Ultra Low-Power,

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

as 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.

Bar-Graph Level Gauge

The high output source capability of the MAX921 series

is useful for driving LEDs. An example of this is the

simple four-stage level detector shown in Figure 7.

(R1 + R2)/R1 volts. The other thresholds are at 3/4 full

scale, 1/2 full scale, and 1/4 full scale. The output

resistors limit the current into the LEDs.

Level Shifter

Figure 8 shows a circuit to shift from bipolar ±5V inputs

to TTL signals. The 10k

Ω resistors protect the

comparator inputs, and do not materially affect the

operation of the circuit.

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

=+

×

++

=

=−

×

++

=

=×

.

R3

(R2 + R3)

R2

.068M

6

k

1.006M

=−

=−

=

11 9

.

Ω

R2

(R1 + R2 + R3)

(V

V )

V

R1

(294k + 1.068M)

(1.182

0.005)

4.5

294k

62.2k

Choose R2

61.9k

(1% standard value).

REF

H

UTH

=×

−

−

=×

−

−

=

=

Ω

Ω

R2

R3

R1

V

V

V

1

294k

5.5

(1.182

0.005)

1

1.068M

OTH

REF

H

+=

×

+

−









=×

+

−









=Ω

MAX923

INB-

REF

HYST

INA+

V-

V+

OUTA

OUTB

10k

R5

2.4M

R1

R2

R3

UNDERVOLTAGE

POWER GOOD

OVERVOLTAGE

R4

+5V

Figure 6. Window Detector