>The behavior you're describing (a large increase in quiescent
>current as the pass transistor goes into saturation due to a
>low-input condition) isn't "spice gone bad"--it's very typical of
>low-dropout regulator ICs as well, although they usually include
>some mechanism for moderating it.

For which well-known reason LDORs meant to be used at near full capacity
use a low Rdson P-FET instead of a bipolar. This usually buys low
quiescent current too. The Seiko LDOs are built like this afaik.

Another feature of these CMOS regulators is the very low temperature
coefficient (wrt discretes and bipolar usual regs).

The simplest CMOS LDO is a depletion type P-FET in series with the load
and an enhancement type FET shunting its gate to GND. Add three
polarization R's (one pulls the pass transistor's gate up to S, and the
other two form a divider from the output voltage to feed the gate of the
error amplifier (the enhancement FET) - of course they are actually just
other FETs in pinch resistor mode when integrated).

A more evil version of this scheme uses a back-diode-less depletion mode
P-FET (I admit I've never seen one of those but they exist), and the
divider alone. The FET must be diode-less because it runs backwards (S to
load, D to +Vcc, G to GND or divider between S and gnd). The FET will
stabilize to its threshold voltage between S and G which is the output
voltage (unless you use a divider for higher V).

And the last and most evil scheme consists of a single NJFET (like BF244
or 245 or 2N equivalents) with G to gnd and D and S to load and Vcc
respectively, and NO other parts. Beware of decounpling input and output
properly as this is also a very popular UHF oscillator configuration. It
works like the single PFET described in the previous paragraph. By
selecting one with Vp ~= 5V (A suffix on BF245 is usually like this), you
have 5V out. This latter scheme has the strange advantage of also working
backwards ! Swap load and supply at will. I have used it to equalize
charge in two small NiCd accumulators. The last two schemes described have
poor regulation but they have a number of advantages for special
applications (besides being nice ideas).

Peter

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