MPXM2102AS datasheet(454/670 Pages) MOTOROLA

454 / 670 page

AN1517

3–308

Motorola Sensor Device Data

www.motorola.com/semiconductors

threshold design, positive feedback can be used to provide

hysteresis for both switching points. The window compara-

tor and the other comparator circuits will be explained in the

following section.

EXAMPLE COMPARATOR CIRCUITS

Several comparator circuits were built and evaluated.

Comparator stages using the LM311 comparator, LM358

Op–Amp (with and without an output transistor stage), and

LM339 were examined. Each comparator was evaluated on

output voltage levels (dynamic range), transition speed, and

the relative component count required for the complete

pressure switch design.

This comparison is tabulated in

Table 2.

Figure 2. LM311 Comparator Circuit Schematic

Vin

RPU

Vout

LM311

VCC

LM311 Used in a Comparator Circuit

The LM311 chip is designed specifically for use as a

comparator and thus has short delay times, high slew rate,

and an open collector output. A pull–up resistor at the output

is all that is needed to obtain a rail–to–rail output. Additionally,

the LM311 is a reverse logic circuit; that is, for an input lower

than the reference voltage, the output is high. Likewise, when

the input voltage is higher than the reference voltage, the

output is low. Figure 2 shows a schematic of the LM311 stage

with threshold setting resistor divider, hysteresis resistor, and

the open–collector pull–up resistor. Table 2 shows the

comparator’s performance. Based on its performance, this

circuit can be used in many types of applications, including

interface to microprocessors.

The amount of hysteresis can be calculated by the following

equations:

REF +

) R2

neglecting the effect of R

REFH +

R1R2

) R2R

R1R2

) R1R

H )

R2R

REFL +

R2R

R1R2

) R1R

H )

R2R

HYSTERESIS

+ V

REF *

REFL

HYSTERESIS

+ V

REFH *

REF

when the normal state is below V

REF

,or

when the normal state is above V

REF

Table 2. Comparator Circuits Performance Characteristics

Characteristic

LM311

LM358

LM358 w/ Trans.

Unit

Switching Speeds

Rise Time

1.40

5.58

2.20

µs

Fall Time

0.04

6.28

1.30

µs

Output Levels

VOH

4.91

3.64

5.00

VOL

61.1

38.0

66.0

Circuit Logic Type

NEGATIVE

POSITIVE

The initial calculation for VREF will be slightly in error due to

neglecting the effect of RH. To establish a precise value for

VREF (including RH in the circuit), recompute R1 taking into

account that VREF depends on R1, R2, and RH. It turns out that

when the normal state is below VREF, RH is in parallel with R1:

REF +

ø R

H )

which is identical to the equation for V

REFH

Alternately, when the normal state is above VREF, RH is in

parallel with R2:

REF +

ø R

) R2 ø R

which is identical to the equation for V

REFL

These two additional equations for VREF can be used to

calculate a more precise value for VREF.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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