Low ESR caps for LDO regulator

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

Reply to
Scott Miller
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regulator - the TI part has a lower dropout voltage,

(though it'd probably toast my board and melt the

operation. I've used LDO regs before with standard

best stability. When I start hunting for low ESR

as low as they go. Am I reading the regulator

wrong place for my parts?

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).

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--Larry Brasfield
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Reply to
Larry Brasfield

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

Reply to
Scott Miller

regulator - the TI part has a lower dropout voltage,

(though it'd probably toast my board and melt the

stable operation. I've used LDO regs before with standard

best stability. When I start hunting for low ESR

about as low as they go. Am I reading the regulator

wrong place for my parts?

Do note that instability is possible for ESR too low as well as too high. ;-)

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

If this is the data sheet you are looking at:

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I think that figure 4 tells you that the ESR of a 10 uf output cap should fall between .024 ohms and .4 ohms, only a 1 to 16 range. Satisfying this in a production situation may cost a lot more than the regulator does. They are not very helpful about how the regulator performs with other capacitances, either, so you will have to sweat what adding additional bypass caps to the rails will do to the stability. I would probably try a fairly good aluminum bypass cap a bit bigger than 10 uf, in parallel with a 1 uf stacked film Panasonic V series) that is in series with a .1 ohm resistor.

Then I would connect the rest of my system and torment the upstream side with pulses to see how the output was behaving. That is a long way from guaranteeing stability with the next thousand regulators, though. The stability of low drop out regulators is not only dependent on the load current and capacitor characteristics, but on the upstream voltage, also. They give me heartburn.

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John Popelish
Reply to
John Popelish

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
Reply to
John Popelish

One reason why Z5U/Y5V can be a problem is temperature range and tempco. In my (industrial) world I have to use X5R or better.

In my experience using small resistors in series with ceramics works well. We are prohibited from using chip tantalums, and the big hermetic tantalums tend to have a fairly high ESR.

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Greg Neff VP Engineering

*Microsym* Computers Inc. snipped-for-privacy@guesswhichwordgoeshere.com
Reply to
Greg Neff

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

Reply to
Andy

Yep - ceramics rule!

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

Reply to
Anders F

Because the LM1117 dislikes ceramic caps? ;-)

/Anders

Reply to
Anders F

I read in sci.electronics.design that Anders F wrote (in ) about 'Low ESR caps for LDO regulator', on Sun, 16 Jan 2005:

Why not put a very low value resistor in series with the ceramic?

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John Woodgate

Scott Miller schrieb:

--> LP 2950

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Tilmann Reh

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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 :).

Cheers Terry

Reply to
Terry Given

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

Reply to
Terry Given

Not so paranoid. Supply rail mechanical-intermittent short is the kind of fault that should cause no damage to a power supply. No small smd film resistors are really appropriate in series with larger caps - but below 220uF and at low voltage, is less than 100mJ per pop.

RL

Reply to
legg

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

Reply to
Anders F

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. snipped-for-privacy@guesswhichwordgoeshere.com
Reply to
Greg Neff

What about L4931CZ50?

Reply to
Andy

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

Reply to
Andy

Reply to
John Popelish

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