Window comparator, Toll-tag circuit – Rainbow Electronics MAX977 User Manual
Page 14
MAX975/MAX977
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 100k
Ω
range are typical.
2) Choose the overvoltage threshold (V
OTH
) when V
IN
is rising, and calculate R2 and R3 with the following
formula:
R2 + R3 = R1 x [V
OTH
/ (V
REF
+ V
H
) - 1]
where V
H
= 1/2V
HYST
.
3) Choose the undervoltage threshold (V
UTH
) when V
IN
is
falling, and calculate R2 with the following formula:
R2 = (R1 + R2 + R3) x [(V
REF
- V
H
) / V
UTH
] - R1
where V
H
= 1/2V
HYST
.
4) Calculate R3 with the following formula:
R3 = (R2 + R3) - R2
5) Verify the resistor values. The equations are as
follows:
V
OTH
= (V
REF
+ V
H
) x (R1 + R2 + R3) / R1
V
UTH
= (V
REF
- V
H
) 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 3
µ
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.
Single/Dual, +3V/+5V Dual-Speed
Comparators with Auto-Standby
14
______________________________________________________________________________________
6
1
3
7
10
R3
V
CC
V
CC
V
IN
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
C
STOB
1/2
MAX977
MAX6120
14
15
2
V
CC
4
3
1
C
STOA
0.1
µ
F
1/2
MAX977
Figure 5. Window Comparator
I/0
I/0
WAKE-UP IRQ
3V
3V
0.1
µ
F
µ
P
0.1
µ
F
1M
4.7k
GND
STAT
LP
STO
C
STO
V
CC
50
Ω
X-BAND
DETECTOR
MAX975
Figure 6. Toll-Tag Reader