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“How does this work? (aka: How does the BJT drop the voltage from BAT+ to 5.5 V?)”
The NPN BJT shown in the BQ76952 schematic is part of a voltage regulator that preregulates the voltage supplied to the BQ76952’s internal LDO(s). The other components that make up the voltage regulator are internal to the BQ76952. Essentially, the output voltage of the voltage regulator is compared to a reference voltage, and the extent to which the NPN BJT is driven varies depending on if Vout is greater than or less than Vref. If Vout > Vref, NPN is driven less and voltage drop across it increases. If Vout < Vref, NPN is driven more and voltage drop across it decreases. In this way, you can use the NPN BJT to drop voltage from BAT+ to the necessary voltage input for REGIN.
“What is the specific working principle of the BJT?”
Because this BJT is an NPN BJT, a current above a certain threshold must be supplied to its gate to allow current to flow from collector to emitter. If the current supplied to the gate is below the threshold, then the NPN BJT will not allow current to flow from collector to emitter.
“In which state does the external BJT work?”
The state of the BJT varies depending on how the output voltage compares to the reference voltage. If a “HIGH” signal is sent to the base of the BJT, the BJT will be “ON” or closed. If a “LOW” signal is sent to the base of the BJT, the BJT will be “OFF” or open.
“How do I select the correct external BJT?”
Please refer to section 12.3.1 and the Notes section beneath in the data sheet for more information on how to select a BJT value to appropriately regulate the voltage.
