MAX975 查看數據表（PDF） - Maxim Integrated

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MAX975

Single/Dual, +3V/+5V Dual-Speed Comparators with Auto-Standby

Maxim Integrated 

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

Comparators with Auto-Standby

R3

VIN

82.1k, 1%

VCC

R2

1 24.9k, 1%

MAX6120 2

3

R1

100k, 1%

VCC

14

4

1/2

MAX977 3

15

1

0.1µF

UNDERVOLTAGE

2

CSTOA

VCC

POWER GOOD

6

5

1/2

11

MAX977

7

9

10

CSTOB

OVERVOLTAGE

(PIN NUMBERS SHOWN ARE FOR QSOP PACKAGE)

0.1µF

3V

X-BAND

DETECTOR

VCC

STAT

MAX975 LP

STO

50Ω

CSTO

GND

3V

4.7k

1M

0.1µF

WAKE-UP IRQ

I/0 µP

I/0

Figure 5. Window Comparator

Figure 6. Toll-Tag Reader

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 50kΩ to 100kΩ range are typical.

2) Choose the overvoltage threshold (VOTH) when VIN

is rising, and calculate R2 and R3 with the following

formula:

R2 + R3 = R1 x [VOTH / (VREF + VH) - 1]

where VH = 1/2VHYST.

3) Choose the undervoltage threshold (VUTH) when VIN is

falling, and calculate R2 with the following formula:

R2 = (R1 + R2 + R3) x [(VREF - VH) / VUTH] - R1

where VH = 1/2VHYST.

4) Calculate R3 with the following formula:

R3 = (R2 + R3) - R2

5) Verify the resistor values. The equations are as

follows:

VOTH = (VREF + VH) x (R1 + R2 + R3) / R1

VUTH = (VREF - VH) x (R1 + R2 + R3) / (R1 + R2)

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 3µA 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.

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