PDX20 EXTERNAL BALLAST TRANSISTOR SETUP

[peteradamcik]

Hi Guys, I'm designing a standalone PDX20 Microcontroller board and I've just ordered the External Ballast NPN Transistor BCP68T. In the PXD20 reference manual page 48-8 it shows how to setup the transistor but there's a resistor and a capactor connected to the transistor. 

What would be the correct values for the resistor and the capacitor to get the 1.2V for the internal CPU voltage (VDD12). I would rather ask the experts than accidentally destroying my chip.

Please help me on this one.

Thanks guys

Pete

[davidtosenovjan · Accepted solution]

At the beginning I would just note PXD20 and MPC5645S are equal MCUs.

PXD20 has been discontinued indeed and I an not sure if its documentation has been updated. However all documents related to MPC5645S applies to PXD20 as well.

Regarding bypass transistor -
Capacitance attached to to emitter of the transistor is in the DS marked as Creg (it should be, but it is not). it is at least 4*10uF

Regarding pull-down resistor (20 kOhm), you may see in TWR-PXD20_SCH, this is optional. Theoretically it reduces overall amount of capacitance required on VDD, however as there is not other specification Cdec above, I probably wouldn't put it there. Maybe I would make a room for the resistor and leave it unmounted.

[peteradamcik]

Hi David, thank you so much for your help on the PDX20. I can say now that I totally appreciate it. So thank you again.

Chip is already mounted on the prototype board, now to setup the JTAG for programming.

Have a great day mate,

Peter.