The schematic concerned is at the end of the microchip document on microchip serial programming
The circuit generates VPP for the programming phase. D1 is the reference of 12.7V and Q2 forms a negative feedback with the opamp. This part I do understand. However, Q2 is strange, what does it do? It seems that it will be in a cut out state because base will be higher than emitter by0.6V due to Q2 ?