UCC27531: The IN pull down resistance to GND of UCC27531DBVR changed after some time

Hi Sam,

Thanks for reaching out to us, my name is Mamadou Diallo, I am the AE supporting this device.

If you're measuring this resistance with the driver connected to the ISO7710, you could be measuring the impedance seen by equivalent resistance from the ISO7710 and the internal pull-down resistance of the driver with some additional resistance on your layout.

Additionally, you may also expect a typical resistor variation in our process of +/-20%.
As a sanity check, I would advise to first probe Pin 2 and Pin 4 with the driver alone (before soldering it to the board).

Errors in resistance measurement may occur due to various reasons including instrument calibration, tolerance, false current injection, etc...

Also what are using to measure this resistance? Are you using flying probe test to measure this resistance?

Thanks.

Hi Diallo,

Thanks for your quick response. At the begining I found the resistance changed is with the UCC27531 in circuit and connect to the ISO7710, but after I solder the UCC27531 out and measured it alone, the resistance is also low to 69K ohm, but after 1 days later I measured the part alone again, the resistance is 118K ohm, I used 8 pcs this UCC27531 in my design, I measure other 7 parts too, they are OK.

I use the Fluke 187 make the measurement, and POS probe connect to Pin2 and NEG probe connect to Pin4.

Regards

Sam

Hi Diallo,

More information, with the fail part I have if I place the POS probe to Pin4 and NEG probe to Pin2, the resistance will be 150k ohm. The part is measured alone.

Regards

Sam

Hi Sam,

The variation in your impedance measurement could be related to many factors including calibration or false current.
The fact that the measurement (with the device soldered out) is varying one day to another may be some false current injected by a the previous energized circuit or possibly leakage causing incorrect readings.

Please let us know if you're still observing the same readings.

Regards,

-Mamadou

Hi Mamadou,

If keep POS probe connect to pin2 and NEG probe connect to pin4, the reading is now around 118k ohm stable, I do not think there is some instrument setup issue, every time I measured at least two parts to make reference.

There is a strange test result, test with the failure part, if I swap the probe, the measurement result is different (118K ohm and 150 K ohm, refer to my comment on Mar 2), I guess there is some internal circuit parallel with this resistor fail, could you please help to make a deep analysis.

Thanks

Hi Sam,

Thanks for the updates.

The result seem strange. Just to confirm, the other 7 units that you mentioned that are "OK" what are the measurements on those?
In the meantime, I have ordered some samples on my end and will look at any variations on those units.

I will update you as soon as I have the samples in hands.

Regards,

-Mamadou

Hi Mamadou,

Please see below picture for test data, I record only the integer part of the reading. I am not very clear when the input resistance changed, so I measured all the PCB boards on hand for tracking.  

Regards

Above test result is with the UCC27531 soldered on PCB board.

Thanks

Hi Sam,

Thanks for the additional information.

I have received the samples and will update you within 24-48hrs.

Regards,

-Mamadou

Hi Sam,

Thanks for your patience.

Please find below measured input resistance from couple of samples, all within the specified range.

Probe + on IN pin and Probe - on GND pin.

Sample #	Input Resistance (measured) kOhms	Nominal Resistance (datasheet) kOhms	Nominal - 20% kOhms	Nominal + 20% kOhms
1	233	230	184	276
2	236.7
3	232.8
4	236.7
5	233.1
6	237.1
7	233.5
8	234.2

Looking at your measurement table though, most of your measurements seems to fall within the +/-20% variation except U13 on one measurement.

Can you please retake the measurement on U13 again while soldered on and off your board to confirm.

Regards,

-Mamadou

Hi Mamadou,

Thanks for the test results. I think new parts will have the pull down resistance within specification, or there will be a serious quality issue from TI. I have the UCC27531 worked in my board for several days and then this happen, I want to know what kind of unapt application will let this happen, may be the pull down resistor is fine but the internal other circuit (paralle with this pull down resistance) in the IC is fail, I need help to understand why this happen, then I can make some design change.

Regards

Sam