SMPS for portable audio

[cheesehead]

I'm hoping to shovea tiny SMPS supply and an amplifier inside a re-cased iPod mini, along with a single 2400mAh lithium ion cell, replacing the original 600mAh cell, to power the whole mess. I haven't got much electronics design experience, but the amplifier itself should be pretty simple - I'm just copying a popular DIY design.

The SMPS is another story. The whole purpose of this exercise is to bypass most of the analog stage of the iPod with higher-quality components (yes, there are plans for this too), and a noisy power supply would defeat the point. Furthermore, the SMPS could also cause a lot of noise to the iPod, either causing noise in the output or simply making it malfunction entirely. Does anyone have any recommendations?

 

[bobdole369]

The EE at my place just found a decent VERY small DC-DC converter on an IC. I'll see if I can get any info on it.

Hmmm, SMPS are generally pretty well regulated - they usually (at least the big ones I used to fix) - use a freq on the order of 100khz or higher as the switched pulse. Thus any noise would be well above your hearing range.

 

[PottedMeat]

How are you building this? Custom PCB or perfboard? If it's a custom PCB the PWM controller datasheet ( the most popular ones at least ) should give you some tips - especially how to minimize switching noise. Things like how to design the power traces ( thicknesses / routing around signal traces / shielding ) and component selection ( you can get things like shielded inductors ).

What kind of supply are we talking about? Digikey sells integrated SMPS in very small forms and a variety of voltage outputs like +15/-15@100mA etc. for like ~$10-20

 

[cheesehead]

I'm going to etch myself a PCB - hopefully, the components won't be too tiny, as I suck at SMD soldering. This will be done with toner trasnfer, so the details can't be too small anyway.

I need to run this off of between 3v and 4.1v - a pretty wide range - and need very low noise in a tiny package. I'd be interested in buying one off the shelf, but I can't see it being cheap.

 

[PottedMeat]

Forgot to ask: So you have 3V-4.1V input? Whats the output voltage and maximum current draw?

 

[cheesehead]

Originally posted by: PottedMeat

Forgot to ask: So you have 3V-4.1V input? Whats the output voltage and maximum current draw?

20v, and approximately 140mA (although it should be much, much less.)

 

[QuixoticOne]

http://www.linear.com/pc/viewC....jsp?navId=H0,C1,C1003

Download their free "switchercad" program.. it'll help you select a part and design the circuit.

Maxim-IC also makes relevant parts for your needs.

Look for something with a VERY low standby / OFF / IDLE power consumption, and high 90% range efficiency while operating, and which is fully or mostly integrated, small form factor solution, et. al.
There should be dozens of good options.

http://www.maxim-ic.com/products/power/

 

[bobsmith1492]

The Ipod runs on 20V? That doesn't sound right somehow... could you verify that?

With a switcher you're not going to have any audible noise. Modern switchers run up to 2.5MHz or so - well beyond audible and super-easy to filter out.

If it IS 20V, here's a good chip. You're probably not going to find a module to do that - most are configured to standard voltages like 3.3V, 5V, 24V, etc. - something like this module may be what you're looking for.

I'd recheck the voltage again first, though.

 

[QuixoticOne]

I figured the 20V was due to his custom(?) amplifier or something.
It is unlikely 20V is used for anything besides maybe EL backlight drive (if they even use EL over LEDs...) in any handheld consumer device. Maybe a few volts for FLASH erasing or something..
Most of it probably runs on +5 or +3.3 or so maybe with the audio amplifiers having a bipolar or higher supply.

Anyway it is often very lossy to step down from 20V to +3.3V or whatever so ideally if you do need both voltages you'll get a dual output chip that generates both at once, or you'll parallel switchers one for 20V and one for X.XV.

 

[Modelworks]

This might do what you need depending on the amp you are using. Here is a very cheap dc-dc converter that outputs -19v from 5V. Which actually might be beneficial if you are using some of the higher precision amps.
http://www.allelectronics.com/...-DC-CONVERTER/-/1.html


If it were me though I wouldn't even use anything that required anything over the dc voltages supplied unless there was not other options.

Plenty of opamps that can do that job and with under 5v.

 

[cheesehead]

The point of the SMPS is to turn between 3v and 4.1v into 20v.

 

[Modelworks]

Originally posted by: Cheesehead
The point of the SMPS is to turn between 3v and 4.1v into 20v.

I think everyone understands that, but the question is why 20v ?

 

[cheesehead]

Originally posted by: Modelworks

Originally posted by: Cheesehead
The point of the SMPS is to turn between 3v and 4.1v into 20v.

I think everyone understands that, but the question is why 20v ?

After I run it through a linear regulator, I'll have 18v. After I split it into +/- 9v. The end result leaves me about 8vac from the headphone amplifier to allow myself adequate headroom from the headphone amplifier.

 

[PottedMeat]

Originally posted by: Cheesehead

Originally posted by: Modelworks

Originally posted by: Cheesehead
The point of the SMPS is to turn between 3v and 4.1v into 20v.

I think everyone understands that, but the question is why 20v ?

After I run it through a linear regulator, I'll have 18v. After I split it into +/- 9v. The end result leaves me about 8vac from the headphone amplifier to allow myself adequate headroom from the headphone amplifier.

Thats a lot of extra work and wasted battery power - the whole point of the switcher is to get a wide range of voltages at high efficiency. Why not go with one inverting ( 4V to -9VDC ) and one boost regulator ( 4V to +9VDC ). I'd use something like 2 MC34063's ( whatever they have nowadays at a higher switching frequency for smaller inductors ).

 

[bobsmith1492]

Originally posted by: Cheesehead

Originally posted by: Modelworks

Originally posted by: Cheesehead
The point of the SMPS is to turn between 3v and 4.1v into 20v.

I think everyone understands that, but the question is why 20v ?

After I run it through a linear regulator, I'll have 18v. After I split it into +/- 9v. The end result leaves me about 8vac from the headphone amplifier to allow myself adequate headroom from the headphone amplifier.

Ah, you should have said +/-9V in the first place.

Here's a chip...

That would appear to be your best bet; build it right into your board and set it directly to +/-9V.

 

[cheesehead]

Originally posted by: bobsmith1492


That would appear to be your best bet; build it right into your board and set it directly to +/-9V.

ooooooh!

SWEET!

Looking at the datasheet...

 

[PottedMeat]

Originally posted by: Cheesehead

Originally posted by: bobsmith1492


That would appear to be your best bet; build it right into your board and set it directly to +/-9V.

ooooooh!

SWEET!

Looking at the datasheet...

Small 4 mm x 4 mm QFN-24 Package - Heh I wouldn't want to hand solder that. I can do TSSOPs with a cheap pencil iron and LCC8s, a QFN would take a long time for me.

 

[bobsmith1492]

Originally posted by: PottedMeat

Originally posted by: Cheesehead

Originally posted by: bobsmith1492


That would appear to be your best bet; build it right into your board and set it directly to +/-9V.

ooooooh!

SWEET!

Looking at the datasheet...

Small 4 mm x 4 mm QFN-24 Package - Heh I wouldn't want to hand solder that. I can do TSSOPs with a cheap pencil iron and LCC8s, a QFN would take a long time for me.

Use some flux and it's not too bad.

For the pad on the botton, I'd bring the ground plane through to the bottom of the board on vias and heat it from the bottom. First put a bit of solder on the pad under the chip, then heat from the bottom and press the chip down until the solder melts. Don't use much though.

That being said, if you're etching yourself, can you even do vias? It'll be tough to lay this thing out without them I bet.

 

[Modelworks]

Originally posted by: Cheesehead

Originally posted by: Modelworks

Originally posted by: Cheesehead
The point of the SMPS is to turn between 3v and 4.1v into 20v.

I think everyone understands that, but the question is why 20v ?

After I run it through a linear regulator, I'll have 18v. After I split it into +/- 9v. The end result leaves me about 8vac from the headphone amplifier to allow myself adequate headroom from the headphone amplifier.

So you are going to convert 4v to +/-9v to power which part # ?

 

[cheesehead]

Originally posted by: Modelworks
So you are going to convert 4v to +/-9v to power which part # ?

The headphone amplifier - a lightly modified PIMETA.

http://tangentsoft.net/audio/pimeta/


I'm removing the cascode for efficiency reasons.

 

[Modelworks]

Originally posted by: Cheesehead

Originally posted by: Modelworks
So you are going to convert 4v to +/-9v to power which part # ?

The headphone amplifier - a lightly modified PIMETA.

http://tangentsoft.net/audio/pimeta/


I'm removing the cascode for efficiency reasons.

I would probably swap out the AD8620 on the amp for an AD8647.
Only requires a single supply of under 5.5V.
http://www.analog.com/static/i...s/AD8646_8647_8648.pdf

Or would likely just use a maxim part in the Direct Drive audio series that has just as good a specs as the pimeta device, but in a single chip form that runs directly off 5v.

 

[cheesehead]

Originally posted by: Modelworks

Or would likely just use a maxim part in the Direct Drive audio series that has just as good a specs as the pimeta device, but in a single chip form that runs directly off 5v.

5v is not nearly enough voltage for 250-ohm Beyerdynamic DT880s.

 

[Modelworks]

Originally posted by: Cheesehead

Originally posted by: Modelworks

Or would likely just use a maxim part in the Direct Drive audio series that has just as good a specs as the pimeta device, but in a single chip form that runs directly off 5v.

5v is not nearly enough voltage for 250-ohm Beyerdynamic DT880s.

Sure it is.
The voltage isn't a problem if you pick the right opamp.
The LME49721 for example could make you deaf with those headphones at 5v.

 

[blahblah99]

Originally posted by: Cheesehead
I'm hoping to shovea tiny SMPS supply and an amplifier inside a re-cased iPod mini, along with a single 2400mAh lithium ion cell, replacing the original 600mAh cell, to power the whole mess. I haven't got much electronics design experience, but the amplifier itself should be pretty simple - I'm just copying a popular DIY design.

The SMPS is another story. The whole purpose of this exercise is to bypass most of the analog stage of the iPod with higher-quality components (yes, there are plans for this too), and a noisy power supply would defeat the point. Furthermore, the SMPS could also cause a lot of noise to the iPod, either causing noise in the output or simply making it malfunction entirely. Does anyone have any recommendations?

Since you're pretty limited on space, I would recommend a boost converter to converter your 2-4V power to around 30V to power your opamps. DC bias the opamp to 15V and you'll still have a +-15V swing to power your headphones.

I wouldn't use a dual supply (+15V and -15V) since more supplies = more components = more real estate space since it seems to be a factor.

Now, if space isn't a factor and sound quality is all you're after, then I would redesign the analog output stage including the DAC using higher quality components.

 

[cheesehead]

Originally posted by: blahblah99


Since you're pretty limited on space, I would recommend a boost converter to converter your 2-4V power to around 30V to power your opamps. DC bias the opamp to 15V and you'll still have a +-15V swing to power your headphones.

I wouldn't use a dual supply (+15V and -15V) since more supplies = more components = more real estate space since it seems to be a factor.

Now, if space isn't a factor and sound quality is all you're after, then I would redesign the analog output stage including the DAC using higher quality components.

You are, perhaps, not familiar with the TLE2024 - an IC that gives you a "ground" between a high and low voltage rail. It works really, really well, and can be buffered for even more current if needed.

I cannot possibly understand how 5v can possibly power a loud headphone amplifier. An op-amp running on 5v will produce 3.5v RMS at the output, which will result in less than 1/20 of a watt into a 250-ohm load - nowhere near the 1/2 watt I'm looking for.

Also, the Wolfson DAC on the iPod is pretty good. The purpose of this mod is to bypass most of the stuff after it by connecting the headphone amp directly to the I/V stage of the DAC, which I may end up replacing as well. This is a fully-documented modification.