STNS01PUR LDO no output

[Orgo]

Hello ST,

Im using STNS01 as a power source in two of my different project prototypes. Unfortunately booth are facing the same problem. When the pcb was produced LDO/charger worked as intended. However after many battery and usb disconnection durring fw debugging, the problem suddenly appeared - LDO do have otput only when USB is connected. After USB disconnect LDO voltage falls to zero eventhrough 4.1V charged li battery  remains still connected. 

Voltages to identify the problem closer:

1) connected the battery(AA sized lithium cell), usb cable and measure voltages(all are references against GND, not battery - terminal which is polyswitch protected):

usb  connected- ntc:0.53V SD:0V IN:5.1V SYS:5.08V BAT:4.13V 

2)then I disconnected USB but kept battery connected

usb disconnected batery backup- ntc:0.0V SD:0V IN:0V SYS:0V BAT:4.14V 

The schematic-power supply part is here: LDO is supplying just low power chips(mcu and sensors). DNP components are soldered manually so they are present.

The schematic suply/mcu/sensor part is here: 

From your datasheet it looks like the internal battery switching mosfets(M3,M4) dont turn on the battery or are damaged(no SYS voltage). 

LDO capacitor: In your datasheet there is stated 1uf in the table 3., Application schematic  above in the same datasheet (table2)recomends 2.2uf cap. Ive added higher value(C17)as well. Might it be causing this problem? There is no range of caps where your LDO is stable writen in the datasheet. However, it looks like its happening before LDO...

So my question is is there a way how to avoid this problem?  What am I doing wrong?

Thank you, best regards Rado.

*I tried to search for similar topics however all topics I found were closed and none of them answered my question

[Peter BENSCH]

Welcome @Orgo, to the community!

Thank you for your detailed, well-founded and remarkably comprehensive investigation of the problem.

It does indeed appear that the anti-series connected MOSFETs M3 and M4 are damaged, which could have several causes, e.g. ESD pulses, as MOSFETs are very sensitive to overvoltage. It is conceivable, for example, that such ESD events occurred when connecting or disconnecting the battery. To avoid such events, it is advisable to connect a TVS to IN, SYS and BAT/BATSNS against GND, which you have already done with U2 and D13. However, if we do not know the possible ESD events and their energy levels, we cannot say whether the peak pulse power of the IP4234CZ6 (100W) and the SMF5V0A (200W) is sufficient to protect the STNS01 from those pulses. You must not ignore the differential resistance of the TVS, which can cause voltages to occur in smaller TVS such as these that exceed the maximum rating of the STNS01 (VIN: 10V, SYS: 6.5V, BAT: 5.5V), meaning that the STNS01 must be considered irreparably damaged. Instead of the SMF5V0A, you could take a look at the SMA6J5.0A, for example, which has comparable voltage values but can provide significantly better protection with 600W, which is why it has (must have) a slightly larger package.

As far as the LDO capacitor is concerned, the relevant specifications are purely recommendations and should be understood as minimum requirements. Whether 1µF, 2.2µF or 4.7µF are connected to the LDO pin is secondary and should not be considered the cause of the observed failure. The important thing here is that you use ceramic capacitors.

Hope that helps?

Regards
/Peter