High output current audio driver

[rickman]

Most op-amps will deliver in excess of 25mA. Typically ~ 30mA. Use a couple of duals or a
quad and connect their outputs together via current sharing resistors. This will deliver
your 100mA.

Graham

Remember that board space issue? It's a killer for sure! That is the
whole reason that I can't use the current approach. The amps are
small, but not microscopic and the passives take up more room that the
amps! If I had to double or quadruple the number of amps I would need
to double the size of the board.

 

[Phil Allison]

"rickman the PRICKMAN"

Remember that board space issue? It's a killer for sure! That is the
whole reason that I can't use the current approach. The amps are
small, but not microscopic and the passives take up more room that the
amps! If I had to double or quadruple the number of amps I would need
to double the size of the board.

** Your absurd PCB space and dissipation requirements are the only issue.

SO YOU FUCKED UP.

Make the PCB bigger.

You FUCKING dickhead !!!!!!!!!!!!



......... Phil

 

[rickman]

"rickman the prickman "

** **** OFF !!!!!!!!!

YOU VILE, ASD FUCKED PIG.


** Nothing at all - since you are vile, autistic pig.

........ Phil

I guess you are trying to show how well you can spell the big words
now. Very good Phil! You get the gold star.

 

[-jg]

Unfortunately, I have not found an audio power amp that will fit all
of my needs. Mostly if they are capable of driving the current and
the voltage I need on the output, they are too large to be useful on
my board.

I may have found a pretty good match to my requirements. Fairchild
FHP3230 will drive 100 mA (typ) and to within 0.35 volts of the rails
with a 150 ohm load. Looking at the graph for output swing for
different loads (at Vcc = 3V) they get to within 0.2 V of the rails
for a 75 ohm load. So I expect that (typ) I can drive a 50 ohm load
to 10 Vpp using a 12 volt supply. It is a very small part and the
short circuit output current is just 120 mA (typ), very close to the
linear output current of 100 mA (typ). I would feel better if they
had max and mins on some of these values and even better if they had a
spice model. Using the lower Vcc, the total power dissipation will be
much lower. I will need to use a small output resistor to improve
stability of the amp, but that shouldn't be a real issue.

I am getting tired of looking and I am running out of vendors. I have
searched TI, ADI, National, Fairchild, Maxim, Microchip and need to
try LT again. I looked at Philips and ST, but their web sites suck so
badly that I gave up trying to figure out if they had anything I could
use.

Any other vendors I should check?

Tightening the current limit seems to be the best approach, so the
Fairchild device looks a good solution.

Because this is only fault protection, you could also add your own
current limiting solution (but that will add area)

Also if your Silicon has thermal shutdoiwn, do not be afraid of
using the resistors to deliberately heat the chip (as so hasten
shutdown, and
lower the resistor sizes).
It sounds like a protection tick-box, so a chip-recovery will
likely be OK.

-jg

 

[-jg]

and now to deflate your balloon: I cannot see thermal shutdown in
that fairchild device, so you'll need to find a device that does offer
that.

Still, the idea was good

-jg

 

[Phil Allison]

"rickman the prickman "


** **** OFF !!!!!!!!!

YOU VILE, ASD FUCKED PIG.

I see ...

** Nothing at all - since you are vile, autistic pig.




......... Phil

 

[David L. Jones]

Remember that board space issue?  It's a killer for sure!  That is the
whole reason that I can't use the current approach.  The amps are
small, but not microscopic and the passives take up more room that the
amps!  If I had to double or quadruple the number of amps I would need
to double the size of the board.

How much height do you have?
Have you thought about using horizontal piggyback boards to give you
effectively twice the surface area? Either one big secondary board, or
multiple boards (e.g. one per channel).
Short height vertical riser boards can often work a treat in this sort
of scenario too.
There is more than one way to skin a cat when it comes to PCB envelope
design.

Dave.

 

[rickman]

"rickman" ...


High resolution? I hope it is not also high accuracy.
A long cable with a 50 Ohm load will have a gain certainly less than one.
And temperature dependent. How are you (is your) customer dealing with that?
You are not doing remote sensing, are you?

"High resolution" is the customer's term for better than voice grade.
I am using a standard 16+ bit codec which can sample at much higher
rates than what is needed for audio. That is what he essentially
asked for. I have been given no details about how the voice grade
mode or the "high resolution" mode will be used. The voice grade mode
I can figure out easily as I have the info on their current design and
voice is pretty standard. It is the 50 ohm mode that I am actually
building for him. But he would like this new card to replace the old
one and pick up the "high resolution" mode as well... if possible.
The issues of driving cables and varying loads aside, this card will
do that. The rest is up to him since I was not given info to work
with.

My only real concern with the Fairchild part meeting the spec for all
of these modes (which can have different build options, btw, so
resistors and such can change) is driving a capacitive load. They say
to use an output resistor to isolate the capacitance. But if I use a
very large one, it will reduce the voltage level. That is why I would
like to simulate it, there are just too many variables to try to get
it right the first time without simulation.

 

[rickman]

How much height do you have?
Have you thought about using horizontal piggyback boards to give you
effectively twice the surface area? Either one big secondary board, or
multiple boards (e.g. one per channel).
Short height vertical riser boards can often work a treat in this sort
of scenario too.
There is more than one way to skin a cat when it comes to PCB envelope
design.

Thanks for the suggestion. But my board is already a daughter card on
a larger board. The customer has an IP network unit with 6U eurocards
as the main boards. He has eight daughter cards on each main board as
the I/O interface. So I am stuck with the size, power supplies and a
lot of other things including the laws of physics. Like some of the
others here, I believe that he is over specing the interface and will
need *either* 10 Vpp or to drive 50 ohm loads, but very unlikely both
together. At this point I am somewhat concerned about the power issue
on the board overall. Some of the posts here got me to take a harder
look at that. So I am leaning toward using a single 12 volt rail.
But that makes it even harder to meet the 10 Vpp requirement into a 50
ohm load.

I will be talking to him in a few days and we will be discussing the
tradeoffs. I just need to know what I can and can't do with available
chips and circuits.

 

[msg]

Phil Allison wrote:

<snip>

Please leave comp.arch.embedded

 

[Jim Stewart]

Phil Allison wrote:

<snip>

Please leave comp.arch.embedded

Crosspost to sci.electronics.design removed.

Agreed. Since Phil seems to find everyone
here so contemptable, perhaps he could find
better company to hang out with.

Just as an aside, comp.arch.embedded is one
of the few places on the internet where I
can find intelligent, on-topic and polite
conversation. sci.electronics.design is
considerably less well-behaved and useful.
I would ask everyone to help maintain the
values here by not crossposting.

 

[James Beck]

Crosspost to sci.electronics.design removed.

Agreed. Since Phil seems to find everyone
here so contemptable, perhaps he could find
better company to hang out with.

Just as an aside, comp.arch.embedded is one
of the few places on the internet where I
can find intelligent, on-topic and polite
conversation. sci.electronics.design is
considerably less well-behaved and useful.
I would ask everyone to help maintain the
values here by not crossposting.

Phil has been in my killfile for so long it is almost humorous.
The first documented case of digital (in both senses) tourettes?
The only time I ever see his ranting anymore is when someone responds to
him.

 

[rickman]

and now to deflate your balloon: I cannot see thermal shutdown in
that fairchild device, so you'll need to find a device that does offer
that.

Still, the idea was good

Thanks for taking a serious interest in this. I'm not sure I need
thermal shutdown in the Fairchild device. It current limits around
120 mA and my circuit is AC coupled. So assuming a max amplitude sine
wave the rms current would be 85 mA and the amp would dissipate 0.44
watts. Assuming the SOIC8 package, at 155 C/W the rise would be 70
C. The part is rated for 150C junction so that puts the max
environment at 80C well above the 70C requirement.

I have also noticed on some parts that you can't rely on the internal
thermal shutdown to fully protect the device. They say that extended
temps above the rated 150 (or sometimes 125) will cause damage to the
package and change the device characteristics.

The more I think about it, the more I like the single 12 volt power
approach. I think that is what I am going to recommend to my
customer.

Anyone have experience with Fairchild support? So far the email
support seems to take a couple of days and the answers seem rather
canned.

 

[Jan Panteltje]

"High resolution" is the customer's term for better than voice grade.
I am using a standard 16+ bit codec which can sample at much higher
rates than what is needed for audio. That is what he essentially
asked for. I have been given no details about how the voice grade
mode or the "high resolution" mode will be used. The voice grade mode
I can figure out easily as I have the info on their current design and
voice is pretty standard. It is the 50 ohm mode that I am actually
building for him. But he would like this new card to replace the old
one and pick up the "high resolution" mode as well... if possible.
The issues of driving cables and varying loads aside, this card will
do that. The rest is up to him since I was not given info to work
with.

My only real concern with the Fairchild part meeting the spec for all
of these modes (which can have different build options, btw, so
resistors and such can change) is driving a capacitive load. They say
to use an output resistor to isolate the capacitance. But if I use a
very large one, it will reduce the voltage level. That is why I would
like to simulate it, there are just too many variables to try to get
it right the first time without simulation.

My personal experience with 'audio' is, that they drive the cable hard.
I once did a 200 Ohm (think it was) audio distribution amp, and had
200 Ohm resistors.
They did not terminate the other end, so the levels were 2x too high.
So better have no resistors, and then that requires a short circuit proof
amp, and some way to dissipate the heat.
Maybe you can protect things by foldback current limit in the supply.

 

[David L. Jones]

Thanks for the suggestion.  But my board is already a daughter card on
a larger board.  The customer has an IP network unit with 6U eurocards
as the main boards.  He has eight daughter cards on each main board as
the I/O interface.  So I am stuck with the size, power supplies and a
lot of other things including the laws of physics.  

Sure, but you will always have a complete 3D envelope available, so
that does not preclude the use of piggyback or riser boards on your
daughterboard if you can do it.
This is precisely one of those times when you should get creative with
your 3D envelope design.
Remember, pigyback and riser cards can even be 0.8mm or 0.5mm boards
if you need to shave an extra 1mm here or there.

Like some of the
others here, I believe that he is over specing the interface and will
need *either* 10 Vpp or to drive 50 ohm loads, but very unlikely both
together.  At this point I am somewhat concerned about the power issue
on the board overall.  Some of the posts here got me to take a harder
look at that.  So I am leaning toward using a single 12 volt rail.
But that makes it even harder to meet the 10 Vpp requirement into a 50
ohm load.

I will be talking to him in a few days and we will be discussing the
tradeoffs.  I just need to know what I can and can't do with available
chips and circuits.

Make sure you get a full 3D envelope spec from him too, not just a
basic maximum height.

Dave.

 

[bg]

rickman wrote in message

You mean to act like a PTC fuse? Interesting idea. It would have to
be sized very carefully to do much good in this application. Most
current limiting devices like this have a fairly wide range between
the operational current and the trip current. I don't know any
details on miniature lamps in that regard, but I expect it would be
tough to fit into a tight spec.

I saw an ad for a Poly something brand of polymer PTC fuse. But it
really isn't a good match. The amps will actually protect themselves,
if you can find one that limits the current to something remotely like
the max operational current. Not much point to protecting the amp if
the rest of the circuit burns up.

I have a lamp here that is used in an automobile radio. It measures 1/8 inch
diameter by 1/4" long. Cold, it measures 38 ohms and at 12volts draws 40ma.
So thats almost 5 watts in a package much smaller that any 5 watt resistor I
know of. I think you would have to do some cut and try engineering for lack
of a spice model, or generate your own.

 

[Eeyore]

Remember that board space issue? It's a killer for sure!

If you can't fit an SMT quad op-amp package and 4 resistors you're TOTALLY screwed anyway ! I
can't imagine ANY other solution will take less space.

Just how much space is there ?

Graham

 

[-jg]

My only real concern with the Fairchild part meeting the spec for all
of these modes (which can have different build options, btw, so
resistors and such can change) is driving a capacitive load. They say
to use an output resistor to isolate the capacitance. But if I use a
very large one, it will reduce the voltage level. That is why I would
like to simulate it, there are just too many variables to try to get
it right the first time without simulation.

Most amps need only relatively small series R to stabilise against
heavy C loads.
It's easy to test on the bench, so I'd grab some of the likely opamps
and breadboard them up, and test them with nasty loads.

Since the fairchild Opamp has no thermal sense, and you are
tightly area/power constrained, I'd look at adding a temperature trip
- devices like AnalogDevices TMP35 are cheap, and small, and
could sense the output area, and so reduce the PCB area for
ressitors / Opamp and copper cooling

-jg

 

[John Devereux]

Crosspost to sci.electronics.design removed.

Agreed. Since Phil seems to find everyone
here so contemptable, perhaps he could find
better company to hang out with.

Just as an aside, comp.arch.embedded is one
of the few places on the internet where I
can find intelligent, on-topic and polite
conversation. sci.electronics.design is
considerably less well-behaved and useful.
I would ask everyone to help maintain the
values here by not crossposting.

I find them about equally useful. Of course in SED there is 10 times
the noise. But if you can just ignore the insults and flamewars there
are some world class experts there. There is a lot of good advice and
ideas to be had - even from Phil, actually!

 

[Peter Bennett]

I have a lamp here that is used in an automobile radio. It measures 1/8 inch
diameter by 1/4" long. Cold, it measures 38 ohms and at 12volts draws 40ma.
So thats almost 5 watts in a package much smaller that any 5 watt resistor I
know of. I think you would have to do some cut and try engineering for lack
of a spice model, or generate your own.

You should re-check that calculation - my calculator says the lamp
only draws 0.5 watts, not 5 watts.