High output current audio driver

[John Devereux]

I have not thought this through... but how about a LM317 or similar
configured as a 100mA constant current source, supplying an
unprotected rail-to-rail output opamp. E.g. AD8531.

(Sorry, not AD8531, you need a 12V capable part. I expect you already
found some.)

 

[Jan Panteltje]

Yes, I understand that. I am using the best heat sinking I can in a
limited design such as this. The parts have a thermal pad and I am
using vias to connect to one of three power planes underneath (four
counting the copper pad on the opposite side). If push comes to
shove, I can layout the board to put the active parts on the top and
use actual heat sinks as will fit in the height provided. But it will
really be up to the customer to provide adequate cooling air.
Besides, the heat generated on board in normal operation is not so bad
at about 0.6 watts total in normal operation (per channel). I think
that is workable and higher heat with a shorted output should be
survivable; I calculate a max junction temp rise of 20C over ambient
which is entirely acceptable. Of course, that depends on the heat
getting off of the board at some point which is the customer's design
problem.

Most, if not all, audio power amps use foldback current limiting, in the output
stage.
This limits dissipation, and current in case of short or overload.
In case of the push pull complementary emittor follower there are 2 sense resistors
in the emittor, the voltage is sensed by 2 transistors that clamp down the base.
There is a resistor bridge to reference ground, in such a way that for
lower output volatges the current limit is lower.
So in case of a shorted output the output voltage is zero, and the current very
limited.
I have done this discrete, but I am almost sure there are audio power amps of
lower wattage with this feature that you could use.

 

[rickman]

--



The opamps I have found that will drive this line (100 mA) have a very
high output short current, typically much higher than this level. The
largest output resistor I can use is 50 ohms when driving a 50 ohm
load to 10 Vpp with these supply levels. This limits the output
current to 200 mA.

What is wrong with capacitively coupling the output so that you don't have
to worry about dc dissipation?

Thanks for posting.

I am capacitively coupling the outputs and I'm not worried about DC
dissipation. I am worried about AC dissipation into a short. Even
then I am only worried about the dissipation in the power resistors
which have to be physically large to dissipate that much power. The
amps typically protect themselves.

 

[rickman]

I have not thought this through... but how about a LM317 or similar
configured as a 100mA constant current source, supplying an
unprotected rail-to-rail output opamp. E.g. AD8531. Opamp is single
supply, feed the LM317 from 12V. If there is a short circuit, the
current limits and the opamp supply is reduced. The LM317 can heat up
and eventually will go into thermal shutdown depending on heat
sinking.

The point is to do the current limit externally, which means you don't
have use a higher supply voltage than needed to drive the load -
although perhaps there is a better way to do it.

Interesting idea, but I don't think I would have the board space.
The rails are + and - 12 volts, so I would have to have a current
limiter for each voltage on each amp which would be eight limiters!

 

[John Devereux]

Thanks for posting.

I am capacitively coupling the outputs and I'm not worried about DC
dissipation. I am worried about AC dissipation into a short. Even
then I am only worried about the dissipation in the power resistors
which have to be physically large to dissipate that much power. The
amps typically protect themselves.

Could you use PTC resistors / polyswitches?

Or use a good groundplane and thermal vias to distribute the heat evenly,
with an overall thermal shutdown. If the amp has it's own safe thermal
shutdown then this might be sufficient by itself.

 

[Phil Allison]

"rickman the prickman "


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

YOU VILE, ASD FUCKED PIG.

I see ...

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




......... Phil

 

[Phil Allison]

"Jan Panteltje"

Most, if not all, audio power amps use foldback current limiting, in the
output
stage.

** Not all, many amp designs use simple current limiting - eg ones that
employ lateral mosfets.

BTW: The term used in relation to audio amplifiers is " VI" limiting.

This limits dissipation, and current in case of short or overload.

** Must be very carefully designed not to cause bad side effects, though.

In case of the push pull complementary emittor follower there are 2 sense
resistors
in the emittor, the voltage is sensed by 2 transistors that clamp down the
base.
There is a resistor bridge to reference ground, in such a way that for
lower output volatges the current limit is lower.
So in case of a shorted output the output voltage is zero, and the current
very
limited.

** The problem is that audio amps are not driving resistors but reactive
loads called *loudspeakers* - hence they must have the ability to
deliver fairly high currents when the output voltage is near zero.

I have done this discrete, but I am almost sure there are audio power amps
of
lower wattage with this feature that you could use.

** None that comply with the prickman's idiot ideas.

Maybe you need to read this:

http://sound.westhost.com/vi.htm



........ Phil

 

[CBFalconer]

"Bob Eld"
.... snip ...

** If the run really is 1000s of metres long, doing that will
not give audio bandwidth with readily available twisted pair
shielded cable.

Delivering audio power down a 50 ohm terminated cable is not a
reasonable use. You are better off delivering a raw signal,
properly equalized for the cable properties, to a remote amplifier.

 

[CBFalconer]

[email protected] says...

Nonsense. The impedance of a "50-ohm cable" may not matter at
audio frequencies but it will still still have a characteristic
impedance of 50-ohms. Telephones don't use "50-ohm cable" which,
of course, cannot be why they use 600-ohms. <boggle>

You have obviously never looked at real lines. Telephones use
twisted pairs, which turn out to be very close to 100 ohm lines,
and quite flat for frequencies in the megacycle range. Down at
audio (say 2000 hz) they are quite close to 600 ohm lines. The
shape of the attenuation vs frequency curve will vary seriously
with the line quality. Also delay.

You can avoid those problems by doing such games as modulating 10
Mhz signals with the audio, and demodulating at the other end.
Such games also allow you to pack many signals onto the one line.

 

[CBFalconer]

"rickman"

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

YOU VILE, ASD FUCKED PIG.

Once we can ignore. But this continuous bad language is just not
acceptable. PLONK.

 

[Eeyore]

The opamps I have found that will drive this line (100 mA) have a very
high output short current, typically much higher than this level. The
largest output resistor I can use is 50 ohms when driving a 50 ohm
load to 10 Vpp with these supply levels. This limits the output
current to 200 mA.

What is wrong with capacitively coupling the output so that you don't have
to worry about dc dissipation?

What's *DC* dissipation got to do with it ?

Graham

 

[Eeyore]

Eeyore a écrit :


For a given output capability it allows using lower resistance so lower
dissipation in that resistance.
Also lower voltage drop in that lower resistance means less supply
voltage needed, hence lower output stage dissipation for the same
driving capability.

Can't be more basic.

OK, I see what you're driving at. It seems the OP has +/- 12V rails though and can't take
advantage of the potentially lower supply voltage with this method. The dissipation will
still be determined by the load current whether he uses real or simulated output Z.

Graham

 

[Eeyore]

Most, if not all, audio power amps use foldback current limiting, in the output
stage.

Not exclusively true and usually only because of secondary breadown issues. Not an
issue with 12V supplies.

Graham

 

[bg]

rickman wrote in message ...

Thanks for posting.

I am capacitively coupling the outputs and I'm not worried about DC
dissipation. I am worried about AC dissipation into a short. Even
then I am only worried about the dissipation in the power resistors
which have to be physically large to dissipate that much power. The
amps typically protect themselves.

You might consider a miniature lamp in series with the line?

 

[YD]

Late at night, by candle light, [email protected] (Don Klipstein)
penned this immortal opus:

So far, all I know is "50 ohms", "audio", and 10 Vpp.

Sounds to me like something the LM386 does well with a 12-15 volt or
so supply. If you need a somewhat lower supply voltage, then use two
LM386's, and drive their inputs out of phase with each other - should work
at 7-8 volts supply voltage but also higher supply voltage such as 12
volts.

This is tolerant of load impedance going significantly from 50 ohms in
either direction, especially upwards.

- Don Klipstein ([email protected])

A TDA2002 or whatever is the modern replacement should do quite
nicely. As other have stated, at audio the line impedance is of little
consequence.

- YD.

 

[rickman]

Most, if not all, audio power amps use foldback current limiting, in the output
stage.
This limits dissipation, and current in case of short or overload.
In case of the push pull complementary emittor follower there are 2 sense resistors
in the emittor, the voltage is sensed by 2 transistors that clamp down the base.
There is a resistor bridge to reference ground, in such a way that for
lower output volatges the current limit is lower.
So in case of a shorted output the output voltage is zero, and the current very
limited.
I have done this discrete, but I am almost sure there are audio power amps of
lower wattage with this feature that you could use.

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?

 

[rickman]

rickman wrote in message ...



You might consider a miniature lamp in series with the line?

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.

 

[Eeyore]

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.

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

 

[rickman]

Could you use PTC resistors / polyswitches?

Or use a good groundplane and thermal vias to distribute the heat evenly,
with an overall thermal shutdown. If the amp has it's own safe thermal
shutdown then this might be sufficient by itself.

Yes, I thought of that. But the real problem is that the amp is rated
for a higher temp than the resistors are. I would need to pull up the
data sheet to make sure, but I think the resistors are only rated for
125C while the junction temp of the amps are mostly 150 and the
shutdown is even higher.

I made a post further down in the thread talking about an amp I found
that is pretty close to being a perfect match to what I need. It
should let me run off of just one power rail cutting my waste heat by
more than half. I feel pretty good about that approach, but the amp
only meets my requirements if I extrapolate the rating points and use
typical data rather than guaranteed data. But at this point I am
willing to shave a little off of my requirements. Heck, they are all
informal and as most people are saying, they are likely not the real
requirements of the end application, just the marketing requirements
that "sound" good. I expect that I can meet the 10 Vpp and the 50 ohm
driving requirements separately as long as the design will come close
to providing them both at the same time. It is just too durn bad that
Fairchild doesn't have a spice model for this part. There were a
number of ADI parts that would have worked with the original approach,
but I didn't feel comfortable using them without a simulation. Since
they have no spice models, I threw 'em back.

The Fairchild parts are nice and small and cheap too!

 

[rickman]

OK, I see what you're driving at. It seems the OP has +/- 12V rails though and can't take
advantage of the potentially lower supply voltage with this method. The dissipation will
still be determined by the load current whether he uses real or simulated output Z.

Yes, it won't reduce the total dissipation, but it will let me use
smaller resistors. But, as I have written in a couple of posts
further down in this thread, I think I found a part that will let me
meet the spec (or come very close to it) running from a single 12 volt
rail. I just would feel a lot better about it if I could simulate
it. Funny, Fairchild tells you that they have models for some of
their amps and when I checked it was just the LM324 and LM324A, that's
IT!!!