Datasheet

VOLTS
V
REF
V
O
TIME
V
IN
V
+
V
REF
V
IN
V
O
-
+
V
-
LMV7271
SNOSA56H –FEBRUARY 2003–REVISED FEBRUARY 2013 www.ti.com
APPLICATION NOTES
BASIC COMPARATOR
A comparator is often used to convert an analog signal to a digital signal. As shown in Figure 28, 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 27. LMV7271
Figure 28. LMV7271 Basic Comparator
RAIL-TO-RAIL INPUT STAGE
The LMV727X 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 curves for V
OS
vs. V
CM
in Typical
Performance Characteristics section. 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 LMV727X ideal in power supply
monitoring circuits, where the comparators are used to sense signals close to ground and power supplies.
OUTPUT STAGE
The LMV7271 and LMV7272 have 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 the 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 section for supply bypassing in the Application Notes for
details.
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