MPXM2102AS datasheet(456/670 Pages) MOTOROLA

456 / 670 page

AN1517

3–310

Motorola Sensor Device Data

www.motorola.com/semiconductors

LM339 Used in a Window Comparator Circuit

Using two voltage references to detect when the input is

within a certain range is another possibility for the pressure

switch design. The window comparator’s schematic is shown

in Figure 6. The LM339 is a quad comparator IC (it has open

collector outputs), and its performance will be similar to that of

the LM311.

Figure 6. LM339 Window Comparator Circuit Schematic

Vin

Vout

LM339

VCC

RPU

RHU

RHL

VREFLW

VREFUW

Obtaining the correct amount of hysteresis and the input

reference voltages is slightly different than with the other

circuits. The following equations are used to calculate the

hysteresis and reference voltages. Referring to Figure 3,

VREFUWistheupperwindowreferencevoltageandVREFLW

is the lower window reference voltage. Remember that

reference voltage and threshold voltage are interchangeable

terms.

For the upper window threshold:

Choose the value for VREFUW and R1 (e.g., 10 kΩ). Then,

by voltage division, calculate the total resistance of the

combination of R2 and R3 (named R23 for identification) to

obtain the desired value for VREFUW, neglecting the effect of

RHU:

REFUW +

R23

) R23

The amount of hysteresis can be calculated by the following

equation:

REFL +

R23R

R1R23

) R1R

HU )

R23R

Notice that the upper window reference voltage, VREFUW,

is now equal to its VREFL value, since at this moment, the

input voltage is above the normal state.

HYSTERESIS

+ V

REFUW *

REFL

where VREFL is chosen to give the desired amount of

hysteresis for the application.

The initial calculation for VREFUW will be slightly in error

due to neglecting the effect of RHU. To establish a precise

value for VREFUW (including RHU in the circuit), recompute

R1 taking into account that VREFUW depends on R2 and R3

and the parallel combination of R1 and RHU. This more

precise value is calculated with the following equation:

REFUW +

R23

ø R

HU )

R23

for the lower window threshold choose the value for VREFLW.

Set V

REFLW +

ø R

HU )

) R3

where R2 + R3 = R23 from above calculation.

To calculate the hysteresis resistor:

The input to the lower comparator is one half Vin (since

R4 = R5) when in the normal state. When VREFLW is above

one half of Vin (i.e., the input voltage has fallen below the win-

dow), RHL parallels R4, thus loading down Vin. The resulting

input to the comparator can be referred to as VINL (a lower in-

put voltage). To summarize, when the input is within the win-

dow, the output is high and only R4 is connected to ground

from the comparator’s positive terminal. This establishes one

half of Vin to be compared with VREFLW. When the input volt-

age is below VREFLW, the output is low, and RHL is effectively

in parallel with R4. By voltage division, less of the input voltage

will fall across the parallel combination of R4 and RHL, de-

manding that a higher input voltage at Vin be required to make

the noninverting input exceed VREFLW.

Therefore the following equations are established:

HYSTERESIS

+ V

REFLW *

INL

Choose R4 = R5 to simplify the design.

HL +

R4R5 V

REFLW *

INL *

(R4

) R5) V

INL *

REFLW

IMPORTANT NOTE:

As explained above, because the input voltage is divided in

half by R4 and R5, all calculations are done relative to the one

half value of Vin. Therefore, for a hysteresis of 200 mV (relative

to Vin), the above equations must use one half this hysteresis

value (100 mV). Also, if a VREFLW value of 2.0 V is desired

(relative to Vin), then 1.0 V for its value should be used in the

above equations. The value for VINL should be scaled by one

half also.

The window comparator design can also be designed using

operational amplifiers and the same equations as for the

LM339 comparator circuit. For the best performance,

however, a transistor output stage should be included in the

design.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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