data/text/DLP3/898207

BQ27750: Some times lost control of external FETs

Part Number: BQ27750 Hello! I has build LiPo batteris packs controlled by BQ27750. It was a big mistake to use the 27750, but it’s too late to change anything. Device work, but some times disconnect batteris from charge or from load OR from both charge and load. When I connect BQ studio to batterie pack BQ27750 says that all fine and FETs are switched on. But if I try measure resistence of external FETs - them are "closed" (switched off) and on GATE pin of FETs is around 2-3 volts. Nothing indicates this condition in BQ Studio. To make device back to life I sending Reset device command to BQ27750 from BQ Studio. And after this action all work fine again.... some time.... I have more than 2000 pcs devices with this problem. Problem became SOMETIMES. I can't find conditios of this. What I do wrong? And how to resolve this problem?

Spider, The bq27750 can disable the FETS for numerous reasons. If after a reset the FETS are enabled then switch to disabled this leads me to believe you are triggering a time based protection event. If you want to investigate yourself, please make sure you are refreshing the registers screen in bqStudio so you can see updates as they happen. Pay specical attention to the "Protection Status" registers. Please provide a log file from bqStudio and a GG file from the data memory. Thanks, Eric Vos

Ok. I'll make log when it happen again. Why voltage at Gate pins is around 2-3 volts? When FETs is off- this value is ZERO, when is on - around 12V. Why sometimes is 3v?

Hello Spider, Can you provide a scope shot and also where you are measuring? Gate pins should be zero with respect from the gate to source (in this case pack+), can you measure that voltage? I believe if the protection FETs are off, that voltage should be zero.

Before Reset but with Closed FETs: After reset and with Opened FETs (see to Log panel): xxxxx.log

Spider, In the above bqStudio image you have CUV tripeed which would cause the discharge FET to turn off. in the 2nd plot it is not set, I believe you are tripping the protection which is killing the FETS. Thanks, Eric Vos

I understand this. But how charge device when both FETs are closed? Be careful! At above bqStudio images Charge FET are opened, BUT in real it is Closed. You can see that on photos. That is my problem! :facepalm:

Spider, this is an issue with your configuration if you are triggering a CTO and CUV. You should change CUV to recover when a charger is detected thus the CUV event will release when charging starts. I suspect the CTO timeout is way too low. this should be in 1000's of seconds. Thanks, Eric Vos

Manual control of FETs from bqStudio not work too. In GUI I see that FET is ON but in real it is closed. It is normal?

Spider, You should not be manually controlling the FETS, but instead let the gauge do the control. You writing to manually control would not work with protection activated. Thanks, Eric Vos

Where you see that protection is activated? I see that Charge FET is ON on first screenshot of bqStudio. But really FET is OFF.