LMV331, LMV393, LMV339

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11

APPLICATION CIRCUITS

Basic Comparator Operation

The basic operation of a comparator is to compare two

input voltage signals, and produce a digital output signal by

determining which input signal is higher. If the voltage on

the non−inverting input is higher, then the internal output

transistor is off and the output will be high. If the voltage on

the inverting input is higher, then the output transistor will

be on and the output will be low. The LMV331/393/339 has

an open−drain output stage, so a pull−up resistor to a positive

supply voltage is required for the output to switch properly.

The size of the pull−up resistor is recommended to be

between 1 k

W and 10 kW. This range of values will balance

two key factors; i.e., power dissipation and drive capability

for interface circuitry.

Figure 19 illustrates the basic operation of a comparator

and assumes dual supplies. The comparator compares the

Figure 19.

+

−

3.0 k

0 V

Time

Comparators and Stability

A common problem with comparators is oscillation due to

their high gain. The basic comparator configuration in

Figure 19 may oscillate if the differential voltage between

the input pins is close to the device’s offset voltage. This can

happen if the input signal is moving slowly through the

comparator’s switching threshold or if unused channels are

connected to the same potential for termination of unused

channels. One way to eliminate output oscillations or

‘chatter’ is to include external hysteresis in the circuit

design.

Inverting Configuration with Hysteresis

An inverting comparator with hysteresis is shown in

Figure 20.

Figure 20. Inverting

Comparator with

Hysteresis

+

−

R3

The hysteresis band (Figure 21) created from the resistor

network is defined as:

respectively.

0

Figure 21.

comparator is pulled up to supply when high. The

comparator is at ground potential when low. The resistances