DRV8305-Q1EVM: Protection the MCU from over-voltage

John Pan

Part Number: DRV8305-Q1EVM
Other Parts Discussed in Thread: DRV8305-Q1,

Hello,

I have a question in the MOTOR & BEMF Sense section in page 4 of the DRV8305-Q1EVM schematics. Three Schottky diodes, D11, D12, and D13 were used to protect the MCU from over-voltage (>3.3V). These Schottky diodes are connected between VSEN A/B/C and VREG/VREF (pin 48 of DRV8305-Q1). My concern is that VREG/VREF is connected to an internal LDO in DRV8305-Q1, which cannot sink current. If the BEMF voltage was over 45V, there would have a current flow through these Schottky diodes to VREG/VREF, which many damage the LDO. Could you please advise whether my understanding is correct?

My approach to fix the problem is to replace these Schottky diodes with three 3V3 Zener diodes and placed them between VSEN A/B/C and GND. Could you please comment on my approach?

Thanks,

John

over 3 years ago

0 Adam Sidelsky over 3 years ago

TI__Guru 61360 points

John,

Could you show us a brief schematic or block diagram showing your proposition? I want to make sure I understand correctly.

Regards,

-Adam

0 John Pan over 3 years ago in reply to Adam Sidelsky

Intellectual 990 points

Hi Adam,

Attached please find the schematics of DRV8305-Q1EVM.

Thanks,

JohnDRV8305-Q1EVM_SCH.PDF

0 Adam Sidelsky over 3 years ago in reply to John Pan

TI__Guru 61360 points

John,

The schotkey diodes you mention are only to protect the voltage divider from exceeding 3.3V and damaging the MCU. The current through the diodes is limited to less than 1mA by the 62k ohm resistor forming the top of the voltage divider.

The VREG should not get pumped up because of the current limiting of the series 62k resistor.

You can also use the 3.3V zener clamp approach to GND if you desire, that should work too.

Regards,

-Adam

0 John Pan over 3 years ago in reply to Adam Sidelsky

Intellectual 990 points

Hi Adam,

Thanks for your reply. I understand the current through the Schottky diode is slightly less than 1 mA. As you said the VREG should not get pumped up due to the small current. But I am still curious where the 1mA current go since the VREG, which is connected to the LDO cannot sink the current. Do you mean that the LDO can sink up to 1 mA current without damage?

If you could, could you please discuss the pros and cons of the schottky approach used in the design reference compared to the Zener diode method?

Thanks again and best regards,

John

0 Adam Sidelsky over 3 years ago in reply to John Pan

TI__Guru 61360 points

John,

The LDO likely cannot sink current in an ideal case but on the rare chance that the schottky diodes conduct then we care most about protecting the MCU and we don't worry if the LDO gets pumped up.

The schottky vs. zener approach is up to you, they both have benefits.

Regards.

-Adam

0 John Pan over 3 years ago in reply to Adam Sidelsky

Intellectual 990 points

Hi Adam,

Thank you very much.

Best regards,

John

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DRV8305-Q1EVM: Protection the MCU from over-voltage