7805 failure mode

Quote "The ESR of the output capacitor should range between 0.3? - 22?."

Nah, thanks, I think I'll pass.

--
Regards, Joerg

http://www.analogconsultants.com

Not much input voltage capability to handle transients- 20V, and (like all LDOs) it's picky about output caps. I still like the 78M05 for applications where it fills the bill. I've never seen a non-mechanical failure.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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I tested some National 1117's as power amps, with an opamp driving the adj pin, and specifically did really abusive voltage diffs between the adjust pin and others, and couldn't blow it up. It's not a real LDO, as it has an emitter-follower output so it has an inherently low output impedance. They seem happy with ceramic output caps. The National datasheet says that ESR should be "below 0.5 ohms."

An LM317 won't quite do a 5.0 to 3.3 volt regulator, but an LM1117 will.

To get the 1.2 and 2.5 volts for a Spartan FPGA, you can ground the adj pin on the lower regulator, and bootstrap the adj of the upper one to the 1.25 output of the lower one, and get 1.25 and 2.5 with no resistors!

John

Nope. Sez 0.3 to 22 Ohms on page 9, paragraph 1.3 Whenever I read something like that goose bumps appear.

At least they openly stated it. For a LM29xx series device there was no mention of some of the pathologies and it became my first black eye from LDOs. Didn't like it when Zin went above about 100 ohms, phsssst ... poof. The answer was pretty much "oops, sorry".

--
Regards, Joerg

http://www.analogconsultants.com

Often a company modifies a datasheet after a customer found a problem. About half way through university I believed datasheets were perfect. Until I found a hardcore bug in one, then another, and another. This includes some top notch manufacturers. Typical scenario, call into app engineering, a group gathers at the other end, more engineers get pulled in, the noise increases, the crackle of lots of paper is heard, then suddenly one hears a muffled "Oh s..t!"

WRT low dropout regulators that whole category is on my banned list no matter from which mfg. I might make an exception if I really, really have to. Like if the earth would stop turning if I didn't, or so.

[...]

--
Regards, Joerg

http://www.analogconsultants.com

[snip]

Crap like the current crop of LDO's are the result of customer demand (*)... minimum number of pins and minimum external components.

With an extra pin, compensating an LDO is trivial.

(*) I run into this from customers all the time, but I tend to be my usual nasty self and tell them that their request is fine... it's no skin off my back if it oscillates ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

What datasheets?

In the old good times, you could have one handbook for *all* electronic components ever made by all companies. And only a small part of those components was really available. The handbooks did have few mistakes, and those mistakes where well known.

It is very unpleasant to discover bugs in the top vendor's hardware or software. But in our days everybody seems to loose any sense of shame. AD Blackfin errata has a couple of lines from me.

You should forgive them for your own sake :-)

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

The older datasheets say 0.5 ohms max, and that larger C values just improve stability. The new ds says 0.3 to 22 ohms, silly on its face, but also says that larger caps "merely improve loop stability", which is silly based on 0.5r min.

Anyhow, they seem happy with 22u ceramics or aluminums. But maybe I should do a little more stability testing.

Lately we avoid alums because of temperature, and tants because of explosions.

It's not clear if 317's are stable with ceramics. All the caps on the datasheet are footnoted as solid tantalums.

John

Current datasheet says it must be within the range of 0.3 ohms to 22 ohms, and at least 10uF (* page 9, 1.3). Presumably (??) it will be stable with ceramic caps if you have a larger electrolytic in parallel.

Yes. That's where I'd use it.

That's cool. The 'sequencing' might even be better that way.

• formatting link

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

Well *now* they state it (current (April 2006) datasheet date, and June 2004 and October 2002 datsheet)

Aha! But *not* in the February 2000 datasheet!

Obviously they found a problem; IIRC in 2000 a 10uF ceramic cap was pretty much unheard of.

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

In was in a databook alright but the bug was not well known. At least not to the manufacturer ....

There may soon be others from me but for the ADN8831 TEC controller chip.

Oh, I forgave, otherwise I should not go to communion on Sundays. But that doesn't mean I won't be careful and a bit suspicious in the future. So I got used to whipping up a switcher whenever there wasn't enough dropout voltage available.

--
Regards, Joerg

http://www.analogconsultants.com

Try that over the full expected temperature range including the low end (unit unloaded from truck in northern Alaska or Siberia and turned on right away). There can be lots of surprises.

--
Regards, Joerg

http://www.analogconsultants.com

Wonder what happens if you were to drive the common pin with a low impedance square wave source? Do you see the spikes on the edges of the square wave? How wide are the spikes?

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

"In theory, there isn\'t any difference between theory and practice.  In 
practice, there is."  - Yogi Berra

I tried a National LM1117 with various loads and outputs caps. I'm injecting a load current step and looking at the transient response.

No cap at all looks fine.

Any tantalum looks good. 10uF looks fine.

It rings with ceramics. Very roughly,

10 uF 50 KHz Q=3

22 uF 40 KHz Q=2

44 uF 20 KHz Q=4

At 22 uF, the esr for critical damping is about 0.2 ohms.

Temperature and load variations don't change things much.

John

Interesting. At least it didn't oscillate. Although ringing can cause brown-out effects in hotrod 3.3V logic or even fry it.

Did you go below freezing?

--
Regards, Joerg

http://www.analogconsultants.com

The last LDO I designed in oscillated beautifully if you left out the

10uF e-cap (just with bypass caps). I know because the proto assembly guy mixed up two regulators, one LDO and one not.

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

Yup. My freeze spray hits -50C.

John

been

least

Some of the newer ldo's are specifically ceramic-stable.

I don't understand exactly why all regs aren't stable for any reasonable c or any reasonable c+esr. Must have to do with the available range of internal compensation parts.

John

Perhaps you should come up with an effective capacitance multiplier, etc., and internally compensate your circuits.