TC1040 Datasheet, PDF (5/14 Pages) Microchip Technology – Linear Building Block . Dual Low Power Comparator and Voltage Reference with Shutdown

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 V_R 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 22µA 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 R_C. Since the
input bias current of the comparator is at most
100pA, the current through R_C can be set to
100nA (i.e., 1000 times the input bias current)
and retain excellent accuracy. The current
through R_C at the comparator’s trip point is V_R /
R_C where V_R is a stable reference voltage.
2. Determine the hysteresis voltage (V_HY) between
the upper and lower thresholds.
TC1040
3. Calculate R_A as follows:
EQUATION 4-1:
R_A
=
R
C


-V----^H----^Y-^
V_{D D}^
4. Choose the rising threshold voltage for V_SRC
(V_THR).
5. Calculate R_B as follows:
EQUATION 4-2:
R_B = ----------------------------1------------------------------
 _----V----^T_----^H_---^R_------
_{VR × RA}


–---1----
R_A
–
---1----
R_C
6. Verify the threshold voltages with these
formulas:
V_SRC rising:
EQUATION 4-3:
V_THR
=
(V_R)(R_A)
 _--1----- ₊  _---1---- ₊  _---1----
 _RA  _RB  _RC
V_SRC falling:
EQUATION 4-4:
V_THF = V_THR –
 ^R_-----^A_----^×_-----^V_----^D_----^D_--
 _RC 
© 2002 Microchip Technology Inc.
DS21345B-page 5