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Old 21st January 2008, 09:24 AM   #1
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Default Maxim MAX9704 - anybody built one?

The only other thread I could find on this chip was asking for a PCB, there were no replies...

I did a Google search 'Tripath TA2024' because I was looking to buy some chips, I found this Maxim chip offering itself as an alternative.

It's a class-D spread spectrun audio amp, with a comparable output, but that's pretty much where the resemblance ends. This runs up to 25V and is filterless, hence not really requiring a PCB as there are just a few caps at the inputs and across the power rails. It just sends the supersonic spread spectrum PWM to the speaker and lets it get on with it. Minimalist or what?

Does anybody know if it is possible to buy the Tripath chip still? I heard the company went out of business and was bought up. I been waiting for a board from sureelectronics, but they said they were waiting for stock. I ordered a Sonic Impact, just to be sure I get at least one.

Anyway, Maxim are sending me two of these chips I hope. They been out for a while so I guess if they were really hot there would be some posts about them. If the Tripath chip turns up in some incarnation, I'll be able to compare them.
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Old 21st January 2008, 04:52 PM   #2
kstrain is offline kstrain  Scotland
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Default short peaker "cables"

IIRC they recommend cables be kept less than 3 inches long (hope I'm thinking about the right chip).

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Old 21st January 2008, 05:15 PM   #3
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This device comes in a 32-Pin TQFN package with a copper paddle. You DO need a proper PCB to use this IC...

For starters, and to quote the datasheet: "The exposed pad is the primary route of keeping heat away from the IC. With a bottom-side exposed pad, the PC board and its copper becomes the primary heatsink for the Class D amplifier."

Secondly, although the design is minimalist and requires very few components, it is still a class-D amp. What this means is that both the gound and Vdd pins, will have high current pulses with ultra-fast rise and fall times. This essentially means that even nanohenries in any wiring around those pins will create large voltage spikes that at best will cause a lot of distortion. At worst, the chip may be damaged.

A double sided board is a bare minimum, with a multilayer board with a dedicated ground and power planes the best.
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Old 22nd January 2008, 01:25 PM   #4
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Not the first time I been caught out by the decreasing size of packages, I just saw the familiar 32 pin square outline and thought it was a QFP32. I've got some charge-pump LED drivers here that I sampled only to discover that these 12-pin devices are only 3x2mm. Those are a bit small for laser-printed PCBs.

3 inches of speaker wire could be a problem too... ...but the datasheet says only 'use a ferrite bead filter... snip ...or when long leads connect the amplifier to the speaker.' The '3 in' reference is to the conditions they employed for EMC test. I don't intend to be submitting for EMC test.

Anyway, I'm not one to give up easily, I bought a nice radiant cooker, I've successfully surface-mounted a few things using it. It's entirely possible to route 8x12 thou traces into each side of a 7mm square, my router doesen't even throw a design rules violation at standard settings. I've forced it to accept much smaller and tighter traces.

And things aren't that bleak, lots of the pins are side-by-side doubled, in fact it might still be possible to solder the components directly to the chip and glue the heatsink on with thermal epoxy.
Unlike the LED driver chip, this one has pins that are exposed on the side of the package. Gluing the heatsink works surprisingly well with Cree and Seoul Semi LEDs and failing all else you can rub the PCB down to a sliver on some abrasive and glue that to the heatsink. As far as components go, there are only the PSU decoupling caps, the DC blocking at the input and the charge pump. In fact, the more I look at it, the more do-able it looks...

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Old 22nd January 2008, 02:07 PM   #5
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I think the MAX9709 would be a better choice; higher output power and it comes in TQFP. Either IC is probably best suited with an output filter nonetheless.
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Old 22nd January 2008, 04:22 PM   #6
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Default Re: Maxim MAX9704 - anybody built one?

Originally posted by wakibaki

It's a class-D spread spectrun audio amp, with a comparable output, but that's pretty much where the resemblance ends. This runs up to 25V and is filterless, hence not really requiring a PCB
Once you go more than a few centimeters you need an output filter.
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Old 22nd January 2008, 04:44 PM   #7
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I'm glad that you are up to the challenge!
Not me, my eyesight is too weak now.

You may use some copper foil in lieu of wires.

Anyways, when you do it, be sure to post some pictures.
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Old 31st January 2008, 03:00 PM   #8
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I got the chips from Maxim but due to some quirk 2xMax9703 arrived, could be finger trouble on my part. These are the mono flavour.

Anyway these are 5mm on an edge w/32 pins. I'm not going to try to solder direct to the chip. In some ways mono is better, I could mount them direct to the drivers and avoid the issues with 3cm lead lengths... ...although I'm fairly sure I've seen more than 3cm of litz as lead-in wire to the coil on many drivers. The issues seem to be more around EMC rather than sonic.

Click the image to open in full size.

Here's a picture of the chip + package outline with a DIL PIC for scale. The layout and sourcing the components is turning out to be a real pain. The chip itself is dwarfed by even SMT caps. You can put 3 traces the width of one of this thing's pads through the space between a 1206 cap's pads. It's not easy to get 25V working in a 0u1 0603. There's the heatsinking copper on the board, plus trying to maintain some decent width in the signal tracks plus trying to keep the inductance of the cap tracks low and the current loop area small.

I guess I'll make the bottom layer of the board GND, connect the chip's heatsink pad to VDD and floodfill the top with that. If I don't route any tracks on the bottom then I don't have any registration problems. I'll punch a via through to GND as close as I can get it to the chip's ground pins, I don't mess with via-in-pad for homemade boards.

In fact here it is, at 2*lifesize...

It's set for spread spectrum and max gain.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

At most, 4 sq cm of copper on the board for direct heatsinking. The thermal relief round the cap pads is impeding the flow into the rest of the layer. Maybe I'll just turn that off, or block up the mask with a pen.

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Old 4th February 2008, 02:57 PM   #9
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Maxim call these 'speaker amplifiers' presumably because they make the most economic sense when mounted very close to the driver.

Click the image to open in full size.

Some DS PCBs.

These things are tiny. Me and my big mouth! ...and my big banana fingers...

I might try etching a solder paste stencil in very thin shim brass or copper foil.

I made the boards this morning, then I remembered about turning off the thermal relief on the cap pads. Never mind, I'll patch in a bit of copper foil.
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Old 4th February 2008, 07:13 PM   #10
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I would like to ask you if you can post HI-res images because I have a few max9704 amps at home and I would like to build one or two.

Regards Aleš
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