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DRV8703-Q1: reason of "nFault"

Soichiro Aoki
Prodigy 40 points
Part Number: DRV8703-Q1

Dear support team,

We use DRV8703-Q1 on our systems, but it outputs "nFault", we need to resolve it.
We considered GAIN_CS, VREF, ICHOP and VDS have some effect for nFault, so we evaluated them, but we do NOT solve the problem.
So we need your advice.

On our systems, it controls DC motor and the maximum current is around 70A.
By the outside circuit, RDS on FET is 1.15mohm, R(SENSE) is 1mohm, VDS is 0.145.
In this condition,
1. In case of GAIN_CS=39.4 and VREF=2.1V (ICHOP by calculation is 48A), system can operate correctly and do not output "nFault".
2. In case of GAIN_CS=19.8 and VREF=2.1V (ICHOP by calculation is 101A), the current is increased to about 70A and DRV8703-Q1 outputs "nFault".
We considered that it may cause by lager ICHOP, so we changed the VREF as below;
3. In case of GAIN_CS=19.8 and VREF=1.07V (ICHOP by calculation is 49A), but DRV8903 output "nFault".

Then we have the question.
Does VDS setting have any relationship for "nFault" judgement?
Or does other setting have any relationship for "nFault" judgement?

Best Regards,

Aoki

  • 0
    TI__Guru 55605 points

    Hello!

    Are you able to read the registers in the device and determine what fault is flagging?  That would help with the debug.

  • 0 in reply to Ryan Kehr
    Prodigy 40 points

    Dear support team,

    We tried to read the registers by SPI, but we could not get them.

    Could you advice the another way to resolve this matter such as your past experience ?

    Best Regards,

    Aoki

  • 0 in reply to Soichiro Aoki
    TI__Guru 55605 points

    I suspect it is a VDS fault.  The VDS monitors look at the voltage drop across the external FETs.  If the voltage is greater than the VDS level you have set, than nFAULT will drop to the low state.

    Voltage drop across FET is Rdson * Current.  Make sure to consider that the Rdson of the FET will increase over temperature and also from device to device.  It is important to use the maximum Rdson value at temperature and over process as specified in the datasheet to make sure you do not trip a Vds fault.