100nA for the thresholds to be accurate. R1 values
up to about 10M
can be used, but values in the
100k
to 1M range are usually easier to deal with.
In this example, choose R1 = 294k
.
3. Calculate R2 + R3. The overvoltage threshold
should be 5.5V when VIN is rising. The design
equation is as follows:
4. Calculate R2. The undervoltage threshold should
be 4.5V when VIN is falling. The design equation is
as follows:
5. Calculate R3.
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 MAX931 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
VIN = (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
resistors protect the
comparator inputs, and do not materially affect the
operation of the circuit.
Two-Stage Low-Voltage Detector
Figure 9 shows the MAX932 monitoring an input
voltage in two steps. When VIN is higher than the
LOW and FAIL thresholds, outputs are high. Threshold
calculations are similar to those for the window-
detector application.
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
REF
=+
×
++
=
×
++
=
=×
.
R3
(R2 + R3)
R2
.068M
6
k
1.006M
Choose R3
1M
(1% standard value).
=
11 9
.
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
=×
=×
=
R2
R3
R1
V
1
294k
5.5
(1.182
0.005)
1
1.068M
OTH
REF
H
+=
×
+
=×
+
=
MAX931-MAX934
Ultra Low-Power, Low-Cost
Comparators with 2% Reference
12
______________________________________________________________________________________
INB+
INB-
INC+
INC-
IND+
IND-
INA+
INA-
OUTA
OUTB
OUTC
OUTD
5
4
7
6
11
10
13
12
750mV
1V
500mV
250mV
250k
182k
9
2
1
16
15
REF
8
V-
V+
GND
14
MAX934
3
+5V
V IN
R1
R2
1.182V
330
330
330
330
Figure 7. Bar-Graph Level Gauge