LMV331 データシートの表示（PDF） - ON Semiconductor

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LMV331

Single, Dual, Quad General Purpose, Low Voltage Comparators

ON Semiconductor 

LMV331, LMV393, LMV339

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

R1

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

the inverting input is higher, then the output transistor will

VIN

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

V+

between 1 kW 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

R2

input voltage (VIN) on the non−inverting input to the

reference voltage (VREF) on the inverting input. If VIN is less

than VREF, the output voltage (VO) will be low. If VIN is

greater than VREF, then VO will be high.

+VCC

−

+

R3

RPULL−UP

VO

RLOAD

Figure 20. Inverting

Comparator with

Hysteresis

VOUT

V+

VREF

0V

VIN

+VIN

+

Time

V+

3.0 k

VO

+VREF

−

Figure 19.

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.

When VIN is less than the voltage at the non−inverting

node, V+, the output voltage will be high. When VIN is

greater than the voltage at V+, then the output will be low.

The hysteresis band (Figure 21) created from the resistor

network is defined as:

DV) + VT1 * VT2

where VT1 and VT2 are the lower and upper trip points,

respectively.

VCC

VO

VT2

VT1

0

VIN

Figure 21.

VT1 is calculated by assuming that the output of the

comparator is pulled up to supply when high. The

resistances R1 and R3 can be viewed as being in parallel

which is in series with R2 (Figure 22). Therefore VT1 is:

VT1

+

ǒR1

VCC R2

ø R3Ǔ )

R2

VT2 is calculated by assuming that the output of the

comparator is at ground potential when low. The resistances

R2 and R3 can be viewed as being in parallel which is in

series with R1 (Figure 23). Therefore VT2 is:

VT2

+

VCCǒR2 ø R3Ǔ

R1 ) ǒR2 ø R3Ǔ

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