Hi Woonje,
In fact, the main purpose of the solution with external PNP transistor is to offload the heat generated by LDO regulator from MCU package.
Since this PNP transistor works in linear mode, the maximum current will be typically limited by the maximum amount of power which transistor could transfer to an ambient environment and simultaneously stay below maximum junction temperature. So, most important parameters will be the maximum ambient temperature, the maximum junction temperature and the junction to ambient thermal resistance for given transistor package with dissipation mounting pad area on the PCB.
In typical case you already know amount of current which will be necessary for powering MCU (see chapter A.3 Supply Currents, the power consumption depends mainly on bus clock value), GDU unit (you would need typically around 12mA for 6 switch events per PWM cycle (100nC total gate charge, 20kHz)) and external loads at VDDX and GPIO pins. In that case, you have to design transistor circuit and their dissipation mounting pad area on the PCB.
Please look at application note AN5207 Hardware Design Guidelines for S12ZVM Microcontrollers
http://www.nxp.com/files/microcontrollers/doc/app_note/AN5207.pdf
and chapter 3.6 Selecting the PNP External Ballast Transistor.
This application note for s12ZVM is slightly newer than AN4721 and contains some more details.
I already saw a presentation with more details about selecting external transistor and calculation of their parameters, but I was not able found it again. I will let you know when I will find anything more.
I hope it helps you.
Have a great day,
Radek
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