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Class D Switching Power Amplifiers and Power D/A conversion

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Old 7th December 2004, 01:13 AM   #1
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Default good audio PWM ICs

I am designing an amplifier for my senior project at Purdue University in Electical and Computer Engineering Technology in the USA. I worked at a highly regarded pro audio company to which I hope to return after graduation and pump out some excellent amplifiers in addition to lots of DIY work for you folks. First, however, I need to graduate

I am fairly familiar with how PWM works, with some experience working with [but not designing] an amplifier using the SG3525. However, I am looking for any alternative schemes for getting my PWM signal. I have a rather complex analog input stage designed outputting 2Vrms to my amplifier. The amp will be powered by 40Vrms single ended. Are there any decent alternatives that work well other than the SG3524, SG3525, TL494, UC3637 in an audio amp? The amp is planned to be a half-bridge using either the IR2011 or IR2110/3 gate drivers. I have not decided on FETs yet.

My only other idea was to convert the analog signal into PCM and then use one of the TASxxxx or Wolfson PCM-PWM chips to do all the hard work for me. This seems totally ridiculous.

Just to make you feel guilty for looking and not giving me any info , I would love to provide the finished schematics and layout files to all interested. I have the input section and power supply nearly finished and, I think, available for viewing. The input section has both RCA and USB inputs and provides LR-4 LP xover, 0-180 phase adjust, and volume control. The power supply is a flyback using Power Integrations' TOPSwitch-GX 250 IC for control. If you are interested, post here and I will try to provide the power supply and input stage schematics shortly.

Thank you, Evan
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Old 7th December 2004, 01:51 AM   #2
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I don't think there is a chip solely for adding a power stage just for Class D apps. The ones that you listed are all power supply chips and are not the most accurate when it comes to audio signals.

A discrete design with high bandwidth components will give good results. I'll post my design a bit later in the clocked feedback thread if you want a starting idea (although I'm no graduate myself).

Cheers
Matt
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Old 7th December 2004, 02:35 AM   #3
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Sounds good. If complexity is not an issue, my vote is towards discrete implementation.

If you'd like to consider Silicon Valley when you graduate, send me a resume.
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Old 7th December 2004, 03:50 AM   #4
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Thank you. I looked at some discrete designs, and after talking with a couple people who have designed Class D amps in commerical products, they said that discrete was too much of a headache with propogation delay matching and such so I hadn't considered discrete until now.

Complexity is not an issue, I have complete faith in myself that I can deliver my project ahead of time regardless of anything.

fr0st, thank you. I have looked at the application in the IR reference amplifier but haven't found anything else that I had great faith in it's performance.

thespeakerguy, YGM
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Old 7th December 2004, 11:41 PM   #5
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Default Class D amps

First study what has already been done so you don't repeat bad history. There are several class-D amp schematics in the Audio circuits section of www.schematicsforfree.mattsoft.net. From indifferent to quite good.

My self I have used thousands of ICE Power modules. They use a balanced modulator with a pair of high precision comparitors (LM319) producing PWM for a pair of the IR2110 driver chips then feeding an H Bridge,
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Old 10th December 2004, 01:37 AM   #6
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Default New Class D FET

Evan, you might be interested in this:

<http://www.irf.com/whats-new/nr041208.html>
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Old 10th December 2004, 02:17 AM   #7
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EF, thanks but this project has to be built by hand or by an old oven, so that FET is out of the question. it looks nice tho

dmfraser, thank you. I have been checking through them and I think I have a good PWM section coming along.

A few more questions for you Class D wizards:

1. I need a comparator for the PWM section and I was looking at MAXIM's offerings. I want to use a push-pull type and it can't be uMAX, BGA, LLP, etc. I also eliminated everything that wasn't rail-to-rail. I can't decide between the MAX985, MAX989, or something really fast that might give me problems like the MAX963. TI didn't have anything that looked better than the MAXIM offerings, unfortunately.

I didn't want to use the LM319 because it is open-collector and, although I haven't had any problems, I have been told to avoid open-collector comparators if possible. Are there any other comparator suggestions?

I noticed that the IR reference amp uses a LT1120. Why would they use an op amp instead of a comparator? Isn't the point of a comparator to react quickly because it doesn't have a dominant pole cap inside it?

2. For my sawtooth generation, I saw the LM556 and MAX038. The MAXIM part doesn't look like what I really want, but the DADJ pin tempts me.

3. By using this discrete design, I will have to have an additional inverter in one of the 2 PWM signals. Won't that add propogation delay? Should I add a delay somehow to even them out?

Thank you, Evan
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Old 10th December 2004, 03:06 AM   #8
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This looks good, but slightly complex http://www.maxim-ic.com/appnotes.cfm/appnote_number/825 I am thinking about replacing the open-collector MAX991 with the push/pull MAX987. And perhaps using the MAX987 for the main PWM comparator also. PSpice, here I come
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Old 10th December 2004, 12:05 PM   #9
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I can't edit for some reason.

Anyway, in question #3 above, I must have been missing something. I understand what is going on now.
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Old 10th December 2004, 12:18 PM   #10
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Quote:
2. For my sawtooth generation, I saw the LM556 and MAX038. The MAXIM part doesn't look like what I really want, but the DADJ pin tempts me.
Friends, don't let your friends use sawtooths for class-d amps !

Quote:
3. By using this discrete design, I will have to have an additional inverter in one of the 2 PWM signals. Won't that add propogation delay? Should I add a delay somehow to even them out?
Pass the signal through two XORs. Feed one input of both gates with the same PWM signal and tie the other input of one of the gates to logic H and the other one to logic L. And voila: you have an inverted and a non inverted version of the same signal, delayed by the same amount of time.

Regards

Charles
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