I have fried the BQ79606AQ1 Chip as well certain EVM components around cell 1 and certain power supply filter capacitors. I believe the issue has to do with the problem described in section 9.2.1.2.6 CB Input of the chip's datasheet, which states:
The Cell Balancing input are connected to internal balance FET through balancing resistor. The resistor sets the balance current. Connect CBn to VCn if not used. The CB pins must NEVER be connected to cell voltages (module connectors) that are expected to be less than the recommended operating condition. The internal FET diode will conduct and likely damage the FET in reverse voltage conditions. CB0 can not be left floating at any condition. If a connection to cell1 negative terminal is open the IC bias current will flow through the CB1/VC1 pins and then to the cell2 negative module terminal, causing CB1 and VC1 pins to go below the minimum voltage recommended with respect to pin AVSS. This violates device spec. If the module connector ground pin can float while the other module terminals are still connected it is recommended that a schottky diode be added between CB1 and device GND (AVSS) to ensure that CB1 and VC1 pin voltage does not violate the absolute maximum limits.
Despite the fact that I have added the schottky diode as specified and have confirmed that this prevents the issue from occurring in the past, I believe the issue has occurred again and I would like an explanation of how this is possible given the conditions of the failure:
- The battery has 6 cells and jumpers J2 and J3 were connected to use the battery as the chip's power supply. The resistor ladder switches of SW2 were all OFF.
- The wires for cells 1-5 were wired to the EVM J1 connector and the cell 6 terminal was left floating. Note that the battery side connections were not made as of yet.
- Cells 1-5 were then all instantly connected to the EVM by connecting the battery side input connector.
- After about 30 seconds, there was smoke and the cells were disconnected from the battery side. Note that the chip was not expected to be operating after the connection was made as the MCU that controls its operation through J8 was powered off the entire time.
- Resistors R34, R35 and R28 were damaged, with R34 being cut in half and R28 shooting up to over 20k ohm.
- Capacitors C4, C5, C9-11, C40, and C55 were all damaged as well and require replacement.
- The conductor from the J1 cell 0 terminal to J3's short to EVM ground was stripped of insulation and severed.
I strongly suspect that the problem described in section 9.2.1.2.6 is the issue because of the damage to resistors and capacitors around the lowest voltage cell that is not replicated anywhere else other than at the power supply filter capacitors. The damage includes external components from both the CB1 and VC1 circuits. Is there an explanation for why keeping cell 6 floating and instantly connecting cells 1-5 would cause this, despite the schottky diode being in place?
