Data Sheet ADCMP391
THEORY OF OPERATION
BASIC COMPARATOR
In its most basic configuration, a comparator can be used to
convert an analog input signal to a digital output signal (see
Figure 19). The analog signal on IN+ is compared to the voltage
on IN−, and the voltage at OUT is either high or low, depending
on whether IN+ is at a higher or lower potential than IN−,
respectively.
Figure 19. Basic Comparator and Input and Output Signals
RAIL-TO-RAIL INPUT (RRI)
Using a CMOS nonRRI stage (that is, a single differential pair)
limits the input voltage to approximately one gate-to-source
voltage (V
GS
) away from one of the supply lines. Because V
GS
for
normal operation is commonly more than 1 V, a single differential
pair input stage comparator greatly restricts the allowable input
voltage. This restriction can be quite limiting with low voltage
supplies. To resolve this issue, RRI stages allow the input signal
range to extend up to the supply voltage range. In the case of the
ADCMP391, the inputs continue to operate 200 mV beyond the
supply rails.
OPEN-DRAIN OUTPUT
The ADCMP391 has an open-drain output stage that requires
an external resistor to pull up to the logic high voltage level
when the output transistor is switched off. The pull-up resistor
must be large enough to avoid excessive power dissipation, but
small enough to switch logic levels reasonably quickly when the
comparator output is connected to other digital circuitry. The
rise time of the open-drain output depends on the pull-up
resistor (R
PULLUP
) and load capacitor (C
L
) used.
The rise time can be calculated by
t
R
= 2.197 R
PULLUP
C
L
(1)
POWER-UP BEHAVIOR
On power-up, when V
CC
reaches 0.9 V, t h e ADCMP391 is
guaranteed to assert an output low logic. When the voltage on
the V
CC
pin exceeds UVLO, the comparator inputs take control.
CROSSOVER BIAS POINT
Rail-to-rail inputs of this type of architecture, in both op amps
and comparators, have a dual front-end design. PMOS devices
are inactive near the V
CC
rail, and NMOS devices are inactive near
GND. At some predetermined point in the common-mode range, a
crossover occurs. At this point, normally 0.8 V and V
CC
− 0.8 V, the
measured offset voltages change.
COMPARATOR HYSTERESIS
In noisy environments, or when the differential input amplitudes
are relatively small or slow moving, adding hysteresis (V
HYST
) to
the comparator is often desirable. The transfer function for a
comparator with hysteresis is shown in Figure 20. As the input
voltage approaches the threshold (0 V in Figure 20) from below
the threshold region in a positive direction, the comparator
switches from low to high when the input crosses +V
HYST
/2. The
new switch threshold becomes −V
HYST
/2. The comparator remains
in the high state until the −V
HYST
/2 threshold is crossed from
below the threshold region in a negative direction. In this
manner, noise or feedback output signals centered on the 0 V
input cannot cause the comparator to switch states unless it
exceeds the region bounded by ±V
HYST
/2.
Figure 20. Comparator Hysteresis Transfer Function
OUT
V
IN
V
REF
IN+
IN–
V
CC
V+
V+
V
REF
V
IN
V
OUT
0V
t
12206-019
OUTPUT
INPUT
0V
V
OL
V
OH
+V
HYST
2
–V
HYST
2
12206-020
Rev. 0 | Page 9 of 15
