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Linear Voltage Regulator filter caps in a battery application.

Linear Voltage Regulator filter caps in a battery application.

Linear Voltage Regulator filter caps in a battery application.

(OP)
While using a genaric LVR such as a 7805 where the supply is from a battery is it typical practice to use a filter cap on the input.  I'm looking at an application where I'll be suppying 100mA peak with a typical operating current of 3mA to a few Digital Panel Meters. The input will be coming from 5AA batteries (7.5V) regulated to 5V.  The spec sheet calls for a 33uF cap on the input but I'm not sure I see a need, maybe the .01uF it calls for on the output ....

Thanks,
Mark

RE: Linear Voltage Regulator filter caps in a battery application.

I've seen these regulators sing at about 1 mhz without a cap.  It doesn't take much of a cap to stop it.

RE: Linear Voltage Regulator filter caps in a battery application.

The rule is if the power lead to the regulator are longer than about 6 inches USE A CAP.

If less then use a smaller or no cap.

RE: Linear Voltage Regulator filter caps in a battery application.

I would use one to avoid future problems. The cost is often very negligible unless your talking tens of  thousands of units.

RE: Linear Voltage Regulator filter caps in a battery application.

Those regulators definitely need to be looking back into a low source impedance at the higher frequencies. Any series inductance from long leads can cause big trouble.

Let the manufacturers application notes be your guide, they have already been through all this. Better safe than sorry.

RE: Linear Voltage Regulator filter caps in a battery application.

I generally don't use something as beefy as a 7805 (my projects tend to use <50mA), but I usually use a 0.33uF on the input side and it's more than adequate.  You may want to consider a smaller regulator that has a better quiescent current if you're running from batteries... I believe the 7805 is something in the 3-4mA range.

Dan
Owner
http://www.Hi-TecDesigns.com

RE: Linear Voltage Regulator filter caps in a battery application.

You need a capacitor.  The AC impedance of batteries is not as small as we would like to think and they are typically inductive at higher frequencies.  The worst problem (maybe) is that for a step current load change, the input voltage can drop below the minimum regulator input voltage.

RE: Linear Voltage Regulator filter caps in a battery application.

All good tips.  macgyvers is a good point the 7805 is truely ancient! And wastes lots of power.

RE: Linear Voltage Regulator filter caps in a battery application.

(OP)
Thanks for all the great suggestions.  I'm going to take some time to research this on the bench with a scope and play a bit.  I was more interested in the space savings of not using the cap as opposed to the cost savings. I admit that the 78xx series is a bit old but I needed to get something done and didn't have the time to play with something new.  So to continue this thread, whats everyone using thats not in a surface mount package?

http://www.national.com/ds/LM/LM78L05.pdf

RE: Linear Voltage Regulator filter caps in a battery application.

Hmmm, that's a tough one for me... usually the beefier parts are the PTH type, but beefier usually means larger quiescent current, too.  If you want the low current draw, SMD parts are the way to go, really.

For example, I use one regulator by Zetex which has a max out of 50mA in an SOT-23 package... I think the quiescent current is about 50uA.  The last project I designed with it used high brightness LEDs tuned down significantly.  Still quite bright, but the processor and crystal took 8mA, 11 LEDs another 5mA, and everything else was negligible.  If I had used a high quiescent current regulator (as I accidentally did for the prototype), my current draw would have been about 30% higher, and that's significant for a battery project.

Dan
Owner
http://www.Hi-TecDesigns.com

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