SLUUCW9 User guide

5.2.5 Cell Undervoltage Protection

The BQ76972 device integrates Cell Undervoltage Protection (CUV), monitoring the voltage of every cell using a comparator-based circuit, and triggering a CUV alert or fault when a cell voltage falls below the CUV threshold. The CUV threshold is programmable from 1.012 V to 4.048 V in 50.6 mV steps and is set by the Protections:CUV:Threshold configuration register. The CUV protection is enabled using the Settings:Protection:Enabled Protections A:[CUV] configuration bit.

The CUV circuitry triggers an alert signal when an undervoltage event is first detected, then triggers a fault after a programmable detection delay, CUV_DLY, which can be set from 10 ms to 6765 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:CUV:Delay configuration register.

When a CUV fault is triggered, the fault recovers if the voltage rises above the CUV threshold by a value of CUV_HYS, which is programmable from 100 mV to 1 V in steps of 50 mV, for Protections:Recovery:Time. This hysteresis level is set by the Protections:CUV:Recovery Hysteresis configuration register.

When CUV is triggered, the device turns off the DSG FET if configured for autonomous FET control, based on the setting in Settings:Protection:DSG FET Protections A[CUV] (the CHG FET remains enabled if already enabled). The device recovers (if configured for autonomous recovery), based on all cell voltages being above CUV threshold + CUV_HYS. Table 5-12 provides further details.

Note: Timing on CUV changes while cell balancing is active, as described in Cell Balancing Loop Slow-down Setting.

Table 5-12 Undervoltage Protection Operation

Status	Condition	Action
Normal	Min cell voltage > *Protections:CUV:Threshold*	Safety Alert A()[CUV] = 0
Alert	Min cell voltage ≤ *Protections:CUV:Threshold*	Safety Alert A()[CUV] = 1
Trip	Min cell voltage ≤ *Protections:CUV:Threshold* for *Protections:CUV:Delay* duration	Safety Alert A()[CUV] = 0 Safety Status A()[CUV] = 1 Alarm Raw Status()[XDSG] = 1 if configured for autonomous FET control
Recovery	Safety Status A()[CUV] = 1 and Min cell voltage > *Protections:CUV:Threshold* + *Protections:CUV:Recovery Hysteresis* for *Protections:Recovery:Time*	Safety Status A()[CUV] = 0 Alarm Raw Status()[XDSG] = 0 if configured for autonomous FET control

When a CUV fault is triggered, a snapshot of all cell voltages is captured and can be accessed through the 0x0080 CUV_SNAPSHOT() subcommand, which has the format shown below.

Table 5-13 0x0080 CUV_SNAPSHOT() Subcommand Format

Subcommand Address	Bytes Within Block	Name	Unit
0x0080	0–1	Cell 1 Voltage at CUV	mV
	2–3	Cell 2 Voltage at CUV	mV
	4–5	Cell 3 Voltage at CUV	mV
	6–7	Cell 4 Voltage at CUV	mV
	8–9	Cell 5 Voltage at CUV	mV
	10–11	Cell 6 Voltage at CUV	mV
	12–13	Cell 7 Voltage at CUV	mV
	14–15	Cell 8 Voltage at CUV	mV
	16–17	Cell 9 Voltage at CUV	mV
	18–19	Cell 10 Voltage at CUV	mV
	20–21	Cell 11 Voltage at CUV	mV
	22–23	Cell 12 Voltage at CUV	mV
	24–25	Cell 13 Voltage at CUV	mV
	26–27	Cell 14 Voltage at CUV	mV
	28–29	Cell 15 Voltage at CUV	mV
	30–31	Cell 16 Voltage at CUV	mV