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DRV8801: Need your answer for nFALUT PIN output voltage fall slowly.

Part Number: DRV8801

Hi team,

thanks for your support before.

We know that when DRV8801 in sleep mode, then we don't care nFault PIN as datasheet said below.

but my customer's test standard that need float current <1mA, but some DRV8801 (which nFault output voltage will fall slowly) float current will > 1mA will fail this test. and we found the way to improve is increase nFault pull-up resistor from 33k ohms to higher value then the current can be reduced very significantly.. like this e2e Umberto said.(+) DRV8801-nFAULT - Motor drivers - INTERNAL forum - Motor drivers - INTERNAL - TI E2E support forums

So far the operation is OK and find the way to improve float current <1mA is increase nFault pull-up resistor's value.

1. I would need your help to confirm that increase pull-up resistor at nFAULT Pin is ok or not? would you give me the limit value?

2. Customer would like to know the reason why when we in sleep mode a few device's nFAULT Pin voltage will fall slowly and float current will >1mA? since this situation haven't happened before. Would you give me some explanation?

BR,

Jimmy

  • Hey Jimmy,

    Thank you for posting your question on E2E.

    1) Increasing the resistor value will result in smaller current as you have seen. It is a little hard to give a hard upper limit without bench testing the exact setup. 

    The best way to tackle your issue would be to consult implement Rick Duncan's comment "The pullup resistor is connected from nSLEEP to nFAULT, and will only consume current when the device is in normal operation and nFAULT is actively pulled low by the DRV8801. When sleep mode is entered nSLEEP is now a logic low, so there is no current draw."

    2) How often is this happening? Has this design been out for some time?

    Best,

    Akshay

  • Hi Akshay,

    thanks for your reply!

    1.

    Did you have any real case has high resistor? would you please let me know to convince customer? since the board is done and might hard to redesign. i think change resistor is the only way to deal with this situation now. I will discuss with customer about pull-up resistor from nSLEEP to nFAULT. In addition, Do we rewrite datasheet's Typical Application Schematic? since the diagram pull-up resistor still connect from VMCU to nFAULT PIN. 

    2. Probably 20pcs board will have 1pcs. and this 1pcs will always have this problem.  

    BR,

    Jimmy

  • Hey Jimmy,

    You can increase the resistance to 100k and I dont expect to see any issues from doing so. 

    The nsleep connection to nfault is a workaround to the high current you are seeing in your system, so not sure if DS change is needed but I will discuss this separately w/ my team.

    I just want to ensure we are not missing anything, so can you show in your schematic where you are measuring the float current. Please send a screen shot with markups for review. Also what is the reason for the capacitor on nFault to GND? Have you tested it w/o the cap to see if you are noticing a difference?

    If circuit connections cannot be changed then can you implement Rick Duncan's suggestion 1? Where you set the MCU GPIO to low or enable the pulldown internally on the GPIO?

    Lastly, I wanted to check if you have performed an ABA swap between the 1/20 Driver where you noticed the current rise and one of the 'good' boards where no issue was noticed? This can help narrow down where the issue might be.

    Best,

    Akshay

  • HI Akshay,

    Thanks a lot!

    Actually we have done the ABA swap also sent to FA and result is Trouble Not Identified (TNI) !

    So I use this result to convince customer that our device is working at normal situation, if you want to pass the test which <1mA at sleep mode. you can increase pull-up resistor or design pull-up resistor between nSLEEP and nFAULT pin.  

    So far so good. if customer have any feedback need to figure out I will call for your help, this discuss I think we can stop here! thanks a lot! Akshay! 

    BR,

    Jimmy

  • Sounds good, Jimmy! 

    Best,

    Akshay