PMOS Gate Driver using BJT
Q3 is operating in one of two distinct modes, depending on whether its collector is higher than or lower than the voltage on the EN signal.

When the collector voltage is higher (right after turn-on), it functions as a constant-current sink, with the current determined by the emitter voltage (which is one diode drop below the base voltage) across R2.

However, when the collector voltage drops too low to sustain this voltage across R2, then the base and emitter voltages also drop and Q3 just functions as a saturated switch.