MAX976/MAX978/MAX998
Single/Dual/Quad, SOT23, Single-Supply,
High-Speed, Low-Power Comparators
Window Comparator
V
CC
R3
The MAX976 is ideal for making a window detector
(undervoltage/overvoltage detector). The schematic
shown in Figure 3 uses a MAX6120 reference and com-
ponent values selected for a 2.0V undervoltage thresh-
old and a 2.5V overvoltage threshold. Choose different
thresholds by changing the values of R1, R2, and R3.
OUTA provides an active-low undervoltage indication,
and OUTB gives an active-low overvoltage indication.
ANDing the two outputs provides an active-high,
power-good signal. The design procedure is as follows:
R1
0.1µF
V
IN
V
CC
OUT
R2
GND
MAX976
MAX978
MAX998
V
REF
1) Select R1. The leakage current into INB- is normally
75nA, so the current through R1 should exceed
1.0µA for the thresholds to be accurate. R1 values in
the 50kΩ to 100kΩ range are typical.
Figure 2. Additional Hysteresis
2) Choose the hysteresis band required (V ). For this
HB
example, choose 100mV.
2) Choose the overvoltage threshold (V
) when V
IN
OTH
is rising, and calculate R2 and R3 with the following
formula:
3) Calculate R1. R1 = R3 x (V / V ). Plugging in the
HB
CC
values for this example,
R
SUM
= R2 + R3 = R1 x [V
/ (V
+ V ) - 1]
REF H
OTH
R1 = 1.2MΩ x (100mV / 5.0V) = 24kΩ
where V = 1/2V
.
HYST
H
4) Choose the trip point for V rising. This is the
IN
threshold voltage at which the comparator switches
3) Choose the undervoltage threshold (V
) when V
IN
UTH
from low to high as V rises above the trip point. In
IN
this example, choose 3.0V.
is falling, and calculate R2 with the following formula:
R2 = (R1 + R ) x [(V - V ) / V ] - R1
SUM
REF
H
UTH
5) Calculate R2 as follows:
where V = 1/2V
.
HYST
H
1
4) Calculate R3 with the following formula:
R3 = (R ) - R2
R2 =
⎛
⎞
⎟
V
1
R1
1
R3
THR
x R1
SUM
−
−
⎜
V
⎝
⎠
5) Verify the resistor values. The equations are as follows:
REF
1
V
OTH
V
UTH
= (V
= (V
+ V ) x (R1 + R2 + R3) / R1
H
REF
REF
R2 =
= 16.2kΩ
⎛
⎜
⎞
3.0V
1
1
1.2M
- V ) x (R1 + R2 + R3) / (R1 + R2)
H
−
−
⎟
⎝1.2 x 24kΩ⎠
24kΩ
R3
82.1kΩ
1%
V
CC
Choose a standard value for R2 of 16kΩ.
6) Verify the trip voltage and hysteresis as follows:
1
2
0.1μF
V
IN
8
1/2
UNDERVOLTAGE
R2
⎛
⎞
1
R1
1
R2
1
R3
24.9kΩ
MAX976
7
V
rising: V
= V
x R1 x
+
+
IN
THR
REF
⎜
⎝
⎟
⎠
5
1%
V
1
CC
⎛
⎞
R1 x V
POWER GOOD
CC
V
falling: V
= V
−
IN
THF
THR
⎜
⎝
⎟
⎠
R3
3
4
2
MAX6120
3
Hysteresis = V
− V
THF
THR
1/2
6
OVERVOLTAGE
MAX976
IR Receiver
The Typical Operating Circuit shows an application using
the MAX998 as an infrared receiver. The infrared photo-
diode creates a current relative to the amount of infrared
R1
100kΩ
1%
light present. This current creates a voltage across R .
D
When this voltage level crosses the voltage applied by the
voltage divider to the inverting input, the output transitions.
Figure 3. Window Comparator
Maxim Integrated
7
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