Application information – Diodes LM2901/ LM2901A/ LM2903/ LM2903A User Manual
Page 8
LM2901/ LM2901A/ LM2903/ LM2903A
Document number: DS36779 Rev. 1 - 2
8 of 15
March 2014
© Diodes Incorporated
LM2901/ LM2901A/ LM2903/ LM2903A
Application Information
General Information
The LM2901/2903 series comparators are high gain, wide bandwidth devices, like most comparators, can easily oscillate if the output lead is
inadvertently allowed to capacitive couple to the inputs via stray capacitance. This shows up only during the output voltage transition intervals as
the comparator change states. Standard PC board layout is helpful as it reduces stray input-output coupling. Reducing the input resistors to < 10kΩ
reduces the feedback signal levels and finally, adding even a small amount (1.0 to 10 mV) of positive feedback (hysteresis) causes such a rapid
transition that oscillations 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 waveform, with relatively fast rise and fall
times, hysteresis is not required. All input pins of any unused comparators should be tied to the negative supply.
The bias network of the LM2901/2903 series comparators establishes a quiescent current independent of the magnitude of the power supply
voltage over the range of from 2.0 V
DC
to 30 V
DC
.
The differential input voltage may be larger than V
CC
without damaging the device (Note16). Protection should be provided to prevent the input
voltages from going negative more than -0.3 V
DC
(@ 25°C). An input clamp diode can be used as shown in the applications section.
The output of the LM2901/2903 series comparators 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 applied to the V
CC
terminal of
LM2901/2903 series comparator 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 the output device can sink is limited by the drive available (which is independent of V
CC
) and the β of this device. When the
maximum current limit is reached (approximately 16mA), the output transistor will come out of saturation and the output voltage will rise very
rapidly. The output saturation voltage is limited by the approximately 60Ω R
SAT
of the output transistor. The low offset voltage of the output transistor
(1.0 mV) allows the output to clamp essentially to ground level for small load currents.