MAX974C/D View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX974C/D Datasheet PDF : 16 Pages
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Ultra-Low-Power, Open-Drain,
Single/Dual-Supply Comparators
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 VIN is rising. The design
equation is as follows:
R2
+
R3
=
R1
×
VOTH
VREF +
VH
−
1
=
294k
×
5.5
(1.182 + 0.005)
−
1
= 1.068MΩ
4) Calculate R2. The undervoltage threshold should be
4.5V when VIN is falling. The design equation is as
follows:
R2 = (R1 + R2 + R3) × (VREF − VH) − R1
VUTH
= (294k + 1.068M) × (1.182 − 0.005) − 294k
4.5
= 62.2kΩ
Choose R2 = 61.9kΩ (1% standard value).
5) Calculate R3:
R3 = (R2 + R3) − R2
= 1.068M − 61.9k
= 1.006MΩ
Choose R3 = 1MΩ (1% standard value)
6) Verify the resistor values. The equations are as
follows, evaluated for the above example:
Overvoltage Threshold :
VOTH
=
(VREF
+ VH) ×
(R1 + R2 + R3)
R1
= 5.474V
Undervoltage Threshold :
VUTH
=
(VREF
− VH) ×
(R1 + R2 + R3)
(R1 + R2)
= 4.484V
where the hysteresis voltage VH
=
VREF ×
R5 .
R4
VIN
R3
1M
VOTH = 5.5V
VUTH = 4.5V
3 INA+
5 HYST
R2
R5
62.2k
10k
R4 6 REF
2.4M
4 INB-
R1
V-
294k
2
+5V
7
V+
1M
OUTA 1
OUTB 8
POWER GOOD
MAX9_3
Figure 5. Window Detector
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.
______________________________________________________________________________________ 11
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