[aprssig] LM7805 and caps

Scott Miller scott at opentrac.org
Sun Aug 26 15:49:29 CDT 2007

Yes, I think any linear regulator datasheet you look at will show 
capacitors on both the input and output.  The input might be optional, 
but I'm pretty sure the output capacitor is required for all of them. 
For low-dropout regulators it's particularly critical and they can be 
very unstable if you get a capacitor with the wrong ESR.

My understanding is that the capacitor is required to dampen the ripple 
in the output voltage, and if you don't do that, the regulator will 
constantly be oscillating, chasing its own tail sort of to try to 
correct that error.

And aside from that, you should get in the habit of adding 0.1uF 
capacitors as close to every digital IC's power input as possible.  As 
the MCU's clock drives its internal workings, it'll draw varying amounts 
of current.  Since the supply traces will always have a certain amount 
of impedance, it'll cause a voltage drop on those traces, and introduce 
noise into the supply that'll cause trouble with analog circuits especially.

So just drop a 0.1uF capacitor between the power input and ground on 
each chip, and it'll have a local source to dampen out those peaks in 
current draw.  They do the same thing on a larger scale in cars, with 
big capacitors on the input to an amp so you don't dim your headlights 
every time it hits a loud bass note.

Some chips will also have separate supplies for analog and digital 
sections, plus maybe I/O drivers, CPU core, and so forth.  All of those 
should be decoupled separately, through their own capacitors placed as 
close as possible to the leads.


Andrew Rich wrote:
> Can someone explain to me why this is so:
> I have been building some projects now that use LM7805 voltage regulators.
> This is now the second time I have had issues with a PIC chip not running.
> It is only when I add caps to the input and the output that it settles down.
> Looking on the CRO, the DC looked noisy.
> I thought that the caps where there to trap AC.
> Do caps on an LM7805 player a larger role ? Is there something inside 
> the reg that needs external C ?
> Curious
> ----------------------------------------------------------------------------
> Andrew Rich VK4TEC
> vk4tec at people.net.au <mailto:vk4tec at people.net.au>
> http://www.tech-software.net <http://www.tech-software.net/>
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