Rainbow Electronics MAX984 - Manual (Page 11)

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:
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 10k
Ω
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
= + 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
MAX971–MAX974/MAX981–MAX984
Ultra-Low-Power, Open-Drain,
Single/Dual-Supply Comparators
______________________________________________________________________________________ 11
MAX9_3
INB-
REF
HYST
INA+ 3
5
6
4
1
8
V-
2
V+
7
1M
OUTA
OUTB
R5
10k
R1
294k
R2
62.2k
R3
1M
POWER GOOD
V IN
V OTH = 5.5V
V UTH = 4.5V
R4
2.4M
+5V
Figure 5. Window Detector