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'low Power Comparators--summary of findings'
1999\10\18@221222 by William K. Borsum

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Since several of you asked for my findings on low power comparators, here
goes:

History:
Original requirement was for a low battery voltage detector circuit for a
PIC based data logger.  9.5 volt batteries could decay to around 6.8 volts
before the combined diode and regulator drops would allow the analog
references to loose stability.  Total power consumption was between 45 and
70 uA in "quiescent" mode, which would give a year or more life off a
standard alkaline radio battery.  Original circuit used the Analog devices
equivalent to the MAX666 regulator with a comparator built in.  The newer
circuits used the TC55 series regulators, and a LMC7225 comparator which
(in theory) would take the quiescent current through those parts from
around ten uA to 2-3 uA--a better than 10% savings in power usage.

However, the LMC7225 turned out to be available ONLY in reels of 1000, with
an 8-12 week lead time--which was not true when the board was designed
several months ago.  (There is a lesson here!)

Findings:
First thought was to go with a 3-pin voltage detector (reset circuit) from
Telcom--the TC54 series which is carried by digikey.  Problem was--dropping
the battery voltage through a divider required about 100uA for the TC54 to
maintain stability due to its low input impedance (3-4 MOhms)--and a nasty
habit of jerking the input impedance around when switching output levels
which led to some very obnoxious oscillations.

One of you all suggested the MAX918 for an open collector unit, which is
what was being replaced.  Ended up with a MAX917--available off the shelf
from Maxim Small Orders (minimum quantity of 25 at $1.90).  Unit has a
built in reference, and a bias current of < 1 nA--which allowed the use of
very large divider resistors, and a divider current of 1-2 uA.  Using a
device with a push-pull output also eliminated the pull up resistor on the
output, and a bit of other power hungry circuitry.  Most of the power
consumption is still in the divider network.

Hope this helps
kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<spam_OUTborsumTakeThisOuTspamdascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\10\19@083147 by wwl

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On Mon, 18 Oct 1999 19:08:39 -0700, you wrote:

>Since several of you asked for my findings on low power comparators, here
>goes:
>
>History:
>Original requirement was for a low battery voltage detector circuit for a
>PIC based data logger.  
Remember you don't need to continuously monitor battery level. You can
use a low-impedance divider, switched on occasionally. This avoids
many of the leakage issues etc.

If using an external comparator, you don't even need a low-power one -
just supply its power from a PIC pin so it's turned off most of the
time.

1999\10\19@130933 by wwl

picon face
On Tue, 19 Oct 1999 11:18:44 -0400, you wrote:

>If you switch the ground instead of the +5 to
>the comparator, and you *also* have the ground
>for the divider resistors connected to this same
>"switched ground", then when the PIC output
>pin is LOW all works as desired, and when the
>PIC output pin is TRISTATED, then the comparator
>is switched off, and current through the divider
>resistors is reduced to almost nil.
except the divider has to come from the battery supply, not 5v, so you
can't just switch the Ov line - it will push current through the input
diode to 5v. This can actually work with some voltage regulators, as
current will flow in the divider _instead_ of the regulator, but it
can get messy...!

I used this to measure battery voltage using the PIc's ADC - you could
obviously adapt it for non a/d parts, either with a comparator, or a
voltage detector chip

+9V
|
+--------------------[5V reg]--->PIC supply
|       |
|      R3
>|     |      C1
 |-----+-R4---||---< output pin
/|
R1
|
+---------+----->a/d input
|         |
R2    [VOLT DET CHIP]--LOGIC O/P
|         |
0V       0V

R1=R2=10K
R3=R4=47K ( I used a digital transistor, so they are internal)
C1 = 10n

When the output pin goes low, the PNP transistor turns on for long
enough to measure the voltage. C1 acts as a level-shifter. This output
pin can often be shared with other functions.
R1/R2 are low enough to avoid errors due to input leakage.
You could even power a comparator like this, but watch out for
settling time.


For brownout, use a reset chip on the processor's supply - smaller
cheaper & simpler than a micropower comparator - Motorola, uem, seiko
and many others do cheap ones (<50c) that draw about a microamp.
beware of using comparators for brownout protection - they can do odd
things at very low voltages, so they don't always hold their outputs
low all the way to 0V.

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