# HUGE capacitor recommended in datasheet for Audio Amp

I'm using the TPA3111 audio amplifier in a circuit of mine and I'm working on adding the appropriate capacitors. However, on page 18/19 of the datasheet TI recommends to use a 220mF capacitor on the power line along with a 220uF capacitor on each of the PVcc pins. Is this entirely necessary? If you take a look at the evaluation board TI offers for this chip they don't do any of that. The same goes for the application case in the datasheet.

I'm already using quite a few capacitors according to the sample application and eval board. The largest so far are 100uF electrolytics and then there are bunch of smaller ceramic caps. Any idea on what dangers I might have if I do not include the additional 220mF or 220uF caps?

Thanks.

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The Example schematic only has 100uF + 0.1uF + 1000pF across the rails. Also, have you considered the "mF" may be a typo? In one place, the datasheet says typically 0.1 mF to 1 uF. I wonder if they meant to type n, and accidentally hit m. Besides, almost nobody uses millifarads, just as very few people use nanofarads. It's Farads -> microFarads -> picoFarads. – Connor Wolf Jul 25 '11 at 10:23
Look at some of TI's similar amp ICs, and compare bypassing. If the TPA3111 capacitance values are not in-line with similar parts, it's almost definitely a typo. – Connor Wolf Jul 25 '11 at 10:25
It's a substitution for uF (microFarad) that occurs in some locations and not others. There is a confusion in the "fonts" or similar in the document that leads to microfarad being abbreviated as uF (mu F) in some locations and as mF in others. If I search the document for mF the search locates use of uF or mF !!! / The first and last paragraphs on page 19 show clearly confusing and inconsistent use of uF and mF - substituting uF for mF in these paragraphs makes them read correctly. I don't recall ever seeing an amplifier cct that specifies capacitors as large as 0.22F for power decoupling. – Russell McMahon Jul 25 '11 at 10:55
@Russell McMahon - I've seen amplifiers which use 0.1F+ for decoupling, but they also needed 240V 50A mains connections, and were rated for (true) 6KW continuous. – Connor Wolf Jul 25 '11 at 11:07

The Example schematic only has 100uF + 0.1uF + 1000pF across the rails.

Have you considered the "mF" may be a typo? In one place, the datasheet says typically 0.1 mF to 1 uF. I wonder if they meant to type n, and accidentally hit m. Also, I copied the u symbol out of the PDF, and it got printed as m when it was pasted. Cut&Paste may be at fault here, it certainly seems to be used within TI's various datasheets.

Furthermore, looking at the TPA3111 Evaluation Kit is informative:
The device is bypassed with two 100uF electrolytics (along with 0.1uF and 1000pF ceramics).

Also, looking at similar parts from the same line is informative. The TPA3110 (15W vs TPA3111's 10W) merely says a larger aluminum electrolytic capacitor of 220 uF or greater placed near the audio power amplifier is recommended. It's worth noting that the same datasheet's example schematics only use two 100uF caps for bypassing.

The same note as in the TPA3111 is present in the TPA3112 datasheet.

It's also worth noting that the TPA3110 and TPA3113 have identical "Power Supply Decoupling" paragraphs, despite the fact that one is half the power of the other (15W vs 6W), which further inclines me to think typo.

The 25W TPA3123 only recommends 470uF of bulk capacitance.

The 100W TAS5121 only recommends 1000uF.

Edit: We will see if it is a typo: "Below is what you submitted to tis-doc-errors@list.ti.com on Tuesday, July 26, 2011 at 04:13:23; E-mail: tis-doc-errors@list.ti.com Lit Number: SLOS618BB Part Number: TPA3111D1 Error Page No: 19 Error Description: Please see this thread: HUGE capacitor recommended in datasheet for Audio Amp"

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"Also, almost nobody uses millifarads, just as very few people use nanofarads. It's Farads -> microFarads -> picoFarads". Not at all. This may have been true 30 years ago, but today milliFarads and nanoFarads are perfectly fine. In engineering we use the multiplier of 1000 such that the number has 1-3 digits left of the decimal point, then compensate with the appropriate units prefix, like milli, micro, nano, etc. – Olin Lathrop Jul 25 '11 at 12:23
@Olin - Perhaps it would be more accurate to state that "Historically, some people didn't (and some still don't) use millifarads or nanofarads, and use Farads -> microFarads -> picoFarads instead"? – Kevin Vermeer Jul 25 '11 at 13:10
Indeed, millifarads aren't used that much: people talk about 1000$\mu$F, not 1mF. But nanofarad is used all the time: a decoupling capacitor is typically 100nF. – stevenvh Jul 25 '11 at 16:46
@Olin Lathrop - I agree nanofarads are somewhat common (and I fixed the line in question), but millifarads? Really? Where, exactly, do you see them used regularly? – Connor Wolf Jul 26 '11 at 3:35
@Fake: Can you show any old schematics or parts lists that show nF and mF. I never saw these at all until the 1990s. Really old stuff sometimes used "m" as a abbreviation for micro. I've even seen "mmF", which meant micro-micro-Farad. Apparently even picoFarads took a while to catch on. In any case, milli, micro, nano, pico is the right way to do things, and is what people should be taught. – Olin Lathrop Jul 26 '11 at 23:35

It all depends on how hard you intend to drive the amplifier. If your goal is to get close to the full 10W over the full audio range then you may need substantial capacitance on the power rail.

Class D amplifiers have very spiky current draw and the gain of the Class D amplifiers is directly proportional to the rail voltage. Any dip in the rail will cause poor frequency response, usually on the low end. For your application this may not be important.

Additionally the part you've chosen is a half-bridge design which can suffer from "bus-pumping" issues which the can cause currents flowing back into the supply to push the rail voltage up to levels dangerous to the circuit.

A common way to deal with this is to use very large decoupling capacitors to "soak up" the pumping. Generally this is more of an issue with high power amplifiers switching under something like 100Khz. However the designer for the datasheet may have added large capacitance as a safety net.

So i would say in summary, unless your pushing this device to its limits, you don't need as much capacitance as they discuss in the datasheet.

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Audio amplifiers are notorious for massive current spikes, especially for lower frequencies. Think about how much more current is consumed when someone bangs the bass drum and it has to move pump extra current into the speaker to replicate that massive one-way movement of the speaker cone.

The big capacitors are there to provide that extra boost of current when it's needed.

The only problem you should see is possibly some distortion when you have higher volumes at lower frequencies, like with drums, and there may be a noticeable drop in power to the rest of the circuitry (lights may dim, etc when you bang that big ol' bass drum) at times.

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I hope the lights don't dim in response to a 10W amplifier :) Really he's not going to get any reasonable response under MAYBE 80Hz with 10W unless he's driving headphones and should just filter it out. – Mark Jul 25 '11 at 8:22
While I agree with the reasons you mention I think this is overkill for an amplifier supplying just a few Watts. What will they suggest for a 50W amplifier? 4.7F? If it's really necessary the amplifier is poorly designed IMO. – stevenvh Jul 25 '11 at 8:28
I completely agree. If this was a big amp, then yes, big caps would be called for. Maybe it's a typographical booboo? – Majenko Jul 25 '11 at 9:50
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