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# DRV8834EVM: Regarding the driving current reduction after the motor becomes heated.

Part Number: DRV8834EVM
Other Parts Discussed in Thread: DRV8834

I conducted measurements of the phase current using the DRV8834 EVM and observed a reduction in driving current after the motor heated up.

Specifically, the current decreased from 207mA to 187mA.

I would appreciate your assistance in understanding this situation. My understanding was that the DRV8834 maintains a consistent current output, so it should ideally remain at the configured value.

For reference, I have attached the current waveforms.

Here are the conditions of my experiment:

1. The driving current was set to 104mA via the EVM GUI.

2. I measured the current from Pin 1 & Pin 6 of the DRV8834.

3. The surface temperature of the motor exceeded 50 degrees during the experiment.

Any insights or guidance you can provide regarding this issue would be greatly appreciated.

Sincerely

webber

• Hey Webber,

A couple of thoughts:

It is likely due to the winding resistance of the motor changing across temperature.  I input your specs into Equation 1 of Thermal behavior of a stepper motor using T = 50C, T0 = 25C (assuming you started around room temperature), and alpha as 0.0039 as given.  This results in R(T) = R0 * (1 + 0.0039 * (50 - 25)) --> R(T) = R(0) * 1.0975.  Using this number with I=V/R and keeping V constant, an initial current of 207mA results in 207mA / 1.0975 = 188.6mA at higher temperature.  This is very similar to the measurement you found.

However, you're also correct that the motor driver should maintain a constant output current given a stable VREF value.  I suspect the motor driver also heated up during this test, or the sense resister warmed up and changed resistance a little bit.  Either of these could cause a slight variation in output current.

Regards,

Jacob

• Hi Jacob,

Thanks for the valuable feedback. Could you share with me anything else that might lead to a decrease in the driving current?

I need to compile a list of all the factors that could affect the driving current.

It seems I can't manage a driving current without a buffer.

For example, if the vendor's specified driving current is 82~125mA, I should include a buffer of at least 10mA. Therefore, the driving current for the configuration of the DRV8834 should be set to a minimum of 92mA.

Sincerely

Webber 2023/9/13

• Hey Webber,

That's pretty much all of it.  The DRV8834 is one of our older parts, you might want to look at something newer like the DRV8411A with improved current sense accuracy and regulation.

Regards,

Jacob

• Hi Jacob,

Got it, thanks.

Sincerely

Webber 20230914