Low ESR caps for LDO regulator

S

Scott Miller

Guest
I'm looking at replacing a 78L05 regulator with a TI TL750L05 low-dropout
regulator - the TI part has a lower dropout voltage, better transient and
reverse bias protection, and can provide more current (though it'd probably
toast my board and melt the enclosure in the process.) Of course, it needs
a low ESR capacitor for stable operation. I've used LDO regs before with
standard tantalums, but this data sheet has me scratching my head.

The data sheet seems to indicate that the ESR needs to be below 0.4 ohms for
best stability. When I start hunting for low ESR tantalums on Digi-Key, I
find that at 10 uF, an ESR of 1.6 ohms or so is about as low as they go. Am
I reading the regulator datasheet wrong, misinterpreting the ESR rating, or
maybe just looking in the wrong place for my parts?

Thanks,

Scott
 
Scott Miller wrote:
I'm looking at replacing a 78L05 regulator with a TI TL750L05 low-dropout
regulator - the TI part has a lower dropout voltage, better transient and
reverse bias protection, and can provide more current (though it'd probably
toast my board and melt the enclosure in the process.) Of course, it needs
a low ESR capacitor for stable operation. I've used LDO regs before with
standard tantalums, but this data sheet has me scratching my head.

The data sheet seems to indicate that the ESR needs to be below 0.4 ohms for
best stability. When I start hunting for low ESR tantalums on Digi-Key, I
find that at 10 uF, an ESR of 1.6 ohms or so is about as low as they go. Am
I reading the regulator datasheet wrong, misinterpreting the ESR rating, or
maybe just looking in the wrong place for my parts?

Thanks,

Scott
Click to expand...
Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
something more standard, like LM1117?
 
Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
something more standard, like LM1117?
Click to expand...
Sorry, I should have specified through-hole. 0.1" radial lead spacing.

Aside from the TL75L05 being quite a bit cheaper, I need something in a
TO-92 package - or at least, something that'll fit in the same footprint.
This is for a kit that I've already produced boards for. I've substituted
other, more expensive LDO regs before for special applications that needed
very low dropout voltage (mostly due to very cold batteries), but the idea
here is mostly to make it more resilient to stupid mistakes (reversed
polarity, for example) and harsh conditions like automotive transients.

I've also got another board revision in the works that gives me some more
flexibility - I've added a transient suppressor, RFI protection, and might
have enough room for a TO-220 regulator. This board will be larger and more
expensive, though, and I'll probably keep both versions available.

Scott
 
"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:9uplu01kbqkk5m3ossmt725b79i4gs0r5q@4ax.com...
On Sun, 16 Jan 2005 13:41:59 -0800, the renowned "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Scott Miller" <scott@3xf.com> wrote in message
news:10ulmudp5bkjmc8@corp.supernews.com...
I'm looking at replacing a 78L05 regulator with a TI TL750L05
low-dropout regulator - the TI part has a lower dropout voltage,
better transient and reverse bias protection, and can provide more
current (though it'd probably toast my board and melt the
enclosure in the process.) Of course, it needs a low ESR capacitor for
stable operation. I've used LDO regs before with standard
tantalums, but this data sheet has me scratching my head.

The data sheet seems to indicate that the ESR needs to be below 0.4 ohms
for best stability. When I start hunting for low ESR
tantalums on Digi-Key, I find that at 10 uF, an ESR of 1.6 ohms or so is
about as low as they go. Am I reading the regulator
datasheet wrong, misinterpreting the ESR rating, or maybe just looking
in the wrong place for my parts?

Thanks,


Use a ceramic cap.

Small sintered tantalums are well known for somewhat
high ESR. If you want to spend more money, a foil
tantalum would have lower ESR. But the ceramic
will be cheaper and more stable in the long term, (at
least if you choose the right dielectric).
Click to expand...
Yep - ceramics rule!

Do note that instability is possible for ESR too low as well as too
high. ;-)
Click to expand...
Yep - and that s*cks big time. Seems I _need_ to put a big nasty
electrolytic cap on my LM1117 in stead of the nice 22uF ceramics I thought I
could use :-(

Cheers,
Anders
 
"Andy" <andy@nodomain.nod> wrote in message
news:41eae305$0$7125$8fcfb975@news.wanadoo.fr...
Scott Miller wrote:
Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
something more standard, like LM1117?
Click to expand...
Because the LM1117 dislikes ceramic caps? ;-)

/Anders
 
Terry Given wrote:

Yep. Very frustrating, the tight ESR range spec. An Al electro aint much
use if the temperature gets down to say -20C, as Al electrolytic ESR
then skyrockets. Sanyo OSCON caps have very low, and (cf Al) very stable
ESR. I normally use X7R caps & series R's, also ensuring the peak pulse
power rating of the resistor can hack it (IOW not an 0402 :).
Click to expand...
The current lasts a very short time and is linited by the regulator,
so I doubt you will ever blow out even an 0402 on start up. But do
you have any trouble finding ESR data for your ceramic caps? I would
probably go with a 1210 size rated for a bit extra voltage (say, 10 or
16 volts, rather than a 6.3 volt unit) to make it more likely that the
ESR was low. And no Y5V or Z5U. X7R or X5R only.

--
John Popelish
 
John Popelish wrote:
Terry Given wrote:


Yep. Very frustrating, the tight ESR range spec. An Al electro aint much
use if the temperature gets down to say -20C, as Al electrolytic ESR
then skyrockets. Sanyo OSCON caps have very low, and (cf Al) very stable
ESR. I normally use X7R caps & series R's, also ensuring the peak pulse
power rating of the resistor can hack it (IOW not an 0402 :).


The current lasts a very short time and is linited by the regulator,
so I doubt you will ever blow out even an 0402 on start up. But do
you have any trouble finding ESR data for your ceramic caps? I would
probably go with a 1210 size rated for a bit extra voltage (say, 10 or
16 volts, rather than a 6.3 volt unit) to make it more likely that the
ESR was low. And no Y5V or Z5U. X7R or X5R only.
Click to expand...
Hi John,

Very true wrt regulator. OTOH a load short will dump the cap into the
series R.....hows that for paranoid :)

I always measure the ESR. And some manufacturers provide good data (eg
TDK, AVX). Good call wrt voltage rating, and especially the dielectric.
Although by now there should be no excuse for SED patrons to use the
evil Z5U/Y5V dielectrics (except as transducers :)

Cheers
Terry
 
On Sun, 16 Jan 2005 14:38:27 -0800, Scott Miller wrote:

Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
something more standard, like LM1117?

Sorry, I should have specified through-hole. 0.1" radial lead spacing.

Aside from the TL75L05 being quite a bit cheaper, I need something in a
TO-92 package - or at least, something that'll fit in the same footprint.
Click to expand...
According to
http://www.revistapcecia.com.br/downloads/datasheets/LM1117.pdf
page 20, the TO-263 package has little leads on nominally .1 centers -
problem is, they're ground, output, input, which is about as
ass-inside-out from a 78L05 as you can get.

Good Luck!
Rich
 
I read in sci.electronics.design that Anders F <af-spam@hi5.dk> wrote
(in <3508vtF4eqc80U1@individual.net>) about 'Low ESR caps for LDO
regulator', on Sun, 16 Jan 2005:
Yep - and that s*cks big time. Seems I _need_ to put a big nasty
electrolytic cap on my LM1117 in stead of the nice 22uF ceramics I
thought I could use :-(
Click to expand...
Why not put a very low value resistor in series with the ceramic?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Scott Miller schrieb:

Sorry, I should have specified through-hole. 0.1" radial lead spacing.

Aside from the TL75L05 being quite a bit cheaper, I need something in a
TO-92 package - or at least, something that'll fit in the same footprint.
Click to expand...
--> LP 2950

--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
http://www.autometer.de
 
"Greg Neff" <greg@nospam.com> wrote:
Terry Given wrote:
[...] I normally use X7R caps & series R's, also ensuring the peak pulse
power rating of the resistor can hack it (IOW not an 0402 :).

In my experience using small resistors in series with ceramics works
well.
Click to expand...
Whart size are you using for what cap sizes? Any tests done?
I actually also considered using a 0805 ferrite. But I'm not quite sure it
will better the EMC on the LDO input anyway...

/Anders
 
On Wed, 19 Jan 2005 22:45:38 +0100, "Anders F" <af-spam@hi5.dk> wrote:

"Greg Neff" <greg@nospam.com> wrote:
Terry Given wrote:
[...] I normally use X7R caps & series R's, also ensuring the peak pulse
power rating of the resistor can hack it (IOW not an 0402 :).

In my experience using small resistors in series with ceramics works
well.

Whart size are you using for what cap sizes? Any tests done?
I actually also considered using a 0805 ferrite. But I'm not quite sure it
will better the EMC on the LDO input anyway...

/Anders
Click to expand...
We have thousands of boards in the field using this approach. Our
boards are put through extensive testing because they are used in rail
vehicle applications. What we usually do is use 1 ohm 1206 resistors
in series with 2.2 to 4.7 uF ceramics (X5R or better). We use
multiples of these RC networks to get the desired capacitance and ESR.

================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
greg@guesswhichwordgoeshere.com
 
Scott Miller wrote:
Sorry, I should have specified through-hole. 0.1" radial lead spacing.

Aside from the TL75L05 being quite a bit cheaper, I need something in a
TO-92 package
Click to expand...
What about L4931CZ50?
 
Anders F wrote:
"Andy" <andy@nodomain.nod> wrote in message
news:41eae305$0$7125$8fcfb975@news.wanadoo.fr...

Scott Miller wrote:
Look for T494C226M016AS at Digi-Key. But why use TL750L05 and not
something more standard, like LM1117?


Because the LM1117 dislikes ceramic caps? ;-)

/Anders
Click to expand...
The datasheet for REG1117 specifies only the upper boundary on the ESR
and the lower boundary on the capacity. Does it mean that one can add
the ceramic caps on the output rail without a stability problem, once
the base 10uF ESR<0.5 Ohm tantalum capacitor is there?

-- Andy
 
Andy wrote:

The datasheet for REG1117 specifies only the upper boundary on the ESR
and the lower boundary on the capacity. Does it mean that one can add
the ceramic caps on the output rail without a stability problem, once
the base 10uF ESR<0.5 Ohm tantalum capacitor is there?
Click to expand...
I don't think so. If the other parallel capacitors added up to much
more than the lumped capacitance at the regulator, they would begin to
dominate the phase of the current at high frequencies.

--
John Popelish
 
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