Datasheet

VOLTS
V
REF
V
O
TIME
V
IN
V
+
V
REF
V
IN
V
O
-
+
V
-
LMV7291
www.ti.com SNOSA86E –FEBRUARY 2004–REVISED MARCH 2013
APPLICATION NOTES
BASIC COMPARATOR
A comparator is often used to convert an analog signal to a digital signal. As shown in Figure 26, the comparator
compares an input voltage (V
IN
) to a reference voltage (V
REF
). If V
IN
is less than V
REF
, the output (V
O
) is low.
However, if V
IN
is greater than V
REF
, the output voltage (V
O
) is high.
Figure 26. LMV7291 Basic Comparator
RAIL-TO-RAIL INPUT STAGE
The LMV7291 has an input common mode voltage range (V
CM
) of −0.1V below the V
−
to 0.1V above V
+
. This is
achieved by using paralleled PNP and NPN differential input pairs. When the V
CM
is near V
+
, the NPN pair is on
and the PNP pair is off. When the V
CM
is near V
−
, the NPN pair is off and the PNP pair is on. The crossover point
between the NPN and PNP input stages is around 950mV from V
+
. Since each input stage has its own offset
voltage (V
OS
), the V
OS
of the comparator becomes a function of the V
CM
. See Figure 3, Figure 4, and Figure 5 in
Typical Performance Characteristics. In application design, it is recommended to keep the V
CM
away from the
crossover point to avoid problems. The wide input voltage range makes LMV7291 ideal in power supply
monitoring circuits, where the comparators are used to sense signals close to gnd and power supplies.
OUTPUT STAGE
The LMV7291 has a push-pull output stage. This output stage keeps the total system power consumption to the
absolute minimum. The only current consumed is the low supply current and the current going directly into the
load. When output switches, both PMOS and NMOS at the output stage are on at the same time for a very short
time. This allows current to flow directly between V
+
and V
−
through output transistors. The result is a short spike
of current (shoot-through current) drawn from the supply and glitches in the supply voltages. The glitches can
spread to other parts of the board as noise. To prevent the glitches in supply lines, power supply bypass
capacitors must be installed. See CIRCUIT TECHNIQUES FOR AVOIDING OSCILLATIONS IN COMPARATOR
APPLICATIONS for details.
HYSTERESIS
It is a standard procedure to use hysteresis (positive feedback) around a comparator, to prevent oscillation, and
to avoid excessive noise on the output because the comparator is a good amplifier of its own noise.
Inverting Comparator with Hysteresis
The inverting comparator with hysteresis requires a three resistor network that are referenced to the supply
voltage V
CC
of the comparator (Figure 27). When V
IN
at the inverting input is less than V
A
, the voltage at the non-
inverting node of the comparator (V
IN
< V
A
), the output voltage is high (for simplicity assume V
O
switches as high
as V
CC
). The three network resistors can be represented as R
1
||R
3
in series with R
2
. The lower input trip voltage
V
A1
is defined as
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