78xx output cap

IIRC the national semi app notes said within 3 inches. I have a little experience with using the caps over 1 inch without problems (but wouldn't call that comprehensive). If its PTH then if it fails you can always tack a cap directly to the IC.

Thanks. I have the NatSemi Linear Application Handbook but haven't seen where they cited the 3-inch figure (haven't looked). Yes, it's TH.

Is it possible to add a smaller cap that near the regulator? I doubt you'll have any serious problems but make sure you bypass that counter. Increase track thickness if you can and properly bypass(low ESR caps). The track length isn't going to be a huge issue if done right.

If your not sure and are worried how bout including some SMT caps near the regulator and near the bypassing just in case? These can be quite small, have low ESR, and you don't have to add them until there is an issue.

At the frequencies the regulator might oscillate, a few inches or a few feet of PCB trace wouldn't matter.

John

--- From National's 78XX data sheet at:

"Considerable effort was expanded to make the LM78XX series of regulators easy to use and minimize the number of external components. It is not necessary to bypass the output, although this does improve transient response. Input bypassing is needed only if the regulator is located far from the filter capacitor of the power supply."

Also, Motorola's (ON Semiconductor) data sheet at:

states, on page 1:

"Cin is required if regulator is located an appreciable distance from power supply filter.

CO is not needed for stability; however, it does improve transient response. Values of less than 0.1 F could cause instability."

So, the bottom lines are:

1. Save yourself some rupees and blow off the output caps if the regulator's transient response is fast enough to detect a large change in load current and keep its output voltage stiff.

1. Save yourself some rupeess and blow off the input caps if the inductance of the input leads/traces isn't high enough to cause the input of the regulator to drop below its regulating threshold voltage when a large change in load current invokes E = Ldi/dt.

--- JF

Oops - that 3" figure might be for LM317

>

Just anecdotal experience, but I've had cases where I've tried to use a

7806 regulator without any bypass capacitors and they've oscillated like mad at a couple of MHz, no matter how close to the input filter they are!

I tried that with a NatSemi 7805 last year.

Went unstable. The next respin included a 22uF OSCON and some 1uF ceramics, and it's fine (a bit ripply perhaps but usable).

I also found an old Analog Devices app note suggesting that to minimise noise, put a ferrite bead between the output of the 7805 and its decoupler. That might have been valid many years ago but it made the regulator oscillate until I replaced the ferrite with a zero ohm resistor.

Nemo

I've never been able to make a 78x05 oscillate under sensible conditions. We shipped >10K units with zero output bypass cap (benign load of a few mA fairly steady).

Putting an inductor on the output without a bypass right at the regulator would not be in my "sensible" category, though.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Since you are using TH mounted components you could consider making some room on the side opposite the 78XX part to place a 0805 or even 0603 sized set of SM pads between the OUT and GND pins of the regulator. These would disrupt the layout in a minimal way and then if it proved necessary to have some cap installed there it can be added at the last minute. For example if the cap that you plan for being ~1 inch away was a 22uF cap the one placed on the SM pads could be maybe 0.1uF. In the

0805 and 0603 sizes there are a whole range of values of capacitance that you can choose.

--

Michael Karas
Carousel Design Solutions
http://www.carousel-design.com

Providing a bit of forward feed?

Making it respond to regulatory stimulus faster.

Put it ON the lead of the regulator where it mounts.

I concur here, if your F above was meant to be microF. (uF, µF ;-)

The last time I used a 7812, it was on a hobbyist prototype, and I hung a 0.1 uF cap on both the input and the output, tightly snugged up against the regulator, with a lead length of about 0.1". But that was when I was young and afraid of everything. :-D

It _did_ serve its purpose, however, of providing 12 VDC to the 9" monitor I used for my "TV Typewriter," distortion-free. :-)

Cheers! Rich

Try using long input leads and an 0.01 uF _input_ bypass. (You really do have to stand on one leg to make a 7800 regulator misbehave. 7900s, not so much.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Thanks for all the replies. I always had the impression that weekends were slow times in technical NGs, but I was pleasantly surprised to see that the two threads I started received quite a bit of attention.To reply to the various posts, some with similar contents:

No problem adding an SMD or a ceramic disc close to the regulator on the copper side. I just don't like 'afterthought' additions if they can be avoided. And yes, there's plenty of room to increase the track sizes which are already 30 and 40 mils wide.

The output cap is a 0.1uF ceramic disc which I've always used as a standard output cap for 78XXes except where the nature of the load makes heavier buffering desireable. Here's the design as it is now:

I don't think you'll have any trouble with that, although personally I would have tried to fit it between the 78xx and the chip, maybe by sliding those power input pads up and down, or by moving the chip left a bit.

It's been years since I've seen a PCB design done from the "bottom view"-- used to do it all the time that way for single-layer boards.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I do most of my designs as single-layer TH. It's only in recent years that we've had online PCB services here in India and the nearest one is literally more than 2000 km away. To avoid weeks of delay and 20x the expense, I still make most of my own PCBs using iron-on transfers. I'm reluctant to use smaller than 15 or

20 mil tracks and I haven't yet found a source for carbide drill bits. So I use single-sided phenolic boards with TH components most of the time.

Why not attempt to do SMT? I have found that it's much easier and faster.

Should the output cap not be as close as possible to the regulator?