Window Comparator

The MAX977 is ideal for making a window detector

(undervoltage/overvoltage detector). The schematic

shown in Figure 5 uses a MAX6120 reference and com-

ponent values selected for a 2.0V undervoltage thresh-

old and a 2.5V overvoltage threshold. Choose different

thresholds by changing the values of R1, R2, and R3.

OUTA provides an active-low undervoltage indication,

and OUTB gives an active-low overvoltage indication.

ANDing the two outputs provides an active-high,

power-good signal. The design procedure is as follows:

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

100nA, so the current through R1 should exceed

10µA for the thresholds to be accurate. R1 values in

the 50k

Ω to 100kΩ range are typical.

is rising, and calculate R2 and R3 with the following

formula:

falling, and calculate R2 with the following formula:

4) Calculate R3 with the following formula:

R3 = (R2 + R3) - R2

5) Verify the resistor values. The equations are as

follows:

Toll-Tag Circuit

The circuit shown in Figure 6 uses a MAX975 in a very

low standby-power AM demodulator circuit that wakes

up a toll tag (part of an automated roadway toll-

collection system). This application requires very long

standby times with brief and infrequent interrogations.

In the awake state, it is capable of demodulating the

typical 600kHz AM carrier riding on the 2.4GHz RF sig-

nal. In this state, the comparator draws its 250µA high-

speed current. After communications have ceased, or

when instructed by the microcontroller, the comparator

returns to its low-power state. The comparator draws

only 3

µA in this state, while monitoring for RF activity.

Typically, this application requires two comparators

and a discrete power-management and signal-

switchover circuit. The MAX975 circuit is smaller, sim-

pler, less costly, and saves design time.

Single/Dual, +3V/+5V Dual-Speed

Comparators with Auto-Standby

14

______________________________________________________________________________________

6

1

3

7

10

R3

5

R2

R1

82.1k, 1%

24.9k, 1%

100k, 1%

2

11

9

OVERVOLTAGE

(PIN NUMBERS SHOWN ARE FOR QSOP PACKAGE)

UNDERVOLTAGE

POWER GOOD

1/2

MAX977

MAX6120

14

15

2

4

3

1

0.1

µF

1/2

MAX977

Figure 5. Window Comparator

I/0

I/0

WAKE-UP IRQ

3V

3V

0.1

µF

µP

0.1

µF

1M

4.7k

GND

STAT

LP

STO

50

Ω

X-BAND

DETECTOR

MAX975

Figure 6. Toll-Tag Reader