LM139/LM239/LM339/LM2901/LM3302
Typical Performance Characteristics
Response Time for Various
Input Overdrives — Negative
Transition
LM2901 (Continued)
Response Time for Various
Input Overdrives-Positive
Transition
DS005706-42
DS005706-43
Application Hints
The LM139 series are high gain, wide bandwidth devices
which, like most comparators, can easily oscillate if the out-
put lead is inadvertently allowed to capacitively couple to the
inputs via stray capacitance. This shows up only during the
output voltage transition intervals as the comparator
changes states. Power supply bypassing is not required to
solve this problem. Standard PC board layout is helpful as it
reduces stray input-output coupling. Reducing this input re-
sistors to
<
10 kΩ reduces the feedback signal levels and fi-
nally, adding even a small amount (1 to 10 mV) of positive
feedback (hysteresis) causes such a rapid transition that os-
cillations due to stray feedback are not possible. Simply
socketing the IC and attaching resistors to the pins will cause
input-output oscillations during the small transition intervals
unless hysteresis is used. If the input signal is a pulse wave-
form, with relatively fast rise and fall times, hysteresis is not
required.
All pins of any unused comparators should be tied to the
negative supply.
The bias network of the LM139 series establishes a drain
current which is independent of the magnitude of the power
supply voltage over the range of from 2 V
DC
to 30 V
DC
.
It is usually unnecessary to use a bypass capacitor across
the power supply line.
The differential input voltage may be larger than V
+
without
damaging the device. Protection should be provided to pre-
vent the input voltages from going negative more than −0.3
V
DC
(at 25˚C). An input clamp diode can be used as shown
in the applications section.
The output of the LM139 series is the uncommitted collector
of a grounded-emitter NPN output transistor. Many collectors
can be tied together to provide an output OR’ing function. An
output pull-up resistor can be connected to any available
power supply voltage within the permitted supply voltage
range and there is no restriction on this voltage due to the
magnitude of the voltage which is applied to the V
+
terminal
of the LM139A package. The output can also be used as a
simple SPST switch to ground (when a pull-up resistor is not
used). The amount of current which the output device can
sink is limited by the drive available (which is independent of
V
+
) and the
β
of this device. When the maximum current limit
is reached (approximately 16 mA), the output transistor will
come out of saturation and the output voltage will rise very
rapidly. The output saturation voltage is limited by the ap-
proximately 60Ω R
SAT
of the output transistor. The low offset
voltage of the output transistor (1 mV) allows the output to
clamp essentially to ground level for small load currents.
Typical Applications
Basic Comparator
(V
+
= 5.0 V
DC
)
Driving CMOS
Driving TTL
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DS005706-5
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