr
    Intellectual 640 points

    Hi Ryan-san,

    Thank you for your reply.

    That's right, you are correct.

    In this case, We are fixing input duty cycle and adjusting chopping current using I0-4 pins.

    However, these results seems unavailable current regulation function.

    Why is there uncontrol point of current regulation function? Is that because of low current device?

    Please advice.

    Thanks,

    Sho

  • 0 in reply to Sho Kobori
    TI__Expert 4955 points

    Hello Kobori-san

    I am sorry that you are experiencing these issues. I am happy to be here to help you. Allow me to conduct some tests in the lab with one of our EVMs to confirm we experience the same behavior. There is a possibility that it is due to the low currents and high sense resistor. 

    I will run similar tests as yours, including the use of a lower sense resistor and a resistive load, I will share the results once obtained. Please let me know if any other questions pop up in the meantime. 

    Best,

    Pedro Arango Ramirez

  • 0 in reply to Pedro Arango Ramirez
    TI__Expert 4955 points

    Hello Kobori-san,

    In the meantime, would you be able to provide us with output current waveforms? They could prove very helpful in diagnosing the problem.

    Best,


    Pedro Arango Ramirez. 

  • 0 in reply to Pedro Arango Ramirez
    Intellectual 640 points

    Hi Pedro-san,

    I'm sorry late reply.

    I can provide waveform data about this case.

    Please refer to attach zip file.

    DRV8842_EVM_TEST1.zip

    Thanks,

    Sho

  • +1 in reply to Sho Kobori
    TI__Expert 4955 points

    Hi Kobori-san,

    I am happy to help, no worries about the late reply. After reviewing your data I think I might have found the issue. The reason that IChop is not being able to regulate current below 80% is that the target current is too low for the device.

    This is due to blanking time, here is the section of it on the datasheet:

    8.3.4 Blanking Time

    After the current is enabled in an H-bridge, the voltage on the ISEN pin is ignored for a fixed period of time before enabling the current sense circuitry. This blanking time is fixed at 3.75 μs. Note that the blanking time also sets the minimum on-time of the PWM.

    Essentially lowest current that the device can achieve via current regulation is limited by the blanking time, once the current is re-enabled by the rising edge of the input PWM signal it needs to stay on for a fixed 3.75uus regardless of what the current is. This is why on the images you presented you can see how the pulse width gets shorter from case 1 to case 2, but past case 3 it stays consistent. It can no longer become any shorter and therefore can't reduce the current any further. 

    If you would like to read more I went into more detail in your other E2E post:

    https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1052825/drv8842evm-regarding-current-regulation

    The solutions to achieve a lower current during current regulations are the same because they stem from the same problem of having too low of a current threshold Itrip for current regulation:

    • If you haven’t already try changing the device’s decay mode to fast decay. This can be done to setting the decay pin to logic high, and should help increase the current decay.
    • If possible lower the input voltage so that current will rise slower through the windings.

    Please let me know if you saw an improvement with the suggested fix or if you have any other questions.

    Best,

    Pedro Arango Ramirez 

  • 0 in reply to Pedro Arango Ramirez
    Intellectual 640 points

    Hi Pedro,

    Thank you for your support.

    My issue was resolved.

    Thanks,

    Sho