Russell McMahon 15:46 2004-05-14:
>capacitors must not have an ESR (equivalent series
>resistance) that is either too high OT too low.

The input cap: *always* the lower ESR the better

> AFAIR the 7805 does not specify this.

Varies between manufacturers.
I have never seen anybody specify more than 220nF input cap.  This must be foil or ceramic, electrolytes have too high internal series resistance (ESR).

Very important is that the total track lengths from the poles of this cap to the regulator is short, not more than 2cm.

I usually use 100nF foil close, and a electrolytic at board power input.  (as to keep it away from the hot regulator)  Where board area is small and i do not need large capacitance i have used one 10uF tantalum instead.

I once used a "professional" to make my layout.  The design was dead, and at analysis i saw she placed the output cap ten cm away from the 78L08, and GND and plus was routed 7cm apart, making a loop inductor, thus creating a nice LC cirquit.  It really was a stable OSCILLATOR!

/Morgan
--
Morgan Olsson, Kivik, Sweden

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