TC1027

DS21284B-page 6

© 2002 Microchip Technology Inc.

Capacitor C1 will charge up through R4. When the

voltage at the comparator’s inverting input is equal to

at ground potential, the value at the non-inverting input

value given by:

EQUATION 4-7:

Using the same resistors as before, capacitor C1 must

now discharge through R4 toward ground. The output

will return to a high state when the voltage across the

period of oscillation will be twice the time it takes for the

RC circuit to charge up to one half its final value. The

period can be calculated from:

EQUATION 4-8:

The frequency stability of this circuit should only be a

function of the external component tolerances.

Figure 4-6 shows the circuit for a pulse width modulator

circuit. It is essentially the same as in Figure 4-4, but

with the addition of an input control voltage. When the

is basically the same as described for the free-running

oscillator. If the input control voltage is moved above or

wave will be altered. This is because the addition of the

control voltage at the input has now altered the trip

points. The equations for these trip points are shown in

Pulse width sensitivity to the input voltage variations

can be increased by reducing the value of R6 from

10k

Ω and conversely, sensitivity will be reduced by

increasing the value of R6. The values of R1 and C1

can be varied to produce the desired center frequency.

FIGURE 4-1:

COMPARATOR

EXTERNAL HYSTERESIS

CONFIGURATION

FIGURE 4-2:

PRECISION BATTERY MONITOR

V

L

V

DD

3

-----------

=

1

FREQ

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

2 0.694

() R4

() C1

()

=

+

–

1/4

TC1027

R2, 330k, 1%

TC1034

1/4

R4, 470k, 1%

R5, 7.5M

1/4

R6, 7.5M

R3, 470k, 1%

R1, 270k, 1%

VR

To System DC/DC

Converter

3V

Alkaline

TC1027

BATTFAIL

BATTLOW

CMPTR1

+

–

CMPTR2

+

–

AMP1

+

–

+