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'powersave during PIC sleep'
1998\03\31@183122 by John R. Sosoka

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I have a PIC controlled circuit that also includes an LM386 audio
amplifier. When I put the PIC into sleep mode, I'd like to eliminate the
power drain of the amp. I can use a relay to do this but I figure that
there is probably some very clean solid state way to accomplish this.
Any ideas?

-John

--
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John R. Sosoka
Neurosmith, LLC
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Suite 206
Long Beach, CA 90803

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1998\03\31@192658 by Bob Blick

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> I have a PIC controlled circuit that also includes an LM386 audio
> amplifier. When I put the PIC into sleep mode, I'd like to eliminate the
> power drain of the amp. I can use a relay to do this but I figure that
> there is probably some very clean solid state way to accomplish this.

Hi John,
Let me rush in here before several other people tell you to replace that
useful inexpensive LM386 chip with some expensive unavailable chip just
because it includes a powerdown pin.

Use a PNP transistor as a switch for the power on the LM386. A base
resistor of 470 ohms will probably work. If the LM386 is running on
something higher than the voltage the PIC is running at, think about using
RA4 as the pin to drive the transistor, before trying something more
complicated, as it is open-drain and will tolerate up to 12 volts.

A PN2907 is not a bad choice for the transistor.

Now everyone else may jump in and tell you to buy a $3 amp chip, or one
that is only available in quantities of 5000 with a 16 week lead time.

Cheers,
Bob

1998\03\31@193323 by ck \The Notes Guy\ Dickinson

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On Tue, 31 Mar 1998 15:10:20 -0800, you wrote:

>I have a PIC controlled circuit that also includes an LM386 audio
>amplifier. When I put the PIC into sleep mode, I'd like to eliminate the
>power drain of the amp. I can use a relay to do this but I figure that
>there is probably some very clean solid state way to accomplish this.
>Any ideas?

John,

Try powering the audio amp through a very low Rds(on) N-channel
MOSFET.  Use a PIC pin to bias the gate high when you want it on, and
make the PIC pin an input and use a 1 Meg resistor to ground to turn
off the MOSFET when you want the amp turned off.

- Rick
   Enterprise ArchiTechs     | Views expressed on topics unrelated
 http://www.eArchiTechs.com  | to messaging are not those of my
NoSpam eMail:.....rtdKILLspamspam@spam@notesguy.com | company, and may not even be mine.

1998\03\31@193738 by Scott Dattalo

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Bob Blick wrote:
>
> > I have a PIC controlled circuit that also includes an LM386 audio
> > amplifier. When I put the PIC into sleep mode, I'd like to eliminate the
> > power drain of the amp. I can use a relay to do this but I figure that
> > there is probably some very clean solid state way to accomplish this.
>
> Hi John,
> Let me rush in here before several other people tell you to replace that
> useful inexpensive LM386 chip with some expensive unavailable chip just

Would that be an LM-Pentium II?


> Use a PNP transistor as a switch for the power on the LM386.
<snip>

Or a Logic Level controlled P-channel MOSFET if the base current
wastes too much power. However, since Bob gave you an extra I/O
pin, you might want to consider using it to power the LM386 -
assuming that you were running the LM386 off of 5 volts...

Scott

1998\03\31@200708 by Mike Keitz

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On Tue, 31 Mar 1998 16:00:25 -0800 Bob Blick <bobspamKILLspamTED.NET> writes:
>> I have a PIC controlled circuit that also includes an LM386 audio
>> amplifier. When I put the PIC into sleep mode, I'd like to eliminate
>the
>> power drain of the amp. I can use a relay to do this but I figure
>that
>> there is probably some very clean solid state way to accomplish
>this.
>
>Hi John,
>Let me rush in here before several other people tell you to replace
>that
>useful inexpensive LM386 chip with some expensive unavailable chip
>just
>because it includes a powerdown pin.

Many chips, including probably the LM386, can be "powered down" by
feeding DC into one of the audio inputs to bias the chip way out of the
normal linear range.  This often reduces the power draw considerably.  An
open-collector PIC pin or a diode can be used to apply the upsetting DC
bias.  Some experiments are in order to see which polarity of bias works
best.

If you decide to cut the power off, it may be easier to switch the ground
(- supply) pin instead with a NPN transistor or N-channel FET.  The AC
grounds should be returned to the + supply in that case.  Depending on
where the audio comes from, it may not be practical, too much noise will
result.

Of course cutting and restoring the power to the chip or applying a DC
bias usually causes the speaker to "pop" or "thump" (depending on how
good the low-frequency response of the speaker is).  The effect can be
minimized by using a small capacitor to couple the speaker and applying
the power or bias slowly.  If the effect is really objectionable, it may
be a reason to go to a newer chip designed to minimize it.  Two LM386's
could be used in a bridge configuration which would offer the ability to
power on and off without sending DC through the speaker.



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1998\03\31@221849 by Hardy e/ou Rafael Pinto

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Hi John


>I have a PIC controlled circuit that also includes an LM386 audio
>amplifier. When I put the PIC into sleep mode, I'd like to eliminate the
>power drain of the amp. I can use a relay to do this but I figure that
>there is probably some very clean solid state way to accomplish this.
>Any ideas?


   Sure!! Use a TIP31 or some other medium to high power bipolar PNP
transistor as a saturated common emmiter. Then tie the base to one of the
PIC's output! Don't forget the biasing! You should not use the divider
configuration; use a single resistor instead. The Vce(sat) commonly is
around 0.4V so the voltage drop will be small. The base current may get a
little high. If it occurs, use a 2N2222 to drive the base (you may use the
Darlington configuration for this).

   The collector will be tied to the Vee pin of the LM386; the emmiter to
the Vdd pin of the PIC. When there is a logical 0 at the base, the collector
will cut the power!


   I'll send some drawings of the  circuit for you... The analisys is quite
straightforward, but if you need help I can explain...



   Rafael Pinto


'powersave during PIC sleep'
1998\04\01@050253 by Russell McMahon
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In one of my products I use a TDA7052 amplifier in
essentially the same way you are describing here. I use a
PNP transistor in the amplifier positive rail supply to turn
it on when required. In low voltage applications you want a
transistor with low saturation voltage and high current gain
(beta). Zetex make some TO92 transistors (and another
similar package) with extremely high current ratings and
high betas at high current for this size package. You can
get Zetex transistors via Farnell - not as cheap as most
anywhere else but available anywhere. The 25 volt Vce ZTX749
has a beta of 100 at 1000ma and a Vsat of 0.5 volt at 2A.
(Farnell 432-970). The 15 volt ZTX788b (Farnell part number
707-790) has a beta of 300 (!) and saturation of 0.45 volts
at 2A

Then again, as others have suggested, you could use a P
channel FET. Beta now irrelevant but watch Rdson. A P
channel FET will be placed in the positive amplifier rail so
the amp is at ground when off. N channel FETS are a little
cheaper and a little more common but will be in the
amplifier ground lead so it floats to + supply when off.
This will not be a problem if you allow for it in your
design.

I have no connection with Farnell or Zetex (except that I
like their products :-) ).

From: Rick "The Notes Guy" Dickinson <.....rtdKILLspamspam.....NOTESGUY.COM>
To: EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Date: Wednesday, 1 April 1998 12:33
Subject: Re: powersave during PIC sleep


On Tue, 31 Mar 1998 15:10:20 -0800, you wrote:

>I have a PIC controlled circuit that also includes an LM386
audio
>amplifier. When I put the PIC into sleep mode, I'd like to
eliminate the
>power drain of the amp. I can use a relay to do this but I
figure that
>there is probably some very clean solid state way to
accomplish this.
>Any ideas?

John,

Try powering the audio amp through a very low Rds(on)
N-channel
MOSFET.  Use a PIC pin to bias the gate high when you want
it on, and
make the PIC pin an input and use a 1 Meg resistor to ground
to turn
off the MOSFET when you want the amp turned off.

- Rick
   Enterprise ArchiTechs     | Views expressed on topics
unrelated
 http://www.eArchiTechs.com  | to messaging are not those
of my
NoSpam eMail:@spam@rtdKILLspamspamnotesguy.com | company, and may not even be
mine.

1998\04\01@061321 by Russell McMahon

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A point to watch is that the supply to the amplifier does
not exceed the voltage tolerance of the PIC. If the PIC and
LM386 run from the same supply there is no problem. However,
if the amplifier is run from a higher supply, when you turn
off the transistor the base lead will float to near the
amplifier supply voltage. The much discussed PIC protection
diodes will then take over and may turn on the transistor
(fully or partially. In this case, use of a resistive
divider in the PIC to base drive cct will keep the PIC pin
below supply.

Even worse, when I did this, the overvoltage caused a cmos
gate to malfunction in an ill defined manner - quite hard to
trace the cause.


{Original Message removed}

1998\04\01@062357 by Hardy e/ou Rafael Pinto

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-----Original Message-----
From: Hardy e/ou Rafael Pinto <KILLspamcppintoKILLspamspamnetrio.com.br>
To: Russell McMahon <RemoveMEapptechTakeThisOuTspamclear.net.nz>
Date: Quarta-feira, 1 de Abril de 1998 08:18
Subject: Re: Re: powersave during PIC sleep


{Quote hidden}

1998\04\01@062607 by Steve Baldwin

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> Many chips, including probably the LM386, can be "powered down" by
> feeding DC into one of the audio inputs to bias the chip way out of the
> normal linear range.

That's a good idea. Even we old farts pick up bits'n'pieces from this
list.

Nobody has mentioned the number one gotcha for powering down bits of
circuits.
Always check that there aren't any inputs with protection diodes or
other internal circuitry that is going to try and power the rest of
the circuit up through a pin. I don't know how many times I've seen
whole boards powered off a single CMOS output when they are
supposed to be in "power down".

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: stevebEraseMEspam.....tla.co.nz      fax +64 9 820-1929
======================================================

1998\04\02@154608 by Martin McCormick

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       One of the pins on the LM386 can be used as a squelch by grounding
it.  The question is whether or not the LM386 draws much current in that
mode.  If I remember correctly, the pin is normally just bypassed to ground
with a .05uF capacitor or sometimes a larger electrolytic cap, but it will
shut the LM386 down if grounded.  In a circuit I built about 10 years ago,
I used an open-collector output on a logic chip to mute the 386.  You might
use a resistor to make the dropping of the pin to ground a little less
abrupt than it would be if one just connects it directly to ground.  I seem
to recall that the thump you get on the LM386 is a little less loud if the
pin goes down gently.  Look for a RC constant of per haps .1 second or so
to soften the thump.  I don't remember how much current the LM386 draws in
this mode, but it was a fraction of normal idling current.  Since the mute
pin is supposed to already be bypassed, you will only need the resistor to
tie it to the open-collector output on your PIC.  Normal audio is with the
output high while a short to ground or a low mutes the 386.

Martin McCormick

1998\04\04@153650 by paulb

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Martin McCormick wrote:

>  One of the pins on the LM386 can be used as a squelch by grounding
> it.  The question is whether or not the LM386 draws much current in
> that mode.  If I remember correctly, the pin is normally just bypassed
> to ground with a .05uF capacitor or sometimes a larger electrolytic
> cap, but it will shut the LM386 down if grounded.

 Indeed, you can short pin 7 to ground and the current draw of the chip
will fall somewhat while the output voltage drops to about 2.5V.  I
think that if the operating voltage is 5 or less, this will shut off the
output drivers and result in a quite low current mode equivalent to a
15K resistor.

 For higher voltages, the burden will be the 15K resistor plus most of
the quiescent current.  It would then be more successful to ground pin 1
(will cause a "plop") and wear a burden of approximately 10K resistance
(1mA at 10V).  The output will float to the supply voltage less 1V so
the output capacitor must be rated accordingly.

 I used this trick on the LM380 some *20* years ago, only a little
differently; you instead pull the bypass pin (1 on the 14-pin package)
to the supply rail which puts a burden of 25K less 1.2V on the supply
but shuts the output drivers down completely and the output voltage
coasts to zero.

 Note that in "bridge" mode for either chip, you can and probably
should common the bypass pins of the two chips as well as or instead of
using a capacitor.

 Cheers,
       Paul B.

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