Datasheet

MAX971–MAX974/MAX981–MAX984
Ultra-Low-Power, Open-Drain,
Single/Dual-Supply Comparators
12 ______________________________________________________________________________________
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 10MΩ can be used, but values in the
100kΩ to 1MΩ range are usually easier to deal with.
In this example, choose R1 = 294kΩ.
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 = 1MΩ (1% standard value)
6) Verify the resistor values. The equations are as
follows, evaluated for the above example:
Battery Switchover Circuit
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.
Level-Shifter
Figure 7 shows a circuit to shift from bipolar ±5V inputs
to single-ended 5V outputs. The 10kΩ resistors protect
the comparator inputs, and do not materially affect the
circuit’s operation.
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
=+×
++
=
=−×
++
=
=× .
RRRR
Mk
M
3232
1 068 61 9
1 006
( + )
. .
.
=−
=Ω−Ω
=Ω
RRRR
VV
V
R
kM
k
k
Choose R k s dard value
REF H
UTH
2123 1
294 1 068
1 182 0 005
45
294
62 2
26191
( + + )
( )
( + . )
(. . )
.
.
. (% ).
=×
−
−
=Ω Ω×
−
−Ω
=Ω
=Ω tan
RR R
V
VV
k
M
OTH
REF H
231 1
294
55
1 182 0 005
1
1 068
.
(. . )
.
+=×
+
−






=Ω×
+
−






=Ω
MAX9_3
INB-
REF
HYST
INA+
3
5
6
4
1
8
V-
2
V+
7
1MΩ
OUTA
OUTB
R5
10kΩ
R1
294kΩ
R2
62.2kΩ
R3
1MΩ
POWER GOOD
V
IN
V
OTH
= 5.5V
V
UTH
= 4.5V
R4
2.4MΩ
5V
Figure 5. Window Detector