MPXM2102AS datasheet(455/670 Pages) MOTOROLA

455 / 670 page

AN1517

3–309

Motorola Sensor Device Data

www.motorola.com/semiconductors

TheusershouldbeawarethatVREF,VREFHandVREFLare

chosen for each application, depending on the desired

switching point and hysteresis values. Also, the user must

specify which range (either above or below the reference

voltage) is the desired normal state (see Figure 3). Referring

to Figure 3, if the normal state is below the reference voltage

then VREFL (VREFH is only used to calculate a more precise

value for VREF as explained above) is below VREF by the

desired amount of hysteresis (use VREFL to calculate RH).

Alternately, if the normal state is above the reference voltage

then VREFH (VREFL is only used to calculate a more precise

value for VREF) is above VREF by the desired amount of

hysteresis (use VREFH to calculate RH).

An illustration of hysteresis and the relationship between

these voltages is shown in Figure 3.

VREF (VREFUW)

VREFL

Hysteresis

VREFH

VREF (VREFLW)

Normal State

Figure 3. Setting the Reference Voltages

LM358 Op Amp Used in a Comparator Circuit

Figure 4 shows the schematic for the LM358 op amp

comparator stage, and Table 2 shows its performance. Since

the LM358 is an operational amplifier, it does not have the fast

slew–rate of a comparator IC nor the open collector output.

Comparing the LM358 and the LM311 (Table 2), the LM311 is

better for logic/switching applications since its output nearly

extends from rail to rail and has a sufficiently high switching

speed. The LM358 will perform well in applications where the

switching speed and logic–state levels are not critical (LED

output, etc.). The design of the LM358 comparator is

accomplished by using the same equations and procedure

presented for the LM311. This circuit is also reverse logic.

LM358 Op Amp with a Transistor Output Stage

Used in a Comparator Circuit

The LM358 with a transistor output stage is shown in Figure

5. This circuit has similar performance to the LM311

comparator: its output reaches the upper rail and its switching

speed is comparable to the LM311’s. This enhanced

performance does, however, require an additional transistor

and base resistor. Referring to Figure 1, note that this

comparator topology was chosen for the pressure switch

design. The LM324 is a quad op amp that has equivalent

amplifier characteristics to the LM358.

Figure 4. LM358 Comparator Circuit Schematic

Vin

Vout

LM358

VCC

Figure 5. LM358 with a Transistor Output Stage

Comparator Circuit Schematic

Vin

RPU

Vout

LM358

MMBT3904LT1

VCC

Like the other two circuits, this comparator circuit can be

designed with the same equations and procedure. The values

for RB and RPU are chosen to give a 5:1 ratio in Q1’s collector

current to its base current, in order to insure that Q1 is

well–saturated (Vout can pull down very close to ground when

Q1 is on). Once the 5:1 ratio is chosen, the actual resistance

values determine the desired switching speed for turning Q1

on and off. Also, RPU limits the collector current to be within the

maximum specification for the given transistor (see example

values in Figure 1). Unlike the other two circuits, this circuit is

positive logic due to the additional inversion created at the

output transistor stage.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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