Application Circuits
Basic Comparator
A basic comparator circuit is used for converting analog sig-
nals to a digital output. The LMV331/393/339 have an
open-collector output stage, which requires a pull-up resistor
to a positive supply voltage for the output to switch properly.
When the internal output transistor is off, the output voltage
will be pulled up to the external positive voltage.
The output pull-up resistor should be chosen high enough so
as to avoid excessive power dissipation yet low enough to
supply enough drive to switch whatever load circuitry is used
on the comparator output. On the LMV331/393/339 the
pull-up resistor should range between 1k to 10kΩ.
The comparator compares the input voltage (V
in
) at the
non-inverting pin to the reference voltage (V
ref
) at the invert-
ing pin. If V
in
is less than V
ref
, the output voltage (V
o
) is at the
saturation voltage. On the other hand, if V
in
is greater than
V
ref
, the output voltage (V
o
) is at V
cc.
.
DS100080-4
DS100080-26
FIGURE 1. Basic Comparator
Comparator with Hysteresis
The basic comparator configuration may oscillate or produce
a noisy output if the applied differential input voltage is near
the comparator’s offset voltage. This usually happens when
the input signal is moving very slowly across the compara-
tor’s switching threshold. This problem can be prevented by
the addition of hysteresis or positive feedback.
Inverting Comparator with Hysteresis
The inverting comparator with hysteresis requires a three re-
sistor network that are referenced to the supply voltage V
cc
of the comparator. When Vin at the inverting input is less
than V
a
, the voltage at the non-inverting node of the com-
parator (V
in
<
V
a
), the output voltage is high (for simplicity
assume V
o
switches as high as V
cc
). The three network re-
sistors can be represented as R
1
//R
3
in series with R
2
. The
lower input trip voltage V
a1
is defined as
When V
in
is greater than Va (V
in
V
a
), the output voltage is
low very close to ground. In this case the three network re-
sistors can be presented as R
2
//R
3
in series with R
1
. The up-
per trip voltage V
a2
is defined as
The total hysteresis provided by the network is defined as
∆V
a
= V
a1
- V
a2
To assure that the comparator will always switch fully to V
cc
and not be pulled down by the load the resistors values
should be chosen as follow:
R
pull-up
<<
R
load
and R
1
>
R
pull-up
.
7
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