NCS2250, NCV2250, NCS2252, NCV2252
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9
APPLICATION INFORMATION
Input Stage
The NCS2250 and NCS2252 have rail−to−rail inputs. The
input common mode voltage range of these comparators
extend 200 mV beyond the rails, allowing voltage sensing
at ground or at the supply voltage.
Output Stage
The NCS2250 has a complementary, push−pull output
stage. When the output transitions between high and low
states, a low resistance path is created between the positive
and negative supply rails, temporarily increasing the supply
current during the transition.
The NCS2252 has an open−drain output stage. This
allows the output to be connected through a pull−up resistor
to another supply voltage for applications where level
translation or level shifting is needed. The output resistor
can be connected to voltages below V
DD
or up to V
DD
+ 0.3
V. Since the NCS2252 relies on an external pull−up resistor
to provide sourcing current, the timing of the output
low−to−high transition is determined by the RC time
constant of the pull−up resistor and the load capacitance.
Hysteresis
When the inputs are near the same voltage, slight voltage
fluctuations due to noise can cause the output to oscillate
between high and low states. If noise−induced switching
behavior is observed at the output, hysteresis should be
added through an external resistor network. This is
particularly the case for NCS2250, as sustained output
oscillations causing increased supply current will result in
elevated junction temperature.
Hysteresis can be added to the circuit by adding one or two
external resistors depending on whether an inverting or
non−inverting configuration is needed. Figure 17 shows the
inverting configuration. In this configuration, the output
voltage adjusts the threshold at the IN+ pin.
Figure 17. Comparator with Hysteresis, Inverting Configuration
+
−
NCS2250
R
F
R
1
R
2
V
IN
For the inverting configuration, the value of the
high−level input voltage which triggers the output to switch
from high to low is given by the following equation:
V
IN_high
+
R
1
R
F
R
1
R
F
) R
1
R
2
) R
2
R
F
V
DD
(eq. 1)
The value of the low−level input voltage which triggers
the output to switch from low to high is given by the
following equation:
V
IN_low
+
R
1
R
F
) R
1
R
2
R
1
R
F
) R
1
R
2
) R
2
R
F
V
DD
(eq. 2)
