SLUSE14A December 2020 – February 2021 BQ76942
The BQ76942 device includes an integrated charge pump and high-side NFET drivers for driving CHG and DSG protection FETs. The charge pump uses an external capacitor connected between the BAT and CP1 pins that is charged to an overdrive voltage when the charge pump is enabled. Due to the time required for the charge pump to bring the overdrive voltage on the external CP1 pin to full voltage, it is recommended to leave the charge pump powered whenever it may be needed quickly to drive the CHG or DSG FETs.
The DSG FET driver includes a special option (denoted source follower mode) to drive the DSG FET with the BAT pin voltage during SLEEP mode. This capability is included to provide low power in SLEEP mode, when there is no significant charge or discharge current flowing. It is recommended to keep the charge pump enabled even when the source follower mode is enabled, so whenever a discharge current is detected, the device can quickly transition to driving the DSG FET using the charge pump voltage. The source-follower mode is enabled using a configuration setting and is not intended to be used when significant charging or discharging current is flowing, since the FET will exhibit a large drain-source voltage and may undergo excessive heating.
The overdrive level of the charge pump voltage can be set to 5.5 V or 11 V based on configuration setting. In general, the 5.5-V setting results in lower power dissipation when a FET is being driven, while the higher 11-V overdrive reduces the on-resistance of the FET. If a FET exhibits significant gate leakage current when driven at the higher overdrive level, this can result in a higher device current for the charge pump to support this. In this case, using the lower overdrive level can reduce the leakage current and thus the device current.
The BQ76942 device supports a system with FETs in a series or parallel configuration, where the parallel configuration includes a separate path for the charger connection versus the discharge (load) connection. The control logic for the device operates slightly differently in these two cases, which is set based on the configuration setting.
The FET drivers in the BQ76942 device can be controlled in several different manner, depending on customer requirements:
|The BQ76942 device can detect protection faults and autonomously disable the FETs, monitor for a recovery condition, and autonomously reenable the FETs without requiring any host processor involvement.|
|The BQ76942 device can detect protection faults and autonomously disable the FETs. When the host receives an interrupt and recognizes the fault, the host can send commands across the digital communications interface to keep the FETs off until the host decides to release them.|
|Alternatively, the host can assert the CFETOFF or DFETOFF pins to keep the FETs off. As long as these pins are asserted, the FETs are blocked from being reenabled. When these pins are deasserted, the BQ76942 will reenable the FETs if nothing is blocking them being reenabled (such as fault conditions still present, or the CFETOFF or DFETOFF pins are asserted).|
|The BQ76942 device can detect protection faults and provide an interrupt to a host processor over the ALERT pin. The host processor can read the status information of the fault over the communication bus (if desired) and can quickly force the CHG or DSG FETs off by driving the CFETOFF or DFETOFF pins from the host processor, or commands over the digital communications interface.|
|When the host decides to allow the FETs to turn on again, it writes the appropriate command or deasserts the CFETOFF and DFETOFF pins, and the BQ76942 device will reenable the FETs if nothing is blocking them being reenabled.|
If the device is in series FET configuration and a single FET is on, it is possible for current to flow through the off-FET body diode. This current can damage the FET if high enough for a long enough time. In this case, when the BQ76942 device is autonomously controlling the FETs, if a current is detected above a programmable threshold, the device will automatically turn on the off-FET to prevent further damage.