Hawker said:

Forgive my stupidity, but why should I bother with a low esr cap if I am

adding .33ohm series resistance? The difference in ESR between say low

ESR tant and say a generic electrolytic will be minimal compared to the

.33ohm resistor you mention.

The low ESR capacitor plus resistor guarantees a stable RC

zero over the life and operating conditions of the system.

Getting the zero from some poorly controlled and unstable

resistance from a high ESR capacitor is a ticking time bomb.

By the way, "low ESR" in this contest means a small (1/4

to 1/10th, say), compared to the added series resistor, so

that the fixed resistor dominates the total resistance, even

if the ESR varies quite a bit.

For that matter a real cheap cap, say 22uF at 16V is going to have an

ESR around around that point anyway. A Nichicon PW series (my cheap

client loves these for cost) is 22uF 16V = 0.60 ohms @100k @ 20c

Does the client expect the circuit to work for several years

and over a range of ambient temperature beyond normal room

temperature? If so, proving that the cheap capacitor is

stable enough to expect success may cost more than using

more predictable parts.

If this circuit is part of a greeting card or toy with a one

month expected life time, then the cheap capacitor is

definitely the way to go.

What is important is to understand the problem and the

stability and cost of its possible solutions. In this case,

the solution not only includes the capacitor and series

resistance at the regulator, but the additional parallel

bypass capacitance distributed at the loads. The data sheet

isn't really helpful about setting up this series parallel

combination inside the stability box. But it has been my

experience that additional parallel capacitance is tolerated

better if you are in the higher resistance half of the graph

shown on the data sheet. But a pulsed load test of the

alternatives is still a good thing to do. The one that

rings the least (in radians of oscillation) is most stable.