LMV331 datasheet(7/21 Pages) NSC

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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 10k

Ω.

The comparator compares the input voltage (V

in)atthe

non-inverting pin to the reference voltage (V

ref) at the invert-

ing pin. If V

in is less than Vref, the output voltage (Vo)isatthe

saturation voltage. On the other hand, if V

in is greater than

ref, the output voltage (Vo)isatVcc..

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

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 < Va), the output voltage is high (for simplicity

assume V

o switches as high as Vcc). The three network re-

sistors can be represented as R

1//R3 in series with R2. The

lower input trip voltage V

a1 is defined as

When V

in is greater than Va (Vin Va), the output voltage is

low very close to ground. In this case the three network re-

sistors can be presented as R

2//R3 in series with R1. The up-

per trip voltage V

a2 is defined as

The total hysteresis provided by the network is defined as

∆V

a = Va1 -Va2

To assure that the comparator will always switch fully to V

and not be pulled down by the load the resistors values

should be chosen as follow:

pull-up << Rload

and R

1 > Rpull-up.

DS100080-26

DS100080-4

FIGURE 1. Basic Comparator

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