Battery-low detect using Voltage Detector and RC

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I thought I had a really clever way of implementing 3-volt battery-low
detection with only 3 parts: a 2.7-volt reset detector, a resistor,
and a capacitor.  The idea was to periodically raise and output which
will would be filtered by the RC filter, and then feed that to the
reset detector.  By timing when the reset detector declares the
filtered voltage to be above 2.7 volts, I could infer battery voltage.
The trouble is those reset detectors (Microchip MCP 130 series) have a
built-in additional power-up delay that swamps out the effect of my RC
filter.

Does anyone know of any tiny 3-terminal device that acts like a
self-contained voltage detector but without any delay?  If so, then
this method could still be salvaged.  I plan on performing an
in-circuit calibration on each piece, and then custom-flashing the PIC
to compensate for variations in threshold voltage and RC values.


-Robert Scott
 Ypsilanti, Michigan

Re: Battery-low detect using Voltage Detector and RC

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What would be wrong with periodically lowering the output and timing when
the reset detector declares the voltage to be below 2.7 volts?



Re: Battery-low detect using Voltage Detector and RC

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By golly, it works!  In a way, it's an even more sensitive measure of
the battery voltage.  The only disadvantage is that the time period is
much shorter, and therefore harder to measure with accuracy.  But on
my target system, a 3.6-volt battery supply takes about 60 usec. to
trigger the reset detector at 2.7 volts using the RC filter I've got
now.  My 4 MHz PIC can measure time to within 1 usec, so I guess I'll
go with it.  Thanks for the suggestion.


-Robert Scott
 Ypsilanti, Michigan

Re: Battery-low detect using Voltage Detector and RC
Hello Robert,

Just in case you get hit with inaccuracies due to internal delay or the
tolerance of the capacitor: Check out the National LMV431 or the TI
counterpart. That is a very precise chip in SOT-23 and you could build
something around it.

Regards, Joerg

http://www.analogconsultants.com

Re: Battery-low detect using Voltage Detector and RC
On Tue, 08 Mar 2005 23:44:41 GMT, Joerg

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I will definitely look into it.  As for inaccuracies, I expect to deal
with most of them with a system calibration.  I will flash a test
program into the completed module after it is potted.  This test
program will determine the correct time delay for a threshold battery
voltage, and then reflash the application program based on the results
of the test.  I won't be relying on any precise part tolerances.


-Robert Scott
 Ypsilanti, Michigan

Re: Battery-low detect using Voltage Detector and RC
Hello Robert,

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Sounds good, as long as the tempcos of the contributors are within your
spec.

Regards, Joerg

http://www.analogconsultants.com

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