MAX974 Maxim, MAX974 Datasheet - Page 12

• Manufacturer Part Number: MAX974
• Description: The MAX971–MAX974 and MAX981–MAX984 single/dual/quad low-voltage comparators feature the lowest power consumption available
• Manufacturer: Maxim

Ultra-Low-Power, Open-Drain,
Single/Dual-Supply Comparators
2) Select R1. The leakage current into INB- is normally
3) Calculate R2 + R3. The overvoltage threshold
4) Calculate R2. The undervoltage threshold should be
5) Calculate R3:
6) Verify the resistor values. The equations are as
12
under 1nA, so the current through R1 should exceed
100nA for the thresholds to be accurate. R1 values
up to about 10M
100k
In this example, choose R1 = 294k .
should be 5.5V when V
equation is as follows:
4.5V when V
follows:
Choose R3 = 1M (1% standard value)
follows, evaluated for the above example:
R
R
Choose R
R
Overvoltage Threshold:
Undervoltage Threshold :
V
V
where the hysteresis voltage V
2
3
2
OTH
UTH
______________________________________________________________________________________
62 2
(
(
1 068
1 006
(
R
294
R
R
to 1M
.
.
1
3
.
2
294
4.484V
2
(V
(V
5.474V
+
k
k
+
REF
M
M
REF
R
k
R
IN
1 068
2
R
294
+ .
61 9
3
1
.
range are usually easier to deal with.
+
)
is falling. The design equation is as
1 068
.
k
R
61 9
k
V )
V )
M
3
R
H
H
can be used, but values in the
.
)
2
V
M
( %
1
k
REF
V
)
(
OTH
s
( .
V
(R1
1 182
(R1
IN
REF
tan
V
V
( .
UTH
1 182
is rising. The design
(R1 + R2)
dard value
H
H
5 5
R2
V
R2
R1
.
H
4 5
1
)
.
0 005
V
REF
.
0 005
.
).
R3)
R3)
R
1
)
)
R5
R4
1
.
The switchover from line-powered DC to a backup
battery is often accomplished with diodes. But this
simple method is sometimes unacceptable, due to the
voltage drop and associated power loss across the
diode in series with the battery. Figure 6’s circuit
replaces the diode with a P-channel MOSFET
controlled by one of the MAX9_3 comparator outputs.
When the DC wall adapter drops below 4V (determined
by R1 and R2), OUTA goes low, turning on Q1.
Comparator B is used to measure the battery voltage,
and gives a “low-battery” indication when the battery
drops below 3.6V.
Figure 7 shows a circuit to shift from bipolar ±5V inputs
to single-ended 5V outputs. The 10k
the comparator inputs, and do not materially affect the
circuit’s operation.
Figure 5. Window Detector
62.2k
294k
1M
R1
R3
R2
V
IN
10k
R4
2.4M
R5
V
V
OTH
UTH
= 5.5V
= 4.5V
3
5
6
4
INA+
HYST
REF
INB-
Battery Switchover Circuit
2
V-
MAX9_3
7
5V
V+
OUTA
OUTB
Level-Shifter
resistors protect
POWER GOOD
1
8
1M