What is the purpose 1k resistor for external ballast MC9S12ZVC

[sreekanthreddyc]

Hello,

A 1 kilo Ohm resistor which is connected between the Base and Emitter of PNP external ballast in MC9S12ZVC. Can we change this resistor value to 100k or more to reduce the current on Input Supply.

Please clarify

Thanks and Regards

Sreekanth

[RadekS]

Hi Sreekanth,

Unfortunately, such change is definitely not recommended.

The voltage regulator drives the current trough this resistor for achieving stable 5V at VDDX/VDDC output voltage level.

The prototype S12ZVM boards used 5k resistor at this place and we met with some issues, therefore the specification was changed to 1k resistor.

Unfortunately, I do not know full details about that issue with 5k resistor, but I suppose that such configuration may sometimes cause unstable output voltage level during fast changes of VDDX/VDDC load.

The external PNP is disabled during STOP mode and the output voltage is generated by the internal RPM (reduced performance mode) regulator. Therefore I suppose that the resistor value change will not help you much with power consumption during sleep mode.

I hope it helps you.

Have a great day,
Radek

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