# Why does the LM1117 data sheet specifically specify tantalum capacitors?

I'm planning on using an LM1117 to regulate 5v to 3.3v. Looking at (any of the several) data sheets for the LM1117, they recommend 10uF Tantalum capacitors between input and ground and between output and ground. I understand the need for the capacitors, but it's not clear to me why these should specifically be Tantalum. I have a bunch of electrolytic 10uF capacitors sitting around here, whereas if it does for some reason need to be Tantalum, I'll have to order those.

Why are they so specific about using tantalum capacitors?

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Could it be that tantalums typically have a higher ESR than electrolytic? – Majenko Aug 18 '11 at 10:12
Matt, having read a little more about it, ESR certainly seems likely to be the reason. ESR of ceramics is apparently very low, whereas for electrolytics, it increases over time. If this is indeed the reason, electrolytics should be fine for now - I'm trying something out, not making 10,000 of them. Still, I'd like to know for sure. – Jon Bright Aug 18 '11 at 10:28
Tantalums have a low ESR! – Leon Heller Aug 18 '11 at 11:42
@leon Not according to what I read on Wikipedia :P – Majenko Aug 18 '11 at 19:12

Tantalum capacitors are completely unnecessary in this application.

• The only reason for choosing tantalum might be lifetime, and this can be designed for with aluminium wet electrolytic caps. It is assumed from here on that lifetime has been properly designed for and is not an issue.

• Using a tantalum capacitor as the input capacitor invites capacitor death at any time if the input power rail can have voltage spikes on it from any source. A spike more than a small fraction above a tantalum capacitor's rated value risks it's total destruction in a high energy circuit, such as this one is.

• The input capacitor is a typical reservoir capacitor, its value is relatively non critical. Tantalum serves no technical purpose here. If ultra low impedance is desired then use of a smaller parallel ceramic is indicated.

• The output capacitor is NOT a filter capacitor in any traditional sense. Its principal role is to provide loop stability for the regulator. (An eg 10 ohm resistor could be placed in series with the capacitor without impeding its functionality. No normal filter cap would tolerate this without impaired functionality).

• The characteristics of aluminum wet electrolytic capacitors of the correct capacitance and voltage rating are well suited to the output capacitor's role. There is no reason to not use them there. This 7 cent capacitor pricing / general data / datasheet would be an acceptable choice in many applications. (Longer lifetime applications may indicate 1 2000 hour/105C part).

The LM1117 datasheet provides clear guidance on the essential and desirable characteristics of the input and output capacitors. Any capacitor which meets these specifications is suitable. Tantalum is an OK choice but is not the best choice. There are various factors and cost is one. Tantalum offers OK cost per capability at capacitances from about 10 uF up. The output capacitor is "safe" against spikes in most cases. The input capacitor is at risk from "bad behavior" from other parts of the system. Spikes above rated value will produce a (literally) flaming melt down. (Smoke, flame, noise, bad smell and explosion all optional -
I have seen one tantalum cap do all of these in turn :-))

Input capacitor

The input capacitor is not overly critical when the regulator is fed from an already well decoupled system bus. Under the diagram on the front page they note "Required if the regulator is located far from the power supply filter" - to which you could add "or another well decoupled portion of the supply". ie capacitors used for decoupling in general may make another one here redundant. The output capacitor is more crucial.

Output capacitor

Many modern low drop out high performance regulators are unconditionally unstable as supplied. To provide loop stability they require an output capacitor which has both capacitance and ESR in selected ranges. Meeting these conditions is essential for stability under all load conditions.

Output capacitance required for stability: Stability requires the output output load capacitor to be >= 10 uF when the Cadj pin does not have an added capacitor to ground and >= 20 uF when Cadj has an added bypass capacitor. Higher capacitances are also stable. This requirement could be met by an aluminum wet electrolytic cap or a ceramic cap. As wet electrolytics are generallt wide tolerance (up to +100%/-50% if not other wise specified) a 47 uF aluminum wet electrolytic would provide adequate capacitance here even when Cadj was bypassed. BUT it may or may not meet the ESR spec.

**Output capacitor ESR required for stabiity: ESR is a "Goldilocks requirement" :-) - not too much and not too little. Required ESR is

    0.3 ohm <= ESR <= 22 ohm.


This is an extremely wide and unusual requirement. Even quite modest ripple currents in this capacitor would induce far larger than acceptable voltag e variations. It's clear that they do not expect high ripple currents and that the capacitor's role is primarily related to loop stability than to noise control per se. Note that "old school" regulators such as eg LM340 / LM7805 often specified no output capacitor or perhaps a 0.1 uF. For example the LM340 datasheet here says "**Although no output capacitor is needed for stability, it does help transient response. (If needed, use 0.1 µF, ceramic disc)".

A tantalum capacitor is not required to meet this specification.
A wet aluminum capacitor will meet this spec with ease. Here are some typical new maximum ESRs for new aluminum wet electrolytic capacitors. The first group are capacitors that might be used in practice in this application at the low end of the capacitance range. The 10 uF, 10V is about half the allowed ESr - perhaps a bit close for comfort across lifetime. The secnind group are what would be used with Cadj bypassed and could be used anyway - ESRs are far away from limits in both directions. The third group are capacitors chosen to approach the lower limt (and they will get higher resistance = better with age). The 100 uF 63V pushes the lower limit - but there wouild be no need to use a 63V part here, and it will get higher (= better) with age. .

• 10uF, 10V - 10 ohm
10 uF, 25V - 5.3 ohm

• 47uF, 10V - 2.2 ohm
47 uF, 16V - 1.6 ohm 47 uF, 25 V, 1.2 ohm

• 470 uF, 10V - 024ohm
220uF, 25V - 0.23 ohm
100 uF, 63V - 0.3 ohm

They say in the LM1117 datasheet

• 1.3 Output Capacitor

The output capacitor is critical in maintaining regulator stability, and must meet the required conditions for both minimum amount of capacitance and ESR (Equivalent Series Resistance).

The minimum output capacitance required by the LM1117 is 10µF, if a tantalum capacitor is used. Any increase of the output capacitance will merely improve the loop stability and transient response.

The ESR of the output capacitor should range between 0.3Ω - 22Ω. In the case of the adjustable regulator, when the CADJ is used, a larger output capacitance (22µf tantalum) is required

ESR is crucial

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 Thank you for the extremely detailed and helpful answer. I'm still a little puzzled about why they specifically call out tantalum so much, but your answer makes clear that I can overlook this. – Jon Bright Aug 18 '11 at 12:21

Electolytics have poor high frequency performance compared to tantalums. With the price of tantalum these days I would recommend using one of your electrolytics with a small ceramic capacitor in parallel - say 100nF. It depends on what you are powering but it's not usually all that critical unless ripple and transient response are especially critical for you.

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 people get in a mindset backwater on occasion - aluminium* electrolytic bad - tantalum good - must use tantalum .. . I'd be pleased to hear anyone critique my reply on technical grounds. If I've missed something or made a significantly incorrect technical assertion then by all means say. But, I don't believe so. * - I've now spelt aluminium "correctly" as I see you are in UK :-). Flame shields up. – Russell McMahon Aug 18 '11 at 12:33 @Russell McMahon - Huh? (1) I posted my answer before you so I wasn't criticising. (2) We seem to agree anyway! (3) Aluminum capacitors are much much worse than aluminium :) – MikeJ-UK Aug 18 '11 at 12:44 mea culpa :-( - I mixed up the comment maker with the questioner - comment was meant to go on the end of my post after Jon's comment :-). I wasn't criticising you - sorry if it seemed that way. I meant to comment on Jon's query why they would spec tantalum given the apparent lack of need for it. Sorry again. – Russell McMahon Aug 18 '11 at 13:18