Other Parts Discussed in Thread: TPS61196,
FROM CUSTOMER:
Our LCD driver PCB is using a this part for 10V power to the LCD, and a TPS61196 LED driver to power the backlight. We are experiencing an issue that is looking more and more to be caused by defective TPS62147 IC's. The symptom is that the backlight of the LCD randomly turns off for 25ms at a rate of anywhere between 3 times an hour and once a day. The
backlight driver enable is driven by the PG signal from the TPS62147. What appears to be happening is the PG signal is dropping to 0V for 1-2us and that is kicking off a soft start cycle with the backlight driver, which takes 25ms. This can be seen in the attached scope trace. Trace #1is the IFB1 pin of the TPS61196 backlight driver, so you can see the backlight turning off. Trace #2 is the 12V line feeding both IC's. Trace #3 is the 10V output of the TPS62147, and trace #4 is it's PG signal. I
have also included portions of the schematic that contain these components. We have also instrumented the IC with a thermocouple and confirmed that it was always below 50C.We have shipped a few hundred boards that have not exhibited the issue to our knowledge. So far all of the TPS62147's have been ordered from Mouser. When we discovered the issue I ordered 10, and replaced them on 8 of our boards. Since then none of those 8 have exhibited the problem. I then ordered 40 more, and all of those that I have used have exhibited the issue. THEY WANT TO START A FAILURE ANALYSIS OR SEE IF TI CAN PROVIDE ANY INSIGHT OR SUGGESTION REGARDING THE BEHAVIOR THEY ARE SEEING. SENT CUSTOMERS EMAIL CHAIN TO PARTS@MOUSER.COM WITH ATTACHMENTS. PLEASE ADVISE IF WE CAN START THE FA PROCESS. REGARDS.
ADDITIONAL INFO FROM THE CUSTOMER TODAY:
We have found that changing the feedback resistors for U1 to 10x lower values (R1 = 10k, R2 = 130k) appears to prevent the PG line from dropping.The unit ran over night without any glitches on the power good signal. With R2 being 1.33M any variation in current over ~40nA into or out of the FB pin will cause the control loop to think the output is out of regulation and cause PG to go low. Lowering R2 to 133k reduces the sensitivity to current fluctuations.The original values were selected based on criteria provided in the TPS62147 datasheet. Can you please ask TI to review this information and provide feedback on the resistor values? Do they believe the updated resistor values are more appropriate for the circuit? CAN WE ASK FOR TI ASSISTANCE ON THIS? SOUNDS LIKE THE PARTS WILL WORK AND ARE NOT DEFECTIVE BUT NEEDED A DIFFERENT RESISTOR. PLEASE CONFIRM.
Please advise.
