BQ76940: BQ76940 used for 12 cell configuration : charge discharge Mosfet ON /OFF issue due to UV

Part Number: BQ76940 Dear Team, We wanted to use BQ76940 used for 12 cell configuration : we are facing problem with charge & discharge Mosfet ON /OFF due to UV. we are able to read all the cell voltage as per cell shorted 12 Series configuration. no issue in cell reading. whatever cells are shorted are reading as 0.050V and it is as expected. We are able to turn on Charge Mosfet but discharge mosfet is not turning on due to again and again fault is getting generated due to UV. 1. How we can avoid UV problem, what will be setting in SW registers? 2. can we turn on both mosfet during charging & discharging? 3. As per my understanding if voltage below 0.5V will not be considered as UV fault, is this understanding correct? Thanks, Devayani

Hi Devayani, The UV problem is likely caused by the low cell reading issue you are investigating in the other thread: <secret URL>/support/power-management/f/196/t/894969 Solving that issue will most likely solve the UV issue you are seeing as well. I will continue to support you in the other thread and close this one. Best regards, Matt

Dear Matt, Other thread , I have closed, because that issue is resolved. can you help for this issue of UV and turning on MOSFET for CHG and DSCG. Now All Cell voltage readings are stable with 10mV accuracy, that is really good. Thanks, Devayani

Hi Devayani, Now that the cell voltages are reading correctly, the UV fault should only occur if the cell voltage is truly low. The under-voltage setting can be adjusted in the UV_TRIP register. An under-voltage condition on one of the cells will always cause the DSG FET to open. If the CHG FET is opening, it may be because of a different fault, so you should read the SYS_STAT register to see which fault is occurring. You can enable the FETs using the SYS_CTRL2 register. Best regards, Matt

Hi Matt, Thanks for feedback. We will test based on your feedback today and update you further progress. Just for an information: we are reading 0.050V cell voltage (All three group 4th cell reading) which is externally shorted at input side to make 12 cell configuration. Meanwhile please clarify below questions; 1. As per our understanding if cell voltage below 0.5V read by BQ then BQ will not trigger UV fault, is this understanding correct? 2. What is minimum voltage limit set in BQ for under-voltage trigger? 3. How BQ will understand that externally 15Cell or 12Cells are connected physically or different cell configuration as per data sheet? , and trigger UV fault? 4. if we do any wrong calculation (adding off set, ADC gain etc..) in CRC while reading cell voltage, is there any chance to trigger UV fault? Thanks, Devayani

Hi Devayani, That is correct - if the voltage is below an ADC reading of 0x0518 (which is around 0.5V), the device will consider this a shorted connection and not a real cell. If you made the shorted connections after connecting the cells, the device may need to be reset. You can either cycle power or send the commands to put the device into SHIP mode, then toggle the TS1 pin to boot the device. Best regards, Matt

Hi Matt, We will test and update you as per your suggestion. meanwhile please answer below questions; 2. What is minimum voltage limit set in BQ for under-voltage trigger? 3. How BQ will understand that externally 15Cell or 12Cells are connected physically or different cell configuration as per data sheet? , and trigger UV fault? 4. if we do any wrong calculation (adding off set, ADC gain etc..) in CRC while reading cell voltage, is there any chance to trigger UV fault? Thanks, Devayani

Hi Devayani, I believe these questions are all answered in the datasheet - see page 25 where there is a detailed discussion on setting under-voltage and over-voltage thresholds. This will answer questions #2, #3, and #4. (See the NOTE in the middle of the page which directly answers question #3). Matt