Hey,
I'm in the design stage of a power supply for a little µC board. Parameters:
LP2992-5.0V
Vin = 7.2 V
Vout = 5 V
Imax = 100 mA
Question is: In the datasheet the ESR for the output capacitor for stable operation is limited from 5 mΩ to about 900 mΩ and it is mentioned, that virtually all ceramic capacitors are in this range. I wanted to use MC0805B475K100CT from Multicomp (4.7 uF) with a D.F. of 10 % @ 1 kHz. When I calculate the ESR from this value with:
ESR = DF * Xc = DF / (2 * Pi * f * C)
I get:
ESR = 0.1 / (2 * Pi * 1 kHz * 4.7 uF) = 3.4 Ω
which is much higher than anticipated. Did I make a gross error or is the capacitor plainly not the right one for this application? (In this case, can you suggest a working alternative. Most capacitors I found have a DF in the region of 2-10 %)
Cheers,
Thor