MPXM2102AS datasheet(459/670 Pages) MOTOROLA

459 / 670 page

AN1518

3–313

Motorola Sensor Device Data

www.motorola.com/semiconductors

PRESSURE SENSOR

Motorola’s MPX5000 series sensors are signal conditioned

(amplified), temperature compensated and calibrated (i.e.,

offset and full–scale span are precision trimmed) pressure

transducers. These sensors are available in full–scale

pressure ranges of 50 kPa (7.3 psi) and 100 kPa (14.7 psi).

With the recommended 5.0 V supply, the MPX5000 series

produces an output of 0.5 V at zero pressure to 4.5 V at full

scale pressure. Referring to the schematic of the system in

Figure 1, note that the output of the pressure sensor is

attenuated to one–half of its value by the resistor divider

comprised of resistors R1 and R2. This yields a span of 2.0 V

ranging from 0.25 V to 2.25 V at the non–inverting terminal of

the comparator. Table 1 shows the electrical characteristics of

the MPX5100.

Table 1. MPX5100DP Electrical Characteristics

Characteristic

Symbol

Min

Typ

Max

Unit

Pressure Range

POP

—

100

kPa

Supply Voltage

—

5.0

6.0

Vdc

Full Scale Span

VFSS

3.9

4.0

4.1

Zero Pressure Offset

Voff

0.4

0.5

0.6

Sensitivity

—

mV/kPa

Linearity

—

– 0.5

—

0.5

%FSS

Temperature Effect on Span

—

–1.0

—

1.0

%FSS

Temperature Effect on Offset

—

–50

0.2

THE RAMP GENERATOR

The ramp generator is shown in the schematic in Figure 1.

A pulse train output from a microcontroller drives the ramp

generator at the base of transistor Q1. This pulse can be

accurately controlled in frequency as well as pulse duration via

software (to be explained in the microcontroller section).

The ramp generator uses a constant current source to

charge the capacitor. It is imperative to remember that this

current source generates a stable current only when it has

approximately 2.5 V or more across it. With less voltage

across the current source, insufficient voltage will cause the

current to fluctuate more than desired; thus, a design

constraint for the ramp generator will dictate that the capacitor

can be charged to only approximately 2.5 V, when using a

5.0 V supply.

The constant current charges the capacitor linearly by the

following equation:

DV + IDt

(1)

where

∆t is the capacitor’s charging time and C is the

capacitance.

Referring to Figure 2, when the pulse train sent by the

microcontroller is low, the transistor is off, and the current

source charges the capacitor linearly. When the pulse sent by

the microcontroller is high, the transistor turns on into

saturation, discharging the capacitor. The duration of the high

part of the pulse train determines how long the capacitor

discharges, and thus to what voltage it discharges. This is how

the dc offset of the ramp waveform may be accurately

controlled. Since the transistor saturates at approximately

60 mV, very little offset is needed to keep the capacitor from

discharging completely.

Figure 2. Ideal Ramp Waveform for the PWM Output Pressure Sensor

Microcontroller

Pulse Train

Exaggerated

Capacitor Discharge

Ramp

Waveform

Ramp Waveform

Offset (100 mV)

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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