SLUUBY1B December 2020 – April 2022 BQ76942
The BQ76942 device integrates Cell Overvoltage Protection (COV), monitoring the voltage of every cell using a comparator-based circuit, and triggering a COV alert or fault when a cell voltage exceeds the COV threshold. The COV threshold is programmable from 1.0 V to 5.5 V in 50 mV steps and is set by the Protections:COV:Threshold configuration register. The COV protection is enabled using the Settings:Protection:Enabled Protections A:[COV] configuration bit.
The COV circuitry triggers an alert signal when an overvoltage event is first detected, then triggers a fault after a programmable detection delay, COV_DLY, which can be set from 10 ms to 6762 ms in units of 3.3 ms, with the actual delay being 3.3 ms × (2 + setting). The setting 0x0 disables the protection. The delay is set by the Protections:COV:Delay configuration register.
When a COV fault is triggered, it recovers if the maximum cell voltage drops below the COV threshold by a COV_HYS hysteresis level, which is programmable from 100 mV to 1 V in steps of 50 mV, for Protections:Recovery:Time. The COV_HYS hysteresis level is set by the Protections:COV:Recovery Hysteresis configuration register.
When an COV fault is triggered, the device will turn off the CHG FET if configured for autonomous FET control based on setting in Settings:Protection:CHG FET Protections A[COV] (the DSG FET remains enabled if already enabled). The device will recover (if configured for autonomous FET control) based on all cell voltages being below COV threshold – COV_HYS for Protections:Recovery:Time.
The BQ76942 device also includes a Cell Overvoltage Latch (COVL) protection, which can create a fault if multiple COV failures occur within a programmable time window. Whenever a COV fault is triggered, the COVL latch counter is incremented. After the device recovers, it decrements the COVL counter after a programmable recovery time window (given by Protections:COVL:Counter Dec Delay) if no further COV faults are detected. If the COVL counter reaches a programmable latch limit (given by Protections:COVL:Latch Limit), it triggers a COVL fault.
The COVL protection recovers after a programmable delay given by Protections:COVL:Recovery Time, but it is important that Protections:COVL:Counter Dec Delay be set shorter than Protections:COVL:Recovery Time; otherwise, COVL would immediately trip again after recovery, since the COVL counter would not have decremented yet. The COVL protection is enabled using the Settings:Protection:Enabled Protections C:[COVL] configuration bit. Table 5-10 provides further details.
| Status | Condition | Action |
|---|---|---|
| Normal | Max cell voltage < Protections:COV:Threshold | Safety Alert A()[COV] = 0 Decrement COVL counter by one after each Protections:COVL:Counter Dec Delay period if COVL counter > 0 |
| Alert | Max cell voltage = Protections:COV:Threshold | Safety Alert A()[COV] = 1 |
| Trip | Max cell voltage = Protections:COV:Threshold
for Protections:COV:Delay duration |
Safety Alert A()[COV] = 0 Safety Status A()[COV] = 1 Alarm Raw Status()[XCHG] = 1 if autonomous FET control is enabled Increment COVL counter |
| Recovery | Safety Status A()[COV] = 1 and Max cell voltage < Protections:COV:Threshold - Protections:COV:Recovery Hysteresis for Protections:Recovery:Time |
Safety Status A()[COV] = 0 Alarm Raw Status()[XCHG] = 0 if autonomous FET control is enabled |
| Latch Alert | COVL counter > 0 | Safety Alert C()[COVL] = 1 |
| Latch Trip | COVL counter = Protections:COVL:Latch Limit | Safety Status C()[COVL] = 1 Safety Alert C()[COVL] = 0 Alarm Raw Status()[XCHG] = 1 if autonomous FET control is enabled |
| Latch Reset | SafetyStatus()[COVL] = 1 for Protections:COVL:Recovery Time duration | Safety Status C()[COVL] = 0 Alarm Raw Status()[XCHG] = 0 if Safety Status A()[COV] = 0 and autonomous FET control is enabled |
When a COV fault is triggered, a snapshot of all cell voltages is captured and can be accessed through the 0x0081 COV_SNAPSHOT() subcommand, which has format as shown below.
| Subcommand Address | Bytes Within Block | Name | Unit |
|---|---|---|---|
| 0x0081 | 0–1 | Cell 1 Voltage at COV | mV |
| 2–3 | Cell 2 Voltage at COV | mv | |
| 4–5 | Cell 3 Voltage at COV | mV | |
| 6–7 | Cell 4 Voltage at COV | mV | |
| 8–9 | Cell 5 Voltage at COV | mV | |
| 10–11 | Cell 6 Voltage at COV | mV | |
| 12–13 | Cell 7 Voltage at COV | mV | |
| 14–15 | Cell 8 Voltage at COV | mV | |
| 16–17 | Cell 9 Voltage at COV | mV | |
| 18–19 | Cell 10 Voltage at COV | mV | |
| 20–31 | Reserved | - |