9

LTC1841/LTC1842/LTC1843

1. Choose the required hysteresis voltage band and

calculate values for R4 and R5 according to the formu-

las in the hysteresis section. In this example,

±5mV of

hysteresis has been added at the comparator input

will be larger because of the input resistor divider.

so the current through R1 should exceed 100nA, to

ensure threshold accuracy. R1 values up to about 10M

can be used, but values in the 100k to 1M range are

usually easier to deal with. In this example choose

R1 = 294k.

3. Calculate R2 + R3. The overvoltage threshold should be

set at 5.5V. The design equation is as follows:

RR

R

V

VV

k

M

UTH

REF

H

23

1

1

294

55

1 182 0 005

1

1 068

+=

+









=

+









=

–

.

..

–

.

4. Calculate R2. The undervoltage threshold should be set

at 4.5V. The design equation is as follows:

RR

R

R

VV

V

R

kM

k

k

REF

H

LTH

21

2

3

1

294

1 068

1 182 0 005

45

294

62 2

=+

+

()

=+

()

=

–

–

.

.– .

.

–

.

Choose R2 = 61.9k (1% standard value)

5. Calculate R3:

R3 = (R2 + R3) – R2

= 1.068M – 61.9k

= 1.006M

Choose R3 = 1M (1% standard value)

6. Verify the resistor values. The equations are as

follows, evaluated for the above example:

Overvoltage threshold:

VV

V

RR

R

R

V

OTH

REF

H

=+

=

12

3

1

5 474

.

Undervoltage threshold:

VV

V

RR

R

RR

V

UTH

REF

H

=

+

=

–

.

12

3

12

4 484

where the hysteresis voltage VV

R

R

H

REF

=

()









5

4