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:
Fully autonomous
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.
Partially autonomous
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).
Manual control
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.
11.5 Protection FETs Configuration and Control
11.5.1 FET Configuration
The BQ76942 device supports both a series configuration and a parallel configuration for the protection FETs in
the system, as well as a system that does not use one or both FETs. When a series FET configuration is used,
the BQ76942 device provides body diode protection for the case when one FET is off and one FET is on.
If the CHG FET is off, the DSG or PDSG FET is on, and a discharge current greater in magnitude than a
programmable threshold (that is, a significant discharging current) is detected, the device will turn on the CHG
SLUSE14B – DECEMBER 2020 – REVISED DECEMBER 2021
Copyright © 2021 Texas Instruments Incorporated
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