Hi Zong,
The current we can draw from the external ballast depends on the base current and gain of the external ballast transistor.
The maximum current we can draw from external ballast transistor and supply VDDX rail is about power dissipation and thermal characteristics.
The user must ensure that the maximum junction temperature is not exceeded.
It depends on ambient temperature, thermal package characteristics and MCU power dissipation.
MCU power has to be calculated for the particular application. This is explained in RM section A.1.6.
For more details about this topic see application note below and section 3.2.2 PNP external ballast transistor:
AN5084, Hardware Design Guidelines for S12ZVL Microcontrollers-Application n ... (REV 1)
Regards,
iggi
