We have approximately fifty modules in the field that use the L6981CDR. The schematic below shows how they are connected. The L6981CDR is used (in combination with other components) to generate an output voltage of 5V. This output voltage is used to power our system. Our system draws a maximum of 700mA from the buck converter. However, most of the time, the current draw is around 5mA in "sleep" mode and 200mA when the system is "awake."
After conducting tests with the buck converter, it seemed to work well, so we put it into production. However, we are now finding that about 5-10% of the systems are returning with the issue that the output voltage of the buck converter stops working after a few days. We measure an output voltage of 0V at the output of the buck converter (point +5V in the schematic) and the current draw is approximately 0mA. We also checked the EN/CLK pin of the L6981CDR, where 12V was present (with a supply voltage of 12.6V). The VIN voltage was also correct. Additionally, we looked at the SW output and saw no pulses, as we normally see with a buck converter. After this phenomenon occurred, we removed and reapplied the supply voltage, and the buck converter worked again.
The cause of this issue is still unknown to us. We have not been able to identify any logic behind it. It occurred in one system after 5 days, but in another system, it happened after just an hour and then did not occur again.
Does anyone have a solution or suggestions for this?
Below are the components we used:
- IC13: L6981CDR
- C5, C6, C10, C25, C28: UMK212B7104KGHT (100nF)
- R8: 10K 0603 1%
- C24: C2012X7R1H105K125AB (1uF 50V 1% X7R)
- C26: C3216X7R1H106K160AC (10uF)
- C27: GRJ32EC71E226KE11L (22uF)
- L3: WE 744393305220 (datasheet)
- R59: 330k 1%
- R60: 68k 1%
- C4: AEH0608220M050R (22uF)
- D7: S1J (Farnell: 4220197)
- D8: SMAJ33A-Q (Farnell: 2839619)
- F2: MINISMDC030F-2 (PTC fuse)
Schematic buck-converter
Top layer
Bottom layer
