MAX981C 查看數據表（PDF） - Maxim Integrated

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MAX981C

Ultra-Low-Power, Open-Drain, Single/Dual-Supply Comparators

Maxim Integrated 

Ultra-Low-Power, Open-Drain,

Single/Dual-Supply Comparators

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 10MΩ can be used, but values in the

100kΩ to 1MΩ range are usually easier to deal with.

In this example, choose R1 = 294kΩ.

3) Calculate R2 + R3. The overvoltage threshold

should be 5.5V when VIN is rising. The design

equation is as follows:

R2

+

R3

=

R1 ×





VOTH

VREF +

VH

− 1

=

294kΩ

×



5.5

 (1.182 + 0.005)

− 1

= 1.068MΩ

4) Calculate R2. The undervoltage threshold should be

4.5V when VIN is falling. The design equation is as

follows:

R2 = (R1 + R2 + R3) × (VREF − VH) − R1

VUTH

= (294kΩ + 1.068MΩ) × (1.182 − 0.005)

4.5

− 294kΩ

= 62.2kΩ

Choose R2 = 61.9kΩ (1% s tandard value).

5) Calculate R3:

R3 = (R2 + R3) − R2

= 1.068MΩ − 61.9kΩ

= 1.006MΩ

Choose R3 = 1MΩ (1% standard value)

6) Verify the resistor values. The equations are as

follows, evaluated for the above example:

Overvoltage Threshold:

VOTH

=

(VREF

+ VH) ×

(R1 + R2 + R3)

R1

= 5.474V

Undervoltage Threshold :

VUTH

=

(VREF

− VH) ×

(R1 + R2 + R3)

(R1 + R2)

= 4.484V

where the hysteresis voltage VH

=

VREF ×

R5 .

R4

VIN

R3

1MΩ

VOTH = 5.5V

VUTH = 4.5V

3 INA+

R2

62.2kΩ

R1

294kΩ

5 HYST

R5

10kΩ

R4 6 REF

2.4MΩ

4 INB-

V-

2

5V

7

V+

1MΩ

OUTA 1

OUTB 8

POWER GOOD

MAX9_3

Figure 5. Window Detector

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 10kΩ resistors protect

the comparator inputs, and do not materially affect the

circuit’s operation.

12 ______________________________________________________________________________________

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