DRV8834: VREFO bypass cap

bowerymarc

Expert 1935 points

Part Number: DRV8834
Other Parts Discussed in Thread: DRV8843

I see in the DRV8843 data sheet that you should not place a bypass capacitor on this pin. I have inherited a design where this directive was ignored and there is a 0.1uF cap on this pin. I'm getting some units where the DRV8843 is failing, though all the input signals are correct (also, motor voltage supply is 8V max). I'm wondering if a bypass cap on this pin (perhaps over multiple power up/down cycles) could cause damage to the chip leading to inability to step?

Related - the design has VREFO going thru a resistor-divider DAC and back to AVREF/BVREF. My question is, does VREFO modulate at all (i.e. for current limiting, during certain phased, etc.) or is it always a constant 2V nominal?

Thanks,

Marc

over 7 years ago

0 Rick Duncan over 7 years ago

TI__Guru** 114905 points

Hi Marc,

VREFO is an internally generated 2,0 V nominal reference voltage. It is designed to be a constant 2V nominal, and drive a load of <400uA.

Adding a capacitor can create inrush current much larger than this.

What current does the resistor divider DAC require?

0 bowerymarc over 7 years ago in reply to Rick Duncan

Expert 1935 points

The part is MCP4726 and it specs input current 210uA typ, 400uA max with the buffered input mode. No buffer, same with additional spec of 210kΩ/29pF input impedance. We've been using it in buffered mode.

0 Rick Duncan over 7 years ago in reply to bowerymarc

TI__Guru** 114905 points

Hi Marc,

Thank you for the confirmation of the resistor divider DAC. The maximum current on the VREF pin is 40uA, so there should no problem there.

The capacitor on the VREFO pin should be removed.

0 bowerymarc over 7 years ago in reply to Rick Duncan

Expert 1935 points

I think you mean 400uA for VREFO?

0 Rick Duncan over 7 years ago in reply to bowerymarc

TI__Guru** 114905 points

Hi Marc,

Yes, it is 400uA for VREFO. The 40uA I mentioned is the absolute max current the MCP4726 VREF pin would sink.

In reality, the current should be ~10uA (2V/210kOhms) typical unbuffered, and probably less buffered.

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DRV8834: VREFO bypass cap