© 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

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.

input bias current of the comparator is at most

100nA (i.e., 1000 times the input bias current)

and retain excellent accuracy. The current

2.

the upper and lower thresholds.

3.

EQUATION 4-1:

4.

5.

EQUATION 4-2:

6.

Verify

the

threshold

voltages

with

these

formulas:

EQUATION 4-3:

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

-------------------------





–

=