MAX975 Maxim, MAX975 Datasheet - Page 14

• Manufacturer Part Number: MAX975
• Description: The MAX975/MAX977 single/dual comparators feature three different operating modes, and are optimized for +3V and +5V single-supply applications
• Manufacturer: Maxim

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
2) Choose the overvoltage threshold (V
3) Choose the undervoltage threshold (V
Single/Dual, +3V/+5V Dual-Speed
Comparators with Auto-Standby
Figure 5. Window Comparator
14
100nA, so the current through R1 should exceed
10µA for the thresholds to be accurate. R1 values in
the 50k to 100k range are typical.
is rising, and calculate R2 and R3 with the following
formula:
where V
falling, and calculate R2 with the following formula:
where V
MAX6120
V
IN
______________________________________________________________________________________
1
3
V
R2 = (R1 + R2 + R3) x [(V
CC
82.1k , 1%
24.9k , 1%
100k , 1%
R2 + R3 = R1 x [V
H
H
R3
2
= 1/2V
= 1/2V
R2
R1
HYST
HYST
(PIN NUMBERS SHOWN ARE FOR QSOP PACKAGE)
14
15
6
7
MAX977
MAX977
10
2
.
.
1/2
1/2
V
V
CC
CC
OTH
4
5
Window Comparator
1
9
REF
C
C
/ (V
STOA
STOB
3
11
0.1 F
REF
- V
UTH
H
+ V
) / V
OTH
UNDERVOLTAGE
) when V
OVERVOLTAGE
H
UTH
) when V
) - 1]
POWER GOOD
] - R1
IN
is
IN
4) Calculate R3 with the following formula:
5) Verify the resistor values. The equations are as
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 A 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.
Figure 6. Toll-Tag Reader
follows:
X-BAND
DETECTOR
V
V
OTH
UTH
= (V
= (V
4.7k
REF
REF
R3 = (R2 + R3) - R2
50
- V
+ V
0.1 F
H
0.1 F
H
GND
) x (R1 + R2 + R3) / (R1 + R2)
MAX975
) x (R1 + R2 + R3) / R1
STO
V
1M
CC
3V
C
STAT
LP
STO
3V
Toll-Tag Circuit
I/0
I/0
WAKE-UP IRQ
P