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Class D Switching Power Amplifiers and Power D/A conversion

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Old 4th January 2012, 08:29 PM   #21
gmarsh is offline gmarsh  Canada
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Damnit, now that I've thought about the thermal idea some more, I'm tempted to replace the ADAU1592 parts with a TAS5414A mounted on the bottom of the PCB. And with the ADAU1592 QFN's being a bitch to solder... hmm.

This project's never getting done at this rate
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Old 4th January 2012, 08:57 PM   #22
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Wouldn't a USB connection presence sensing circuit be really easy to make and would be extremely low powered. It's really just sensing whether there's termination between + and GND, and if so turns on the higher amp circuit to provide, and as soon as connection is lost, powers down again. 5V at about 5-10mA should suffice.

As long as muted power consumption is held beneath the battery self discharge there's really no need to go further as up to that level is basically free anyways. I mean, the self discharge rate will discharge the batteries regardless of the power up to that rate is being used by the circuit or not.

Last edited by Saturnus; 4th January 2012 at 09:03 PM.
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Old 4th January 2012, 10:48 PM   #23
gmarsh is offline gmarsh  Canada
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From my understanding of SLAs, you don't get "free" current out of them up to the self discharge current - I'd love to be corrected on this though as it'll make the design a lot easier.
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Old 4th January 2012, 11:01 PM   #24
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Self-discharge is unwanted chemical reactions in the battery happening even when current is supposedly not flowing. From my experience with both SLAs and LiFePO4 batteries both will have a "free current" point up to about full (for SLAs) and 2/3rd (for LFPs). It's easily tested though. Just take 2 identical batteries, charge them to the same voltage, and put a low power circuit on one, being lower than the discharge rate, and the other will just reamin unconnected. After a month, measure both batteries. For a 12V 7Ah self discharge is about 15mA, so about the same as a low power single LED.
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Old 5th January 2012, 04:28 PM   #25
gmarsh is offline gmarsh  Canada
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I dunno about 15mA... 7Ah/.015A = 466 hours = 19 days. The "general google concensus" is that 40%-50% self discharge happens in a year... going with the 50% number (3.5aH) that's 3.5Ah/(365*24h) = 400uA.

Granted, self discharge isn't constant. I imagine it varies with battery voltage, number of cycles on the battery, temperature, etc.

I've been killing myself trying to make my card do 100uA standby, but I'll probably move that up to 500uA or even 1mA.
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Old 5th January 2012, 05:18 PM   #26
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You might be right. I've only tested it with freshly recharged batteries and only for a few days. Maybe self-discharge is hyperbolic instead of linear. Or maybe it's because I've only tested with batteries in parallel which could also affect the result.

Either way, aiming for less than 1mA is pretty pointless as that would mean 291 days of standby. Any battery should be recharged at least every 3-4 months, or 3-4 times a year to avoid damage anyways.
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Old 5th January 2012, 05:25 PM   #27
Atilla is offline Atilla  Norway
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Oh, so far it sounds like a really good idea. Now that summer is coming (in diy timekeeping) I'm really interested in portable summertime audio again
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Old 5th January 2012, 06:29 PM   #28
gmarsh is offline gmarsh  Canada
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Using LT3680 for the +5V output, running all the time, estimated power consumption is 334uA. If the micro senses the battery voltage is getting low, it will shut that down and power consumption drops to 260uA. Most of that is the ADAU1592's (82uA each) and the LT3652HV solar charger (85uA). Think I'll go with this method... only things on the user interface will be a pair of USB jacks for charging, an audio input using jack switching for the enable, and perhaps a bicolor blinkenled which blinks with the battery state.

If I go with TAS5414A, shutdown consumption is around 100uA. I'm really debating this change since it means not having to solder QFNs - the next worst parts on the board are exposed pad MSOPs and they're not all that bad to do. Unfortunately the TAS pulls 240mA idle, 160mA with two channels active instead of four... while the two ADAUs pull 44mA/pop idle.

ADAUs are nice because they've got decent current draw, fast mute/unmute (planning on using this if the audio's connected but not playing, to save a bit of power), two of them won't beat together since they're self-oscillating, etc. Only real issue with them is the package.

And I just realized they're going obsolete. Dangit.
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Old 13th January 2012, 03:10 PM   #29
gmarsh is offline gmarsh  Canada
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99% done on the PCB now - just have to hook up the microcontroller I/O and fix a couple of thermals. It's a ridiculously tight fit, but I managed to keep a decent layout.

PCB dimensions are 6x10cm (fits in a Hammond 1590N, barely) with 12-pin 3.5mm terminal blocks on each short side and four mounting holes in each corner. It's a 2 layer board, compatible with the free Eagle tools.

I've kept two ADAU1592s for the audio output, despite their obsolescence and annoying package. No other chip was jumping out at me with the same performance, operating/shutdown current, etc.

I'm also making a slight variation on the board with one terminal block replaced with "user interface" connectors (dual USB jack, audio connector, bicolor LED) which I'll be using myself. Hoping I can keep this within 6x10cm also to allow use with free Eagle, I haven't laid the board out yet with these changes.

I'll post the design files on here when it's done, most likely later tonight. This will be under a permissive license, probably Creative Commons, letting anyone change the design for their own purposes.
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Old 13th January 2012, 03:26 PM   #30
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Sounds really great. And I really can't thank you enough for doing this. My skills at electronics design are unfortunately woefully inadequate for such a task. Loudspeaker design is another matter entirely

I will naturally try and see if I can't make one if my soldering skills are up to the task and use it for the signature Boominator version I'm building for this years festival season.

I also very much appreciate you putting all this effort into fitting it into the Hammond case although I know it must have been a herculean task. But it would be a shame if people who have already built it and future builders who are building it after the drawings available would be excluded from using it.
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