Rainbow Electronics MAX924 User Manual
Page 12
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
12
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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 10M
Ω
can be used, but values in
the 100k
Ω
to 1M
Ω
range are usually easier to deal
with. In this example, choose R1 = 294k
Ω
.
3. Calculate R2 + R3. The overvoltage threshold
should be 5.5V when V
IN
is rising. The design
equation is as follows:
4. Calculate R2. The undervoltage threshold should
be 4.5V when V
IN
is falling. The design equation is
as follows:
5. Calculate R3.
Choose R3 = 1M
Ω
(1% standard value)
6. Verify the resistor values. The equations are as
follows, evaluated for the above example.
Bar-Graph Level Gauge
The high output source capability of the MAX921 series
is useful for driving LEDs. An example of this is the
simple four-stage level detector shown in Figure 7.
The full-scale threshold (all LEDs on) is given by V
IN
=
(R1 + R2)/R1 volts. The other thresholds are at 3/4 full
scale, 1/2 full scale, and 1/4 full scale. The output
resistors limit the current into the LEDs.
Level Shifter
Figure 8 shows a circuit to shift from bipolar ±5V inputs
to TTL signals. The 10k
Ω
resistors protect the
comparator inputs, and do not materially affect the
operation of the circuit.
Overvoltage threshold :
V
(V
V )
(R1
R2
R3)
R1
5.474V.
Undervoltage threshold :
V
(V
V )
(R1
R2
R3)
(R1 + R2)
4.484V,
where the hysteresis voltage V
V
R5
R4
OTH
REF
H
UTH
REF
H
H
REF
=
+
Ч
+
+
=
=
−
Ч
+
+
=
=
Ч
.
R3
(R2 + R3)
R2
.068M 6
k
1.006M
=
−
=
−
=
1
1 9
.
Ω
R2
(R1 + R2 + R3)
(V
V )
V
R1
(294k + 1.068M)
(1.182
0.005)
4.5
294k
62.2k
Choose R2
61.9k (1% standard value).
REF
H
UTH
=
Ч
−
−
=
Ч
−
−
=
=
Ω
Ω
R2
R3
R1
V
V
V
1
294k
5.5
(1.182
0.005)
1
1.068M
OTH
REF
H
+
=
Ч
+
−
=
Ч
+
−
=
Ω
MAX923
INB-
REF
HYST
INA+
V-
V+
OUTA
OUTB
10k
R5
2.4M
R1
R2
R3
UNDERVOLTAGE
POWER GOOD
OVERVOLTAGE
V
IN
V
OTH
= 5.5V
V
UTH
= 4.5V
R4
+5V
Figure 6. Window Detector