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Hi 2K,
Normally you can´t go below the regulators reference voltage.....but.
If you use the same Vref. to create a negative mirror voltage you can now use this as your ground or minus output.
You have to use 723 in so called "floating configuration".
Basically, you need 2 independent DC sources.
One is rated with your output voltage, say 30V/2A. This voltage is connected to the series pass transistors (internal series transitor + external power transistors in darlington configuration).
The other dc source is say 15V/200 mA. This voltage is used to supply power for 723.
Connect the neg. pole of +30V reg to positive pole of 15 V reg. So, output voltage of the entire regulated power supply is floating on the top of the '723 power supply. Connect the output voltage divider for the feedback between output voltage of the entire regulated power supply and neg. pole of '723.
This might seem complicated but it isn't. If this description is not clear just draw yourself a block diagram.
This technique is used all comercial variable power supplies based on '723 that go down to 0 V
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