> A simple zener regulator is probably a good example; given reasonably
> accurate ideas of minimum and maximum current consumption of the
> circuit, you can design a zener circuit that is "maximally efficient
> for a linear regulator."

I'm afraid I have to disagree here.
Zener shunt regulators can be surprisingly tricky BECAUSE of their low
component complexity.

A zener shunt regulator works by ALWAYS drawing at least the maximum current
(via the dropping resistor) that the circuit will EVER need to draw. eg if
you have a circuit that draws from 0 mA to 10 mA then the zener regulator
MUST draw at least 10 mA at all times. The current not used by the load is
shunted through the zener. A linear regulator in the same application will
be more efficient than the zener regulator whenever the load current falls
more than Q mA below 10 mA where Q is the regulators operating current (ie
the current sent down its ground lead rather than through the load. Modern
regulators such as the LM317 (modern only compared with eg 7805 family)
operate their internals from the regulator differential voltage and only
have a small and relatively constant ground lead current so are potentially
more efficient than a zener across a wide range.

Zeners are also potentially poor when the input voltage varies widely and
sometimes closely approaches the output voltage. In such cases the series
pass element has to be sized to pass Imax with little voltage drop. When Vin
rises with no or low load the current in the zener rises to above Imax_out -
sometimes many times Imax_out.



        Russell McMahon

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