DRV8811 drive current decay

Gregory Dierksen

Prodigy 70 points

Other Parts Discussed in Thread: DRV8811

Hello,

I am using a DRV8811 chip to drive a NanoTech LP2515S0104. The DRV8811 circuitry is as follows:

VMA = VMB = 12V

ISENA = ISENB = 2.87 Ohms

RCA = RCB = (220 pF) * (14.7 kOhms)

VREF = 3.0V

Decay mode is set to fast decay.

During testing I've noticed that when driving the motor at low speeds (~50 steps/sec) the drive current has a significant decay from it's initial value (see figure below). What gives rise to this decay? Is it normal or can it be prevented somehow?

This figure was obtained using a current probe set to 100mV/A. I wrapped the motor coil wire around the current probe twice so all amplitudes in this figure should be divided by two. The top and bottom cursors are positioned at 200mA and -200mA, respectively.

over 9 years ago

0 Rick Duncan over 9 years ago

TI__Guru** 114905 points

Hi Gregory,

I am assuming you have SRn set to a logic low, and USM1 and USM0 are set to a logic low. Is that correct?

If so, please try setting SRn to a logic high. I think you will see better results.

0 Gregory Dierksen over 9 years ago in reply to Rick Duncan

Prodigy 70 points

Rick,

You are correct that USM1 and USM0 are set to logic low. However, SRn is set to logic high.

I did, however, make a mistake in the original post: the decay mode is slow. But I don't believe this could be producing the behavior I'm seeing, correct?

Thanks,

Greg

0 Rick Duncan over 9 years ago in reply to Gregory Dierksen

TI__Guru** 114905 points

Hi Greg,

Sorry for the delay getting back to you.

I have seen this before, but not under the conditions you are describing. Typically, it is when SRn is set low. Is it possible that SRn is floating, and therefore a logic low? Did you measure the voltage at the SRn pin?

By the way, is this the TI EVM or your board? If your board, do you happen to have a second board to try to confirm this is repeatable?

Thanks

0 Gregory Dierksen over 9 years ago in reply to Rick Duncan

Prodigy 70 points

Rick,

Thanks for the response. I confirmed that the SRn pin is high.

However, our in-house electrical engineer was able to take a look at the setup and determined that the measurement was not being taken accurately. Therefore I believe we have resolved this particular issue.

Thank you again for your help.

0 Rick Duncan over 9 years ago in reply to Gregory Dierksen

TI__Guru** 114905 points

Hi Gregory,

I am glad to hear that your issue has been resolved.

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DRV8811 drive current decay