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TC1040 Datasheet(PDF) 5 Page - Microchip Technology

Part No. TC1040
Description  Linear Building Block . Dual Low Power Comparator and Voltage Reference with Shutdown
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Maker  MICROCHIP [Microchip Technology]
Homepage  http://www.microchip.com
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TC1040 Datasheet(HTML) 5 Page - Microchip Technology

 
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© 2002 Microchip Technology Inc.
DS21345B-page 5
TC1040
4.2
Precision Battery Monitor
Figure 4-2 is a precision battery low/battery dead
monitoring circuit. Typically, the battery low output
warns the user that a battery dead condition is
imminent. Battery dead typically initiates a forced
shutdown to prevent operation at low internal supply
voltages (which can cause unstable system operation).
The circuit in Figure 4-2 uses a single TC1040, one-
half of a TC1029, and only six external resistors.
COMPA
and
COMPB
provide
precision
voltage
detection using VR as a reference. Resistors R2 and
R4 set the detection threshold for BATT LOW, while
Resistors R1 and R7 set the detection threshold for
BATT FAIL. The component values shown assert
BATT LOW at 2.2V (typical) and BATT FAIL at 2.0
(typical). Total current consumed by this circuit is
typically 22
µA at 3V. Resistors R5 and R6 provide
hysteresis for comparators COMPA and COMPB,
respectively.
4.3
External Hysteresis (Comparator)
Hysteresis can be set externally with two resistors
using positive feedback techniques (see Figure 4-3).
The design procedure for setting external comparator
hysteresis is as follows:
1.
Choose the feedback resistor RC.Since the
input bias current of the comparator is at most
100pA, the current through RC canbeset to
100nA (i.e., 1000 times the input bias current)
and retain excellent accuracy. The current
through RC at the comparator’s trip point is VR /
RC where VR is a stable reference voltage.
2.
Determine the hysteresis voltage (VHY) between
the upper and lower thresholds.
3.
Calculate RA as follows:
EQUATION 4-1:
4.
Choose the rising threshold voltage for VSRC
(VTHR).
5.
Calculate RB as follows:
EQUATION 4-2:
6.
Verify
the
threshold
voltages
with
these
formulas:
VSRC rising:
EQUATION 4-3:
VSRC falling:
EQUATION 4-4:
R
A
R
C
V
HY
V
DD
-----------


=
R
B
1
V
THR
V
R
R
A
×
---------------------
 1
R
A
-------
1
R
C
-------
-----------------------------------------------------------
=
V
TH R
V
R
() R
A
()
1
R
A
-------


1
R
B
-------


1
R
C
-------


++
=
V
THF
V
THR
R
A
V
DD
×
R
C
-------------------------


=


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