SLUUCG7 April 2024 BQ76922
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 alive. At initial power up, the Settings:Configuration:Power Config[SLEEP] configuration bit determines whether the device can enter SLEEP mode. After initialization, SLEEP mode can be allowed or disallowed using the 0x0099 SLEEP_ENABLE() and 0x009A SLEEP_DISABLE() subcommands. The 0x12 Battery Status()[SLEEP_EN] bit indicates whether the device is presently allowed to enter SLEEP mode or not, while the 0x12 Battery Status()[SLEEP] bit indicates whether it is presently in SLEEP mode or not.
When the magnitude of the CC1 Current measurement falls below a current threshold given by Power:Sleep:Sleep Current, the system is considered in relax mode, and the BQ76922 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 every Power:Sleep:Voltage Time seconds. While in SLEEP mode, the device also begins a 4-s, low-power current measurement 1 s—after completing voltage and temperature measurements—which is used for accumulated charge integration while in SLEEP mode. When the Power:Sleep:Voltage Time timer expires and a set of measurements is to be taken, the 4-s, low-power current measurement is stopped to enable the full measurement set to begin. To optimize the accumulated charge integration, the value of Power:Sleep:Voltage Time can be set to 5 s or (4 × n + 1) seconds, which minimizes the time while the 4-s, low-power current measurement is stopped. All temperature protections use the ADC measurements taken at the Power:Sleep:Voltage Time intervals, so they update at a reduced rate during SLEEP mode.
The BQ76922 device exits SLEEP mode if a protection fault occurs, a current begins flowing, a charger is attached, the 0x009A SLEEP_DISABLE() subcommand is sent, or if the RST_SHUT pin is asserted for < 1 second. 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 is not restarted until the next 1-s boundary occurs within the device timing. Therefore, new data may not be available for up to ≈1 second 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 at periodic intervals and, if it exceeds Power:Sleep:Wake Comparator Current in magnitude, the device quickly transitions back to NORMAL mode. In addition to this check, if the 4-s current measurement taken at each Power:Sleep:Voltage Time interval exceeds Power:Sleep:Sleep Current, the device exits SLEEP mode.
The rate at which the coulomb counter ADC operates in this WAKE COMPARATOR mode is programmable using the Settings:Configuration:Power Config[WK_SPD_1:0] configuration bits. By changing the conversion rate, the noise level of the resulting ADC conversions is also modified, with higher noise levels at faster conversion rates. The tables below summarize the settings.
| WK_SPD_1 | WK_SPD_0 | Current Measurement Speed | Measurement Noise Level (One Sigma)(1) | Comment |
|---|---|---|---|---|
| 0 | 0 | 48ms | ≈6 μV | Recommended option for best accuracy |
| 0 | 1 | 24ms | ≈10 μV | |
| 1 | 0 | 12ms | ≈25 μV | Not recommended |
| 1 | 1 | 6ms | ≈100 μV | Recommend only when used with thresholds > 1000 μV |
The setting WK_SPD[1:0] = 0x0 provides the lowest noise level, with measurements exhibiting a sigma of approximately 6 μV, and a measurement taken every 48ms. If a faster measurement rate is desired, then the setting 0x1 results in a sigma of approximately 10 μV and a measurement taken every 24ms. If an even faster measurement rate is needed, the setting 0x3 can be used to provide a measurement every 6ms. However, the sigma of this measurement is approximately 100 μV, which can result in unintentional wakeup from SLEEP mode if the threshold is set too low. For this reason, this setting is recommended only for use if the Power:Sleep:Wake Comparator Current is set so that the differential voltage |VSRP – VSRN| > 1000 μV. Note that a six-sigma excursion using the 0x3 setting can cause a wakeup with a current approximately 600μV below the programmed threshold. The setting 0x2 (which is the default setting) may exhibit a large offset level and should not be used.
The device also monitors the PACK pin voltage and the top-of-stack voltage at each Power:Sleep:Voltage Time measurement interval. If the PACK pin voltage is higher than the top-of-stack voltage by more than Power:Sleep:Sleep Charger PACK-TOS Delta and the top-of-stack voltage is less than Power:Sleep:Sleep Charger Voltage Threshold, the device exits SLEEP mode. The BQ76922 device also includes a hysteresis on the SLEEP mode entrance, in order to avoid the device quickly entering and exiting SLEEP mode based on a dynamic load. After transitioning to NORMAL mode, the device does not enter SLEEP mode again for a number of seconds given by the Power:Sleep:Sleep Hysteresis Time setting.
During SLEEP mode, the DSG FET can be driven either using the charge pump or in source-follower mode, as described in Section 5.2.2. The CHG FET can be disabled or driven using the charge pump, based on the setting of Settings:FET:FET Options[SLEEPCHG].