I am not sure if understand what you are saying. If the electrolytic capacitor has a temperature range, does the ESR change with temperature?The 2940 - 2941 series can be cranky about the ESR of the output capacitor. Aluminum electrolytics will make it oscillate at cold temps because the ESR is too high.
Absolutely. The ESR of an aluminum cap goes skyward at temps below about 10C. Cap makers show the curves.I am not sure if understand what you are saying. If the electrolytic capacitor has a temperature range, does the ESR change with temperature?
A cap has a certain amount of inductance, capacitance, and resistance at any frequency. ESR is the resistance (real) part of the impedance. You can read ESR directly on a network analyzer by sweeping out until the impedance goes minimum (the resosnance point) where inductive and capacitive parts are equal so the cap looks purely resistive. Above resonance, it looks more inductive. Below it looks more capacitive.I see on page 2 an impedance at certain frequencies. Doesn't the impedance of the capacitor have a bearing on the ESR?
I think you're right.... put 3 one Ohm resistors in parallel.Please excuse the geekitude but, the graph in post 8 is a perfect example of a question I have thought about before.
When looking at a printed log graph, the physical center between (2) 1's is about pi/10.
Would the center of the useful range in that graph be closer to .5 or .314?
Or have I been standing too close to my nephew while he was smoking something?
I will understand the resistor part of the answer, but are you sure about 22uF ceramic capacitors. I have seen 1.0uf, but 22uF.......I don't know.Absolutely. The ESR of an aluminum cap goes skyward at temps below about 10C. Cap makers show the curves.
Best way to go is get a good X7R ceramic cap of the size and voltage needed which will have almost zero ESR, and add external resistance in series to satisfy the ESR requirement of the IC regulator.
Here is the ESR limits for the output cap printed on the 2941 data sheet: note that about 0.5 Ohms is dead perfect. Two 1 Ohm resistors paralleled in series with a 22uF ceramic is perfect.
Read the LM2940 data sheet for the value. I recall 22uF was the minimum but verify on the data sheet to be sure. There are high capacitance ceramics available from makers like Taiyo-Yuden.I will understand the resistor part of the answer, but are you sure about 22uF ceramic capacitors. I have seen 1.0uf, but 22uF.......I don't know.
YES! The capacitor is for loop compensation. It is absolutely required and the ESR must be in the specified range. I would (seriously) estimate I must have gotten thousands of calls over 20 years regarding parts oscillating because the wrong capacitor was used.One other question if the input to the regulater is pure DC, like from a battery do we still need the capacitor on the output?
Thanks,
Ned
It doesn't matter if you get the exact "center value", it just has to be in the stable range.Are you cereal? I could kick this up to the math forum if you're just joking with me. We have some major math geeks here.