LMV761MA NSC [National Semiconductor], LMV761MA Datasheet - Page 8

• Manufacturer Part Number: LMV761MA
• Description: Low Voltage, Precision Comparator with Push-Pull Output
• Manufacturer: NSC [National Semiconductor]

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Application Hints
Basic Comparator
A basic comparator circuit is used to convert analog input
signals to digital output signals. The comparator compares
an input voltage (V
ence voltage (V
V
than V
Hysteresis
The basic comparator configuration may oscillate or produce
a noisy output if the applied differential input is near the
comparator’s input offset voltage. This tends to occur when
the voltage on one input is equal or very close to the other
input voltage. Adding hysteresis can prevent this problem.
Hysteresis creates two switching thresholds (one for the
rising input voltage and the other for the falling input volt-
age). Hysteresis is the voltage difference between the two
switching thresholds. When both inputs are nearly equal,
hysteresis causes one input to effectively move quickly past
the other. Thus, moving the input out of the region in which
oscillation may occur.
Hysteresis can easily be added to a comparator in a non-
inverting configuration with two resistors and positive feed-
back Figure 2. The output will switch from low to high when
V
The output will switch from high to low when V
where V
The Hysteresis is the difference between V
REF
IN
= ((V
rises up to V
the output (V
REF
IN2
REF
, the output voltage (V
is calculated by
(R
V
IN2
FIGURE 1. Basic Comparator
1
+R
REF
IN1
V
= (V
2
O
IN1
IN
))/R
) at the inverting pin. If V
, where V
) is low (V
) at the non-inverting input to the refer-
∆V
REF
= (V
2
= V
IN
)-((V
(R
= V
REF
CC
1
+R
REF
IN1
IN1
OL
(R
R
2
(R
1
) – V
). However, if V
O
1
is calculated by
/R
- V
+R
) is high (V
1
+R
2
IN2
2
))/R
CC
2
)) - (V
R
2
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1
IN1
)/R
OH
CC
IN
IN
2
and V
IN
).
falls to V
is less than
R
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1
is greater
))/R
IN2
2
.
)
IN2
,
8
Input
The LMV761/762 have near zero input bias current. This
allows very high resistance circuits to be used without any
concern for matching input resistances. This also allows the
use of very small capacitors in R-C type timing circuits. This
reduces the cost of the capacitors and amount of board
space used.
Shutdown Mode
The LMV761 features a low-power shutdown pin that is
activated by driving SD low. In shutdown mode, the output is
in a high impedance state, supply current is reduced to 20nA
and the comparator is disabled. Driving SD high will turn the
comparator on. The SD pin should not be left unconnected
due to the fact that it is a high impedance input. When left
unconnected, the output will be at an unknown voltage. Also
do not three-state the SD pin.
The maximum input voltage for SD is 5.5V, referred to
ground and is not limited by V
logic to drive SD while V
as 3V. The logic threshold limits for SD are proportional to
V
Board Layout and Bypassing
The LMV761/762 is designed to be stable and oscillation
free, but it is still important to include the proper bypass
capacitors and ground pickups. Ceramic 0.1µF capacitors
should be placed at both supplies to provide clean switching.
Minimize the length of signal traces to reduce stray capaci-
tance.
CC
FIGURE 2. Non-Inverting Comparator Configuration
.
CC
operates at a lower voltage, such
CC
. This allows the use of 5V
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