Rainbow Electronics MAX970 - Manual (Page 11)

MAX965–MAX970
Single/Dual/Quad, Micropower,
Ultra-Low-Voltage, Rail-to-Rail I/O Comparators
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3) Calculate R1: R1 = (R3 + R4) x (V HB / V CC ). Putting
in the values for this example, R1 = (2.2M
Ω
+ 10k) x
(50mV / 5.0V) = 22.1k
Ω
.
4) Choose the trip point for V IN rising. This is the
threshold voltage where the comparator output tran-
sitions from low to high as V
IN rises above the trip
point. For this example, choose 3.0V.
5) Calculate R2 as follows:
where V THR is the rising-voltage trip threshold.
Choose a standard value of 15k
Ω
.
6) Verify trip voltages and hysteresis as follows:
where V THR is the rising-voltage trip point, and V THF
is the falling-voltage trip point.
Circuit Layout and Bypassing
Power-supply bypass capacitors are not needed if sup-
ply impedance is low, but 100nF bypass capacitors
should be used when supply impedance is high or
when supply leads are long. Minimize signal lead
lengths to reduce stray capacitance between the input
and output that might cause instability.
IR Receiver
Figure 5 shows an application using the MAX965 as an
infrared receiver. The infrared photodiode creates a
current relative to the amount of infrared light present.
This current creates a voltage across R1. When this
voltage level crosses the reference voltage applied to
the inverting input, the output transitions. Optional R3
provides additional hysteresis for noise immunity.
2-Cell to TTL Logic-Level Shifter
Figure 6 shows an application using the MAX965
to convert a 2-cell voltage-level signal into a TTL-
compatible signal. The supply voltage for the compara-
tor comes from the 2-cell supply. The output is pulled
up to a 5V supply.
V ri g
V V x R x
R R R R
V falling
V V
R x V
R R
Hysteresis V V
IN
THR REF
IN
THF THR
CC
THR THF
sin :
:
= + +
+

 

= −
+

 

= −
1
1
1
1
2
1
3 4
1
3 4
R
V
V x R R R R
R
V
x k k M k
k
THR
REF
2
1
1
1
1
1
3 4
2
1
3 0
1 2 22
1
22
1
2 2 10
14 76
=

 

− −
+
=

 

− −
+
=
.
. .
.
GND
V
CC
V
CC
V CC V CC
0.1 µ F
OUT
R3
R D
R PULL-UP
REF
MAX965
HYST
Figure 5. IR Receiver
GND
V
CC
+5V 2 CELLS 0.1 µ F
OUT
INPUT
REF
MAX965
HYST
Figure 6. 2-Cell to TTL Logic-Level Translator