MAX976/MAX978/MAX998
Single/Dual/Quad, SOT23, Single-Supply,
High-Speed, Low-Power Comparators
7
Maxim Integrated
2) Choose the hysteresis band required (V
HB
). For this
example, choose 100mV.
3) Calculate R1. R1 = R3 x (V
HB
/ V
CC
). Plugging in the
values for this example,
R1 = 1.2MΩ x (100mV / 5.0V) = 24kΩ
4) Choose the trip point for V
IN
rising. This is the
threshold voltage at which the comparator switches
from low to high as V
IN
rises above the trip point. In
this example, choose 3.0V.
5) Calculate R2 as follows:
Choose a standard value for R2 of 16kΩ.
6) Verify the trip voltage and hysteresis as follows:
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
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.
Window Comparator
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:
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.
2) Choose the overvoltage threshold (V
OTH
) when V
IN
is rising, and calculate R2 and R3 with the following
formula:
R
SUM
= R2 + R3 = R1 x [V
OTH
/ (V
REF
+ V
H
) - 1]
where V
H
= 1/2V
HYST
.
3) Choose the undervoltage threshold (V
UTH
) when V
IN
is falling, and calculate R2 with the following formula:
R2 = (R1 + R
SUM
) x [(V
REF
- V
H
) / V
UTH
] - R1
where V
H
= 1/2V
HYST
.
4) Calculate R3 with the following formula:
R3 = (R
SUM
) - R2
5) Verify the resistor values. The equations are as follows:
V
OTH
= (V
REF
+ V
H
) x (R1 + R2 + R3) / R1
V
UTH
= (V
REF
- V
H
) x (R1 + R2 + R3) / (R1 + R2)
