Low ESR caps for LDO regulator

Scott Miller · 2005-01-16T21:28:59+00:00

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

Jan 16, 2005 22 Replies

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John Popelish 21 years ago

I think that this is the big problem with LDO regulators. Their stability is highly load dependent. When I have used them, I performed stability tests (injected disturbances), varying all relevant component values over a wider range than tolerances would account for, in the actual layout, and using several different regulator units from different production batches. This means that the board layout had to include provisions for alternatives.

But I always worry about the generality of the solution I arrive at, especially if the regulator or load undergoes any significant revision.

John Popelish

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Andy 21 years ago

Okay, how is one supposed to evaluate the stability / dynamic characteristics of a real circuit powered by a LM/REG/LD/LT/NCP/...1117? No model seems to be available from the manufacturers. Only through test and trial??

Suppose I am powering by an LM1117 an IC, with the tantalum as required at the output of the regulator. The IC has 10 ceramic 0.1uF caps spread around, from the power rail to ground. Is there a minimum trace inductance to observe between the regulator and the first 0.1uF, that would preserve good dynamic response of the reg.?

I understand that there may be no response easily available. If this is a really grey area, and everyone has to make his own tests, why not simply go with some AnyCAP from AD?

Thank you.

-- Andy

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Anders F 21 years ago

How would one determine the "mJ" capacity of standard 0603/0805/1210 resistors?

One could hope for a decent ramp-up of the LDO and put it in series with the load (?). Of course it will see the entire DC-load and drop the voltage a bit at high loads... But it won't get the "AC-kicks" and limit the caps.

/Anders

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