Recently I have designed a new board using TPS74801RGWT, the following picture shows the schematics.
Actually this LDO was a backup circuit, I did NOT solder L48 for use, that means this TPS74801 was almost no load. During my debug work on these boards (13 pieces in total), I found TPS74801 could not output 2.5V voltage normally on two boards, while on the other 11 boards this LDO worked OK. Then the following test/change were done:
1. Test the SS, EN, IN, OUT voltage during power up on failed board, the SS voltage would get down slowly after charged to about 0.8V. While on a board with good output, the SS voltage would maintain 0.8V after power up.
2. Remove SS capacitor C523 and retest all four voltage above, SS voltage still getting down after power up but with less time to 0V. Then enlarge SS capacitor C523 to 47nF, and SS voltage still getting down but with more time to 0V.
3. Solder a resistor between output and GND to enlarge the output current to 125mA, then power up, and found SS voltage still getting down.
4. Connect the SS pin of TPS74801 to a 0.8V power source from outside, the failed TPS74801 would output 2.5V normally.
5. Replaced one failed TPS74801 in one circuit board with a brand new one and the new one worked well while this did not work in the other circuit board.
Besides, I checked the solder condition and found it seems OK. And all the temperature was around 25℃ during these experiments.
Now I have some questions here--
- Can you help to review my design, is there any problem according to the schematic shown above?
- How could I confirm whether the chips were OK, I somewhat doubt there might be something wrong with this batch of TPS74801.
- According to the experiments on the SS pin, the problem seems to related to the soft-start discharge block inside the chip, could you help to explain more details about the mechanism of the SS pin work inside?
- Any other suggestions on debugging?