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'[EE]:Turn off 3 pin regulator?'
2002\01\31@132451 by Ian Hynes

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PICers,
That thread about battery backup reminded me of something ...

I have a 5V PIC cct and an 8v driver cct. (Coupling is by 4N25
optocouplers). To save power I'd want to switch off the 8V regulator,
say a 7808 in this case.

OK, Let's say I put an NPN transistor (say BD139) in the earth leg of
the 3pin 7808 & switch it on/off with a 4N25. No worries, that kills
the 7812 ... BUT the o/p voltage then floats to supply, 12V in this
case.

So what you need is some simple way to switch off the regulator and
have its o/p volts go to zero when it's OFF = when the npn tarnsistor
is OFF.

Any ideas?

Thanx - Ian


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2002\01\31@134133 by David VanHorn

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>
>So what you need is some simple way to switch off the regulator and have
>its o/p volts go to zero when it's OFF = when the npn tarnsistor is OFF.

P Channel mosfet on the output, or use a reg with a shutdown pin.

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2002\01\31@134348 by Spehro Pefhany

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At 04:22 AM 2/1/02 +1000, you wrote:

>So what you need is some simple way to switch off the regulator and
>have its o/p volts go to zero when it's OFF = when the npn tarnsistor
>is OFF.

A P-channel logic MOSFET, a resistor, and an open-collector driver
(2N4401 + a resistor), in series with the input of the regulator.
+a zener and another resistor if the 12V might have transients that
could kill the 20V MOSFET gate. Or use a PNP 'tarnsistor' instead of
the MOSFET.

But, really, it would be better to just use an LM2941 (in TO-220-5
or TO-263-5) that has shutdown built-in, is LDO and costs about 85
cents in 100's (adjustable output voltage).

Best regards,

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2002\01\31@141932 by Ian Hynes

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Original message from: Spehro Pefhany <EraseMEspeffspam_OUTspamTakeThisOuTINTERLOG.COM>
>
>But, really, it would be better to just use an LM2941 (in TO-220-5
>or TO-263-5) that has shutdown built-in, is LDO and costs about 85
>cents in 100's (adjustable output voltage).
>

Thanks guys ... that sounds like thge way to go.
Ian


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2002\01\31@145244 by Don Hyde

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I've done this sort of thing several times.

Use that strange "open drain" PIC output -- RA4, I think.  Hook it to the
base of a PNP transistor via a resistor (10K or so).  Hook emitter to +8,
collector to the + input of the regulator.

When your PIC wakes up, the output is tri-state, so no current flows thru
the transistor.  Program the pin to output and set it low, then the PNP
transistor turns on, with pretty low drop of Vcesat, so you only lose a
couple hundred millivolts.

Switch the output high, the transistor turns back off, and the regulator
output goes to zero.   Remember that the regulator will not switch on and
off instantly, since it needs a cap on its output to be stable, and that cap
will take at least a few microseconds to charge and discharge, and maybe
quite a few milliseconds.

Depending on the circuits it's feeding, it may take a very long time for the
output to go to zero.  On one design turning a transmitter on and off, it
was necessary to add another transistor to short the output of the regulator
to ground when it was supposed to be off, so that the transmitter's
oscillator would really stop and not interfere with a nearby receiver.

> {Original Message removed}

2002\01\31@182537 by Ian Hynes

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Original message from: Don Hyde <@spam@DonHKILLspamspamAXONN.COM>
>
>I've done this sort of thing several times.
>..... Remember that the regulator will not switch on and
>off instantly, since it needs a cap on its output to be stable, and
that cap
>will take at least a few microseconds to charge and discharge, and
maybe
>quite a few milliseconds.

Hey this sounds like a good idea. will try it. Decay time won't be a
problem - the drain is around 600mA

Thanx - Ian


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2002\01\31@183851 by Russell McMahon

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> >So what you need is some simple way to switch off the regulator and
> >have its o/p volts go to zero when it's OFF = when the npn tarnsistor
> >is OFF.

> But, really, it would be better to just use an LM2941 (in TO-220-5
> or TO-263-5) that has shutdown built-in, is LDO and costs about 85
> cents in 100's (adjustable output voltage).


The LM2941 has a regrettably high quiescent current in shut down mode.
The LM2936 cannot be shut down but has a much lower quiescent current  -
around 10uA in practice AFAIR - still too high in many cases.


       RM

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2002\01\31@192600 by Spehro Pefhany

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At 08:49 AM 2/1/02 +1300, you wrote:
>
>The LM2941 has a regrettably high quiescent current in shut down mode.

I don't see it specified. It isn't the horrible regular Iq is it? 8-(
It's a lateral-PNP type that draws a LOT of current when Vin-Vout gets
small and at high output currents.

>The LM2936 cannot be shut down but has a much lower quiescent current  -
>around 10uA in practice AFAIR - still too high in many cases.

More expensive, but there is the LP3964 which uses an internal MOSFET
pass transistor. 15uA in shutdown., but  doesn't have the automotive
resistant specs of the LM2941. Too high a shutdown current to eliminate
the on/off switch in many applications, but a heck of a lot better than
the few-mA Iq of a 78xx.

Best regards,
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\01\31@220311 by Ian Hynes

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Original message from: Spehro Pefhany <RemoveMEspeffspamTakeThisOuTINTERLOG.COM>
>
>At 08:49 AM 2/1/02 +1300, you wrote:
>>
>>The LM2941 has a regrettably high quiescent current in shut down
mode.
>
>>More expensive, but there is the LP3964 which uses an internal
MOSFET
>pass transistor. 15uA in shutdown., but  doesn't have the automotive
>resistant specs of the LM2941. Too high a shutdown current to
eliminate
>the on/off switch in many applications, but a heck of a lot better
than
>the few-mA Iq of a 78xx.

Hey guys - I just tried that configuration with the PNP transistor in
the i/p of the 3T regulator. Works like a charm! (transistor base low,
o/p = 8V, base high, o/p= 0V)
Uh .. didn't measure total Iq. Better do that ...   :)

Regards - Ian


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'[EE]:Turn off 3 pin regulator?'
2002\02\01@013900 by Russell McMahon
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> >The LM2941 has a regrettably high quiescent current in shut down mode.
>
> I don't see it specified. It isn't the horrible regular Iq is it? 8-(
> It's a lateral-PNP type that draws a LOT of current when Vin-Vout gets
> small and at high output currents.

The datasheet in the 1993 NatSemi Power IC's databook that I have has a
diagram at the very end showing a pullup resistor on the on/off enable line
(low = on, high = off).
A note says "To assure shutdown, select resistor R3 [the pullup] to
guarantee at least 300 uA of pullup current when S1 is open. Assume 2v at
the on/off pin"

The data sheet proper has a value for On/Off threshold current of 50 uA
typical and 00 uA max at Von/off = 2v and Iout  <= 1 amp. (Clearly Iout =
0ish when it's off but this is preeumably the curremt one is switching off
with this Ion/off current. )

So the answer is

   - at least 300 uA (appnote)
   - typically 50 uA (data sheet)
   - worst case         (data sheet)

Choose one :-)

This is a MUCH worse current drain than the LM2936's always on quiesecent
current of 9 uA tyoical, 15 uA max (at load of 100 uA - lower presumably at
no load current.

Turning the supply off with a FET or even a PNP transistor you should be
able to get off state quiescent current down to leakage levels - certainly
far far far below 1 uA. Cost is 2 transistors max (1 NPN, 1 PNP and 2
resistors)(1 transistor if you use the PIC RA4 pin trick to drive the PNP).



           Russell McMahon

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2002\02\01@032701 by Vasile Surducan

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On Thu, 31 Jan 2002, David VanHorn wrote:

> >
> >So what you need is some simple way to switch off the regulator and have
> >its o/p volts go to zero when it's OFF = when the npn tarnsistor is OFF.
>
> P Channel mosfet on the output, or use a reg with a shutdown pin.
 like LM317 for positive voltage, or a simple npn in series with the
output reg.

 Vasile

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2002\02\01@045146 by Russell McMahon

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That should have said



> > >The LM2941 has a regrettably high quiescent current in shut down mode.

>     - at least 300 uA (appnote)
>     - typically 50 uA (data sheet)
>     - worst case 100 uA (data sheet)    <====================== changed
>
> Choose one :-)

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2002\02\01@101045 by Roman Black

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> > OK, Let's say I put an NPN transistor (say BD139) in the earth leg of
> > the 3pin 7808 & switch it on/off with a 4N25. No worries, that kills
> > the 7812 ... BUT the o/p voltage then floats to supply, 12V in this
> > case.
> >
> > So what you need is some simple way to switch off the regulator and
> > have its o/p volts go to zero when it's OFF = when the npn tarnsistor
> > is OFF.
> >
> > Any ideas?


Hi, there's a circuit here:
centauri.ezy.net.au/~fastvid/self_swi.htm
that might do what you want. :o)
-Roman

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2002\02\01@230047 by Ian Hynes

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Original message from: Roman Black <EraseMEfastvidspamEZY.NET.AU>
>
>
>Hi, there's a circuit here:
>centauri.ezy.net.au/~fastvid/self_swi.htm
>that might do what you want. :o)
>-Roman
>
Yeah that cct looks cool. If the PIC did draw less than 5mA, though,
you might want a resistor from 12V rail down to the SCR's anode to
keep its  current >Ido when it's in the on state? Just a thought ...

Ian


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2002\02\02@060829 by Roman Black

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Ian Hynes wrote:
>
> Original message from: Roman Black <RemoveMEfastvidEraseMEspamEraseMEEZY.NET.AU>
> >
> >
> >Hi, there's a circuit here:
> >centauri.ezy.net.au/~fastvid/self_swi.htm
> >that might do what you want. :o)
> >-Roman
> >
> Yeah that cct looks cool. If the PIC did draw less than 5mA, though,
> you might want a resistor from 12V rail down to the SCR's anode to
> keep its  current >Ido when it's in the on state? Just a thought ...
>
> Ian

Actually it's better to ensure the PIC (and
other 5v stuff) draw more than 5mA when you
choose, allowing the PIC to turn everything off
by reducing the current below Ih.
One interesting method is to put a 330 ohm
resistor from a PIC output pin to gnd, when
the PIC sets the pin high it will draw about
10mA. Turning the pin low will drop the 10mA
and may alone be enough to break Ih and turn
the entire circuit off.
Also, a decoupling cap may be needed across
the A and K of the scr to avoid breaking hold
with rapid current changes. :o)
-Roman

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2002\02\02@144556 by Ian Hynes

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Original message from: Roman Black <RemoveMEfastvidspam_OUTspamKILLspamEZY.NET.AU>
>
>>Actually it's better to ensure the PIC (and
>other 5v stuff) draw more than 5mA when you
>choose, allowing the PIC to turn everything off
>by reducing the current below Ih.
>One interesting method is to put a 330 ohm
>resistor from a PIC output pin to gnd, when
>the PIC sets the pin high it will draw about
>10mA. Turning the pin low will drop the 10mA
>and may alone be enough to break Ih and turn
>the entire circuit off.
>Also, a decoupling cap may be needed across
>the A and K of the scr to avoid breaking hold
>with rapid current changes. :o)
>-Roman

Hmm .. all very true.
Getting back to the transistor turn on/off method, I had a diode from
the 12V battery to the regulator i/p to protect against polarity
reversal. With the emitter of the transistor seeing the high side &
collector to regulator i/p, I guess you could dispense with the
protecting diode?

But a transistor's reverse Vbe doesn't want to be much more than 5V so
should I keep the diode? With diode forward volts of, maybe 0.5V &
VceSAT of maybe 0.4V, supply=12 (nominal) & regulator o/p=8V you're
cutting it fine with the regulator's dropout level. To keep the diode
or not to keep it ... that is the question    :o

-Ian


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2002\02\02@184617 by Roman Black

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Ian Hynes wrote:
{Quote hidden}

I can't remember the circuit Ian, can you re-post
it?
-Roman

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2002\02\03@211324 by Ian Hynes

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part 1 1224 bytes content-type:text/plain; charset="us-ascii" (decoded 7bit)

Original message from: Roman Black <EraseMEfastvidspamspamspamBeGoneEZY.NET.AU>
>
>Ian Hynes wrote:
>>
>>
>> Getting back to the transistor turn on/off method, I had a diode
from
>> the 12V battery to the regulator i/p to protect against polarity
>> reversal. With the emitter of the transistor seeing the high side &
>> collector to regulator i/p, I guess you could dispense with the
>> protecting diode?
>>
>> But a transistor's reverse Vbe doesn't want to be much more than 5V
so
>> should I keep the diode? With diode forward volts of, maybe 0.5V &
>> VceSAT of maybe 0.4V, supply=12 (nominal) & regulator o/p=8V you're
>> cutting it fine with the regulator's dropout level. To keep the
diode
>> or not to keep it ... that is the question    :o
>
>I can't remember the circuit Ian, can you re-post
>it?
>-Roman

OK - it's an attached 2k gif. Hope there's enough res left to make it
out.
I reckon you could dispense with the blocking diode?
Thanks - Ian.
>


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part 2 1926 bytes content-type:image/gif; name="PWR01.gif" (decode)


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