Data General NOVA 4/S Manual page 91

The +12V regulator draws power directly from the
battery. It is a simple linear series pass design and has no
built-in protection circuits.
The +5V regulator draws power from either the +12V
main supply or from the battery. It is a buck switching
regulator and is very similar to the one in the main supply.
When the low voltage switch turns on, current flows from
the source, through the primary winding of the flyback
transformer and on to the output. The pulse width
modulator controls the duty cycle of the switch to regulate
the output voltage. If too much current flows in the
primary winding, the current limiter reduces the
modulator's duty cycle.
The -5V regulator draws power from the +5V regulator
via the flyback transformer. It is a simple linear series pass
design and has built-in current limiting.
the +12V regulator, and current flows from the +12V
over to battery operation. As long as the battery retains
discharges to a dangerously low level, the under-voltage
dectector negates the MEM OK signal, which in turn
entirely. (Note that the entire supply also shuts down