TIDUF77 June 2024 MSPM0G1507
The reference board has a diode bridge in series with the electrolytic capacitor C67, there is a large inrush current to charge C67 when powered on. A inrush current protection circuit is designed to limit this inrush current. When board power is on, the capacitor current is limited by RT1, a 50ohm PTC resistor. After successful power-on, bias power supply provides 3.3V, then the microcontroller starts to work outputting a low level signal to pull down base of Q2 after a 2 second delay, then Q2 is turned off to turn on Q1 to provide a low resistance path for current of C67. Drian to source break down voltage of Q1 must be high enough since voltage drop on RT1 is very high, see Figure 3-40 for this voltage. And delay time can be set in software.