SLUSE14A December 2020 – February 2021 BQ76942
SLEEP mode is a reduced functionality state that can be optionally used to reduce power dissipation when there is little or no system load current or charging in progress, but still provides voltage at the battery pack terminals to keep the system active. At initial powerup, a configuration bit determines whether the device can enter SLEEP mode. After initialization, SLEEP mode can be allowed or disallowed using subcommands. Status bits are provided to indicate whether the device is presently allowed to enter SLEEP mode or not, and whether it is presently in SLEEP mode or not.
When the magnitude of the CC1 Current measurement falls below a programmable current threshold, the system is considered in relax mode, and the BQ76942 device autonomously transitions into SLEEP mode, if settings permit. During SLEEP mode, comparator-based protections operate the same as during NORMAL mode. ADC-based current, voltage, and temperature measurements are taken at programmable intervals. All temperature protections use the ADC measurements taken at these intervals, so they will update at a reduced rate during SLEEP mode.
The BQ76942 device exits SLEEP mode if a protection fault occurs, current begins flowing, a charger is attached, if forced by a subcommand, or if the RST_SHUT pin is asserted for < 1 s. When exiting based on current flow, the device quickly enables the FETs (if the CHG FET was off or the DSG FET was in source-follower mode), but the standard measurement loop does not restart until the next 1-s boundary occurs within the device timing. Therefore, new data may not be available for up to ≈1 s after the device exits SLEEP mode.
The coulomb counter ADC operates in a reduced power and speed mode to monitor current during SLEEP mode. The current is measured every 12 ms and, if it exceeds a programmable threshold in magnitude, the device quickly transitions back to NORMAL mode. In addition to this check, if CC1 Current measurement taken at each programmed interval exceeds this threshold, the device exits SLEEP mode.
The device monitors the PACK pin voltage and the top-of-stack voltage at each programmed measurement interval. If the PACK pin voltage is higher than the top-of-stack voltage by more than a programmable delta and the top-of-stack voltage is less than a programmed threshold, the device exits SLEEP mode. The BQ76942 device also includes a hysteresis on the SLEEP mode entrance to avoid the device quickly entering and exiting SLEEP mode based on a dynamic load. After transitioning to NORMAL mode, the device will not enter SLEEP mode again for a number of seconds given by the hysteresis setting.
During SLEEP mode, the DSG FET can be driven either using the charge pump or in source-follower mode (as described in High-side NFET Drivers). The CHG FET can be disabled or driven using the charge pump, based on the configuration setting.