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'[OT]: How to reduce power consumption for battery '
2001\07\22@121748 by John Waters

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Hi All,

For a battery operated circuit, we want the power consumption to be as low
as possible, but if the circuit involves a microcontroller, we will need a
voltage regulator like the 78L05 or 7805 to keep the 5V stable. Since the
supply voltage is at least 2V higher than the 5V, part of the current will
be bypassed through the regulator, in other words, the regulator "consumes"
some power even when there is no or little current demanding by the
microcontroller circuit itself.

Is using the regulator unavoidable? What is the usual methods to reduce
power consumption for a battery operated microcontroller circuit?

Thanks in advance!

John





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2001\07\22@131828 by SkinTech

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Hi John,

Actually if the current drain is zero, the regulator consumes only its own
standby current, it doesn't 'syphon off' the excess voltage in any way. It
does carry the same current as the load, so if you draw 1 amp, the excess 2
volts (which will appear across the reg) will waste 2v * 1 amp = 2 watts.
The load gets 5v * 1 amp = 5 watts, giving you some 71% efficiency (only).
This is independent of the load current. You get much better mileage with
one of the modern switching low voltage regs, they have almost 100%
efficiency (well yes, almost...).

Cheers, Jan Didden

{Original Message removed}

2001\07\22@132705 by Kyrre Aalerud

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The switching regs use ac don't they ?
If so, how would you run them from battery ?

----- Original Message ----- From: "SkinTech" <spam_OUTjan.diddenTakeThisOuTspamSKINTECH.BE>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Sunday, July 22, 2001 7:26 PM
Subject: Re: [PICLIST] [OT]: How to reduce power consumption for battery operated circuits?


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2001\07\22@133743 by David VanHorn

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At 07:26 PM 7/22/01 +0200, Kyrre Aalerud wrote:
>The switching regs use ac don't they ?
>If so, how would you run them from battery ?

They use pulsed DC, and are perfectly happy running from batteries.
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2001\07\22@133954 by Dave Dilatush

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John Waters wrote...

>For a battery operated circuit, we want the power consumption to be as low
>as possible, but if the circuit involves a microcontroller, we will need a
>voltage regulator like the 78L05 or 7805 to keep the 5V stable. Since the
>supply voltage is at least 2V higher than the 5V, part of the current will
>be bypassed through the regulator, in other words, the regulator "consumes"
>some power even when there is no or little current demanding by the
>microcontroller circuit itself.
>
>Is using the regulator unavoidable? What is the usual methods to reduce
>power consumption for a battery operated microcontroller circuit?

7805/78L05 regulators take a fair amount of quiescent current just for
their own internal biasing needs, and for battery operated devices
they're pretty wasteful.

National Semiconductor, Linear Technology, and Maxim all have
micropower voltage regulators that have very low quiescent current
drain suitable for battery operation.  The one I use most often in my
designs is National's LP2951, which takes about a hundred microamps
for internal use.  The nice thing about this part is that it also
provides a handy "low-voltage inhibit" error output which can be used
as a reset signal for microprocessors, etc.

DD

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2001\07\22@135610 by Mike Kendall

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The difference between the supplied voltage and the regulated voltage is
called the "head" voltage.  It will not necessarily be 2 watts regulator
dissipation for a head voltage of 2volts and drain of 1 watt.  It depends on
the type of regulator.  That is why switching power supplies have such high
efficiency.  I used some Maxim smart charging chips a few years back for
NiCad batteries.  They could withstand a large head voltage because they
were a switching device, similiar to a DC-DC converter.  I'm guessing the
78L05 is linear (they're pretty old and I'd have to look up the specs) and
cannot withstand a high head voltage.  There is certainly some more
efficient ways of doing it if efficiency is worth paying the extra money
for.  I used a very small DC-DC converter to field charge laptops off of
lithium batteries a couple of years ago and the price was not that high.
73,
Mike/ke6cvh

{Original Message removed}

2001\07\22@160258 by Herbert Graf

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Although more expensive then the 7805 try using a switcher, they are much
more efficient. TTYL

> {Original Message removed}

2001\07\22@161929 by Douglas Wood

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Does the circuit need to be pwered up all the time? I am currently working
on a circuit that uses a DS11305 RTC to wake up the power based on a
schedule. The RTC interrupt output turns on the power to the micro, which
does its thing and then shuts down the power until the next wake up event
occurs. We are expecting to get something like 10 years of operation out of
our Li batteries by doing this...

Douglas Wood
Software Engineer
EraseMEdbwoodspam_OUTspamTakeThisOuTkc.rr.com

Home of the EPICIS Development System for the PIC and SX
http://epicis.piclist.com

> > {Original Message removed}

2001\07\22@174214 by Robert E. Griffith

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Hi John,

I have just been spending hours going over Linear's web site looking for
regulators for my power supply needs.

Check out
http://www.linear.com/prod/datasheet?datasheet=441&product_family=power . It
seems to be an example of what other people who posted before me are
referring to. 1.5v from one battery in -> regulated 3.3v out.  They have
lots of variations (5V out for example). The top of the power product tree
is here http://www.linear.com/prod/pft.html (to find other devices similar
to the one above.


New, Related Question:
Do you have to be more concerned with EMI when you use a switching DC-DC
convert / regulator?  I am considering this linear device
www.linear.com/prod/datasheet?datasheet=569&product_family=power
In a nutshell its 5V in, -12V @150ma out.  Its overkill for what I need (2ma
at -12V for an LCD), but the chip is only a couple bucks, and I am afraid of
discrete components:)

The data sheet actual has a sample board layout, warning, "If you don't lay
out your PCB carefully, its not going to work".

Is this normal?  Are switching regulators trickier to work with?  I gather
that a DC-DC convert is similar to a switching regulator; the difference
being that the output of the DC-DC convert will vary with changes in Vin. Is
that correct?

--BobG

> > {Original Message removed}

2001\07\22@192808 by Dave Dilatush

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BobG wrote...

>New, Related Question:

>Do you have to be more concerned with EMI when you use a switching DC-DC
>convert / regulator?  
Yes, you do; possible interference to sensitive circuits in your own
device is always an issue, as are possible radiated and/or conducted
emissions that could interfere with other devices.

The LT1931 device you linked to below operates around a megahertz,
which could cause a lot of interference with nearby radios.

>I am considering this linear device
>www.linear.com/prod/datasheet?datasheet=569&product_family=power
>In a nutshell its 5V in, -12V @150ma out.  Its overkill for what I need (2ma
>at -12V for an LCD), but the chip is only a couple bucks, and I am afraid of
>discrete components:)

It's overkill, and at the very low currents your application requires,
it won't be very efficient either.  Take a look at the efficiency
curve for the "+5V to -12V Inverting Converter" circuit (I assume this
is the one you were thinking of); see how the efficiency drops off
sharply at low output currents?

From what I see on a quick read of the datasheet, it looks like this
device was optimized to give high output power in a VERY small circuit
footprint; hence, the high operating frequency to make the inductors
small.  It doesn't seem to be targeted at low current applications.

>The data sheet actual has a sample board layout, warning, "If you don't lay
>out your PCB carefully, its not going to work".

And they REALLY mean it, too: if you have a layout that doesn't have
the right ground topology (i.e., the right arrangement of the current
paths in the circuit) the chip won't work.

>Is this normal?  
Unfortunately, yes.

>Are switching regulators trickier to work with?  
Sometimes yes, sometimes not so tricky.  At best, they take
considerably more care to design with than the trusty ol' 7805 linear
regulator.  At worst, they are unbelievably nightmarish to design.

For applications requiring a couple of watts of power, there are many
IC switching regulators that are fairly easy to design with.  National
Semiconductor's "Simple Switcher" chips are the ones I've used most
often, and they have the design procedure down to a cookbook approach
where you just plug some numbers into a few formulas, look up a few
component values in a table, and you're done.  Component selection,
particularly filter capacitors, is critical, and so is circuit layout;
but other than that it's straightforward.

For applications requiring more power, design becomes progressively
more and more difficult as the power increases.  Above a few dozen
watts, things get tricky; above a few hundred, it becomes "don't try
this trick at home".  Above a few thousand watts, it gets real scary.

For lower-power applications, the situation gets even trickier;
designing a switching regulator to convert 10 milliwatts at high
efficiency can be a real challenge because every piddling little power
loss within the circuit has to be minimized.

In any case, yes; switching regulators are generally more difficult to
design with than linear regulators.

>I gather
>that a DC-DC convert is similar to a switching regulator; the difference
>being that the output of the DC-DC convert will vary with changes in Vin. Is
>that correct?

Yes and no.  In some cases (such as the LT1931 you cited), the terms
are used interchangeably.

Here's a suggestion, for whatever it's worth:

The easiest way to generate a negative voltage for your LCD bias would
be with one of the RS232 interface chips (like a MAX232A) that
generate their own + and - RS232 levels using capacitive charge pumps.
Just ignore the RS232 transceiver circuits (ground the unused inputs
or connect them to Vcc, whichever results in the least current drain)
and use only the power section.

The efficiency will be a lot better than the LT1931, and so will the
price tag.

The only drawback is that you won't get a full -12 volts out of it,
only around -9V to -10V.  Would that be enough?

Hope this helps...

Dave

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2001\07\22@194257 by David VanHorn

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>
> >Do you have to be more concerned with EMI when you use a switching DC-DC
> >convert / regulator?
>
>Yes, you do; possible interference to sensitive circuits in your own
>device is always an issue, as are possible radiated and/or conducted
>emissions that could interfere with other devices.
>
>The LT1931 device you linked to below operates around a megahertz,
>which could cause a lot of interference with nearby radios.

True, up to a point.
If it's a 10 watt converter, then you're going to have 10 watts in
circulating AC, at the switching frequency, with edge rates out to 10X that
and more.
However, they can be built properly, with just some attention to where the
current is going.
If you do this, you can barely tell they are on, with a sensitive receiver.


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2001\07\22@211236 by Robert E. Griffith

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>>Hope this helps...

Wow, it sure does.  Thanks for detailed explanation.

Here is another, related question: I have two enclosures making up my
device - one powering the other through a cable that also carries serial
Tx/Rx.  The first is powered from a wall transformer.  Can I supply
regulated 5V to the remote device (6-12 ft), or am I better off sending the
higher, unregulated voltage to the remote device and give it its own, second
5v reg?  The remote unit draws 50ma typical, 120ma in surges.

--BobG


Details of what I am doing if you are curious:

Wall Transformer:        Main Enclosure:          LCD Enclosure:
12V unregulated:     ->          --------12V----->
                        5V@250ma                 5V@100ma
                        3V@40ma                  -12V@2ma

My breadboarded prototype (which works) uses a 12v unregulated wall
transformer and I have lots of regulators around.  The 12v wall wart
connects to the main enclosure where a 5V linear reg supplies about 250ma.
This enclosure also has a 3V linear reg that drops the 5V down for about
50ma at 3v.

A twisted pair cable takes the +12V unregulated and serial signals to the
second enclosure that is basically a serial LCD.  In fact the prototype uses
a serial LCD I bought from http://www.earchLCD.com. It's a nice product, but too
expensive for this project and with a lot of extra features I am not using.
This serial LCD board has another 5v linear reg to supply the logic (100ma)
and an LTC1044A converts the +12 to -12V for the 2ma Vee for the LCD.  Since
I am making a new, leaner serial LCD board from scratch, I figured might be
able to reduce the components.

Also, the more important thing I am trying to solve with changing the power
supply method is the heat generated by the 5V linear reg in the main
circuit. It has well over 7 volts of head voltage @ 1/4 amp, so it tends to
get hot.  I want to put it in an enclosure without a fan.

So, I figured that I could use a 5V transformer (over-rated so the reg sees
at at least 5+(its drop out) at full load) and use only one 5v reg in the
main enclosure.  I could send the regulated 5V over the serial connection to
power so that the display enclosure's would not need another.  Then at the
display enclosure boost and invert the +5v to -12v.  The -12V really needs
to be set between -10V and -8V to adjust the contrast for a particular LCD.
The data sheet for the LCD suggests a simple voltage divider with a pot
since the current is so small.  I am thinking of putting in a variable reg
so that the adustment will be more stable for the end user.




{Original Message removed}

2001\07\22@221129 by Dave Dilatush

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BobG wrote...

>>>Hope this helps...
>
>Wow, it sure does.  Thanks for detailed explanation.

You're welcome; glad I could be of some help.

>Here is another, related question: I have two enclosures making up my
>device - one powering the other through a cable that also carries serial
>Tx/Rx.  The first is powered from a wall transformer.  Can I supply
>regulated 5V to the remote device (6-12 ft), or am I better off sending the
>higher, unregulated voltage to the remote device and give it its own, second
>5v reg?  The remote unit draws 50ma typical, 120ma in surges.

Hmmm...  Depends on the size of the wire, and how tightly you need the
5 volt supply to be regulated at the end of the 6-12 feet of cable.  
If you have any analog stuff out there in the remote unit that needs
an accurate supply voltage, sending 12 volts to the remote and then
regulating it down to 5 volts locally might be the best choice.

Otherwise, if there's only digital stuff out there, I think I might be
tempted to just ship the 5V down the cable (with a generous bypass
capacitor at the receiving end).

{Quote hidden}

Now, that might be a candidate for a switching regulator: you've got a
modest amount of power there, and a heat problem you want to solve.

Check out National Semiconductor's LM2671, which is rated at a
half-ampere.  That might do the trick; it will certainly run cool, as
they project about 92% efficiency for operation at 12V in/5V out at
500 mA.

>So, I figured that I could use a 5V transformer (over-rated so the reg sees
>at at least 5+(its drop out) at full load) and use only one 5v reg in the
>main enclosure.  I could send the regulated 5V over the serial connection to
>power so that the display enclosure's would not need another.  Then at the
>display enclosure boost and invert the +5v to -12v.  The -12V really needs
>to be set between -10V and -8V to adjust the contrast for a particular LCD.
>The data sheet for the LCD suggests a simple voltage divider with a pot
>since the current is so small.  I am thinking of putting in a variable reg
>so that the adustment will be more stable for the end user.

You might not need a regulator; but you could always design one in,
and bypass it if it proves unnecessary.

Regards,

Dave

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2001\07\22@222725 by Robert E. Griffith

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Thanks for pointing out the National line and in particular the LM2671. That
gives me a quick, easy, and reasonably inexpensive option for first hardware
iteration.  And, more importantly, I am learning a lot:)

--BobG



{Original Message removed}

2001\07\23@091716 by Don Hyde

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Mostly it depends on what other than the microcontroller you need to power.

KISS would dictate running straight from the battery without any regulators.
Depending on your circuit, it can often be done.

If you look through the specs of PIC's, you will find that there are a
number of them (especially the ones with L for low-voltage in their part
numbers) that are low enough voltage and tolerant enough of voltage
variations that they will work directly from a battery.

When designing this way, you need to study the discharge voltage
characteristics of the battery chemistry you are planning on, and make sure
that fresh cells won't blow up your circuit, and that the voltage where your
circuits give up is low enough that you can use most of the battery's
available energy.

For instance, we use primary lithium cells (readily available for digital
cameras).  New ones are around 3.2V per cell, and to use up 90% of the
useful energy, you have to take them down to around 2.1V.

One gotcha you need to be aware of is just what does your circuit do when
the battery dies?  The voltage will eventually taper off to the point that
it won't run, but on the way, it may do strange things.  That's why a lot of
newer PIC's have stuff like programmable brown-out detectors.

> {Original Message removed}

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