DRV8350: n_Fault Output Capability

Hi Brian,

Has the customer tested ESD with the 1nF cap in place, is it still failing?

If so can they try intermediate values between 1nF and 1uF? (Try 47nF, 100nF, 500nF . . .) to notice point of failure.

Also, if the RC filter is not strong enough to protect from ESD strikes, has the customer considered using TVS diode in order to protect pins from ESD?

Thanks,

Joseph

Hello Joseph,

Thanks for comment.

However, customer would like to focus on the R/C filter and device itself capability, not workaround for ESD solution.  

Below waveform was short-circuit test on U-V or U-W phases of the DRV8350. When performing short-circuit, the nFAULT pin is high -> Low -> high ->  Low..., not very stable. The question is that how the behavior is with 1uF filter? Why 1nF doesn't?

Customer suspected if the external capacitor has relationship with nFAULT pin or not? Could you help on this clarification in advance! Very Thanks. 

Scope setting:

CH1: nFAULT pin of the DRV8350, CH2: GD_FAULT_C (Conneted to MCU I/O pin) 

Pull-High Resistor (Rpu) is 10Kohm(default)

We have tried Rpu that was changed from 10K-OHm to 1K-OHM, but there is no improvement.  

Filter C: 1uF

Regards

Brian 

Hi Brian,

Yes I suspect the capacitor is causing the behavior the customer sees in the waveform. 

1nF: OK -- Smaller cap value allows nFAULT to be pulled down in shorter time when fault is detected. Correct behavior.

1uF: NG -- Bigger cap value is harder for device to pull down in time, holds charge from Rpu, which causes bouncing (high, low, high, low..) effect.

Thank you,

Joseph

Thanks. 

So it looks like the problem is not related to the internal open drain of the device, that is related to the RC filter and Rpu, correct?

Rpu may decide the current amounts to charge the filter capacitor at startup. Why it will be bouncing?   

Regards

Brian

Hi Brian,

Yes, behavior is related to the RC filter, not device internals.

Can the customer test different values here?

What is wrong with the 1nF behavior?

Thank you,

Joseph

Thanks!

the reason behind is that 1uF filter is able to solve system ESD. That’s a purpose designed. However it has side effect as PG could be bouncing. Therefore customer is trying to understand what happened with the Circuit and IC. 

for this bouncing behavior, except RC filter ! Can Rpu have contribution on this bouncing? 

Hello Joseph,

Here please find the additional information and questions as attached file. 

For the bouncing behavior with 1uF, customer still don't understand very details. Here're two configuration of the R/C filter in the below slide. and you can see the different behavior of the nFAULT signal. Could you help to clarify this behavior more! very thanks. 

DRV8350 Issue July16, DELTA.pptx

Hi Brian,

1. Can DELTA also provide waveforms of the supply of the pull up (5V) to see if the supply is dipping during the testing?

2. Can they try more values less than 1uF? (Try 47nF, 100nF, 500nF . . .)

Please note that it is not recommended to put such large capacitors on nFAULT pin, this is why customer sees incorrect behavior. 

Thank you,

Joseph

Hi Joseph,

1. The measured 5V waveform is shown in the attached image. It looks fine.

2. We believe there’s no need to test other values.

Could you explain the internal operation logic of this pin and why the bouncing occurs?

Thank you

Hello,

The device's internal pull down structure is not designed to have any extra current sink through the nFAULT pin other than the small amount of current from the 10k pull up resistor.

There is no way around this, and hence why you see this behavior with the 1uF capacitor.

The recommended ESD protection is to use a TVS diode with a breakdown region above operating voltage, but low enough that it allows for a path to ground during an ESD strike. I have tested this method as a successful way to protect our devices from ESD.

Please understand that the device's internals are NOT compatible with a large capacitor on the nFault pin. The large sink current and therefore large amounts of ringing on internal pulldown structures caused by the cap is most definitely the source of the bouncing.

Thank you,

Joseph