Ultra-Low-Power, Open-Drain,

Single/Dual-Supply Comparators

12

______________________________________________________________________________________

2) Select R1. The leakage current into INB- is normally

under 1nA, so the current through R1 should exceed

100nA for the thresholds to be accurate. R1 values

up to about 10M

Ω can be used, but values in the

100k

Ω to 1MΩ range are usually easier to deal with.

In this example, choose R1 = 294k

Ω.

3) Calculate R2 + R3. The overvoltage threshold

equation is as follows:

4) Calculate R2. The undervoltage threshold should be

follows:

5) Calculate R3:

Choose R3 = 1M

Ω (1% standard value)

6) Verify the resistor values. The equations are as

follows, evaluated for the above example:

Battery Switchover Circuit

The switchover from line-powered DC to a backup

battery is often accomplished with diodes. But this

simple method is sometimes unacceptable, due to the

voltage drop and associated power loss across the

diode in series with the battery. Figure 6’s circuit

replaces the diode with a P-channel MOSFET

controlled by one of the MAX9_3 comparator outputs.

When the DC wall adapter drops below 4V (determined

by R1 and R2), OUTA goes low, turning on Q1.

Comparator B is used to measure the battery voltage,

and gives a “low-battery” indication when the battery

drops below 3.6V.

Level-Shifter

Figure 7 shows a circuit to shift from bipolar ±5V inputs

to single-ended 5V outputs. The 10k

Ω resistors protect

the comparator inputs, and do not materially affect the

circuit’s operation.

Overvoltage Threshold :

V

(V

V )

(R1

R2

R3)

R1

5.474V

Undervoltage Threshold :

V

(V

V )

(R1

R2

R3)

(R1 + R2)

4.484V

where the hysteresis voltage V

V

R5

R4

OTH

REF

H

UTH

REF

H

H

REF

=+

×

++

=

=−

×

++

=

=×

.

RR

R

R

Mk

M

32

3

2

1 068

61 9

1 006

(

+

)

.

.

.

=−

=Ω −

Ω

=Ω

RR

R

R

VV

V

R

kM

k

k

Choose R

k

s

dard value

REF

H

UTH

21

2

3

1

294

1 068

1 182

0 005

45

294

62 2

261 9

1

(

+

+

)

(

)

(

+

.

)

(.

.

)

.

.

.

( %

).

=×

−

−

=Ω

Ω ×

−

−Ω

=Ω

=Ω

tan

RR

R

V

VV

k

M

OTH

REF

H

23

1

1

294

55

1 182

0 005

1

1 068

.

(.

.

)

.

+=

×

+

−









=Ω ×

+

−









=Ω

MAX9_3

INB-

REF

HYST

INA+

3

5

6

4

1

8

V-

2

V+

7

1M

Ω

OUTA

OUTB

R5

10k

Ω

R1

294k

Ω

R2

62.2k

Ω

R3

1M

Ω

POWER GOOD

R4

2.4M

Ω

5V

Figure 5. Window Detector