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11.5 Protection FETs Configuration and Control
11.5.1 FET Configuration
The BQ769142 device supports 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
BQ769142 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
FET, to avoid current flowing through the CHG FET body diode and damaging the FET. When the current rises
above the threshold (that is, less discharge current flowing), the CHG FET will be turned off again if the reasons
for its turn-off are still present.
If the DSG FET is off, the CHG or PCHG FET is on, and a current in excess of a programmable threshold (that
is, a significant charging current) is detected, the device turns on the DSG FET to avoid current flowing through
the DSG FET body diode and damaging the FET. When the current falls below the threshold (that is, less
charging current flowing), the DSG FET is turned off again if the reasons for its turn-off are still present.
When a parallel configuration is used, the body diode protection is disabled.
11.5.2 PRECHARGE and PREDISCHARGE Modes
The BQ769142 device includes precharge functionality, which can be used to reduce the charging current for an
undervoltage battery by charging using a high-side PCHG PFET (driven from the PCHG pin) with series resistor
until the battery reaches a programmable voltage level. When the minimum cell voltage is less than a
programmable threshold, the PCHG FET will be used for charging.
The device also supports predischarge functionality, which can be used to reduce inrush current when the load is
initially powered, by first enabling a high-side PDSG PFET (driven from the PDSG pin) with series resistor, which
enables the load to slowly charge. If PREDISCHARGE mode is enabled, whenever the DSG FET is turned on to
power the load, the device will first enable the PDSG FET, then transition to turn on the DSG FET and turn off
the PDSG FET.
The PCHG and PDSG drivers are limited in the current they can sink while enabled. As such, it is recommended
to use 1 MΩ or larger resistance across the FET gate-source.
11.6 Load Detect Functionality
When a short circuit in discharge latch or overcurrent in discharge latch protection fault occurs and the DSG FET
is off, the device can be configured to recover when load removal is detected. This feature is useful if the system
has a removable pack, such that the user can remove the pack from the system when a fault occurs, or if the
effective system load that remains on the battery pack is higher than ~20-kΩ when the DSG FET is disabled.
The device periodically enables a current source out of the LD pin and recovers the fault if a voltage is detected
at the LD pin above a 4-V level. If a low-impedance load is still present on the pack, the voltage the device
measures on the LD pin is generally below 4 V, preventing recovery based on load detect. If the pack was
removed from the system and the effective load is high, such that the current source generates a voltage on the
LD pin above a 4-V level, then the device can recover from the fault.
Copyright © 2021 Texas Instruments Incorporated
The BQ769142 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 BQ769142 device will reenable the FETs if
nothing is blocking them being reenabled.
Product Folder Links:
SLUSE91A – SEPTEMBER 2020 – REVISED FEBRUARY 2021
BQ769142
BQ769142
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