Other Parts Discussed in Thread: TPS61196
Per customer: Our LCD driver PCB is using a TPS62147 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 #1
is 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.
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.
-Brandon
