LDO substitute for LM7805

[ZAGNUT]

always hear that the LM2940 regulator is very picky about capacitors, but how picky is it really?

attached is a regulator circuit that normally runs with an LM7805 type regulator. current draw is around 40mA. it has 5 22uF electrolytes and 5 0.015uF ceramic caps. C1, C2 and C3 are right next to the regulator while the rest are spread around the board (about 2"x4"). will this be good enough for the LM2940? and what about the diode, will it be needed?


appreciate any help,
dave

[AndyOne]

Zagnut,

Just do what it says in the datasheet when it comes to decoupling, the manufacturers should know best.

http://www.datasheetarchive.com/search.php?q=lm2940&sType=part&ExactDS=Starts

Most of that decoupling won't be for the benefit of the regulator it will be for the other devices.

Andy.

[Chippie]

Seems a bit ott as far as caps go imho....I guess you cant have enough decoupling :p

The diode is there purely to prevent damage due to reverse polarity....

Obviously the current rating of the diode needs to be greater than the fuse rating to be effective...

[ZAGNUT]

yes, all that decoupling is for lots of other stuff like an 80 pin pic.

dug through the digikey catalog and found a small 22uF SMT tantalum with low ESR. at only 3.5mm long i'm thinking i can just solder it right to the regulator's legs and leave the rest of the caps on the board as they are. if i have the minimum capacitance needed and the ESR is dead nuts in the middle of the recommended range the rest of the caps being there shouldn't matter, right?


dave

[ZAGNUT]

Seems a bit ott as far as caps go imho....I guess you cant have enough decoupling :p

The diode is there purely to prevent damage due to reverse polarity....

Obviously the current rating of the diode needs to be greater than the fuse rating to be effective...


sounds right but the diode is a 1 amp 1N4004 and i have burnt the 2 amp fuse in a moment of stupidity. no damage to the diode at all.


dave

[Dan Baldwin]

sounds right but the diode is a 1 amp 1N4004 and i have burnt the 2 amp fuse in a moment of stupidity. no damage to the diode at all.


dave

That's what's supposed to happen. While the 1N4001 is rated at 1 amp continuous forward current, it is rated at 30 amps for an 8.3 MS surge.

Dan

[village_idiot]

Why not drop a switching regulator in there?

That circuit sure does have a lot of filtering, though not really sure why they didn't use a larger value set of caps.

[ZAGNUT]

Why not drop a switching regulator in there?

That circuit sure does have a lot of filtering, though not really sure why they didn't use a larger value set of caps.

if i was going lipo i would do just that: mount a dimension engineering mini any-volt off the board and use a single cell.

but i've already sworn off lipos after puffing a pack in my older microstar. now i just want to keep things relatively simple with a 5 cell 4/5A nicad pack and have already made room for it by going to a thin glass display and keeping all parts on the board 5mm or less in height.

i could find room for another cell or two and stick with the regular LM7805 but that seems like a waste since the encoder needs only 5 volts and the XPS module even less.



dave

[village_idiot]

OK, here's a crude idea... What about a 5.1 volt Zener diode?

Hmmm.... I hadn't thought of ordering low profile components for my M*, guess I'll wait and see what is in the kit before I order my switches.

And I decided that I won't bother with a Lipo in mine either, Nimh just seems like a better choice in a TX. 2500Mah+ AA cells are cheap, and they should last plenty long. I'm still going to go with 8 cells though, it just fits the design better. I may drop in a switch regulator though, not sure yet.

How are your stocks of wire/cable? http://www.rcgroups.com/forums/showthread.php?t=687711#post7479173

[Acetronics]

Hi Zagnut

I use Micrel LDO's ( MIC 29xxx) for years without any pain ... even With Low ESR Big capacitors ... ( 1000µ !!! )

A REALLY BIG Advantage ... is their Adj devices show an ON/OFF pin that makes, with a 2N7000 inverter a wonderful security switch ... ( Switch open = Power ON )

Alain

[ZAGNUT]

thanks ace,

i found the MIC2937A in the TO-220 package at digikey. a bit pricey but still cheaper than the LM2940 and low ESR (about 0.5ohm) tantalum together.

data sheet says ESR should be about 5ohm or less so i think i'll be ok with the caps i already have.



dave