MPXM2102AS datasheet(460/670 Pages) MOTOROLA

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AN1518

3–314

Motorola Sensor Device Data

www.motorola.com/semiconductors

The PWM output is most linear when the ramp waveform’s

period consists mostly of the rising voltage edge (see

Figure 2). If the capacitor were allowed to completely

discharge (see Figure 3), a flat line at approximately 60 mV

would separate the ramps, and these “flat spots” may result in

non–linearities of the resultant PWM output (after comparing

it to the sensor voltage). Thus, the best ramp waveform is

produced when one ramp cycle begins immediately after

another, and a slight dc offset disallows the capacitor from

discharging completely.

Exaggerated

Capacitor Discharge

Figure 3. Non Ideal Ramp Waveform for the PWM Output Pressure Sensor

Microcontroller

Pulse Train

Ramp

Waveform

The flexibility of frequency control of the ramp waveform via

the pulse train sent from the microcontroller allows a

programmable–frequency PWM output. Using Equation 1 the

frequency (inverse of period) can be calculated with a given

capacitor so that the capacitor charges to a maximum

∆V of

approximately 2.5 V (remember that the current source needs

approximately 2.5 V across it to output a stable current). The

importance of software control becomes evident here since

the selected capacitor may have a tolerance of

± 20%. By

adjusting the frequency and positive width of the pulse train,

the desired ramp requirements are readily obtainable; thus,

nullifying the effects of component variances.

For this design, the ramp spans approximately 2.4 V from

0.1 V to 2.5 V. At this voltage span, the current source is stable

and results in a linear ramp. This ramp span was used for

reasons which will become clear in the next section.

In summary, complete control of the ramp is achieved by the

following adjustments of the microcontroller–created pulse

train:

• Increase Frequency: Span of ramp decreases.

The dc offset decreases slightly.

• Decrease Frequency: Span of ramp increases.

The dc offset increases slightly.

• Increase Pulse Width: The dc offset decreases.

Span decreases slightly.

• Decrease Pulse Width: The dc offset increases.

Span increases slightly.

THE COMPARATOR STAGE

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. As Figure 1

shows, the pressure sensor output voltage is input to the

non–inverting terminal of the op amp and the ramp is input to

the inverting terminal. Therefore, when the pressure sensor

voltage is higher than a given ramp voltage, the output is high;

likewise, when the pressure sensor voltage is lower than a

given ramp voltage, the output is low (refer to Figure 5). As

mentioned in the Pressure Sensor section, resistors R1 and

R2 of Figure 1 comprise the voltage divider that attenuates the

pressure sensor’s signal to a 2.0 V span ranging from 0.25 V

to 2.25 V.

Since the pressure sensor voltage does not reach the

ramp’s minimum and maximum voltages, there will be a finite

minimum and maximum pulse width for the PWM output.

These minimum and maximum pulse widths are design

constraints dictated by the comparator’s slew rate. The

system design ensures a minimum positive and negative

pulse width of 20

µs to avoid nonlinearities at the high and low

pressures where the positive duty cycle of the PWM output is

at its extremes (refer to Figure 4 ). Depending on the speed of

the microcontroller used in the system, the minimum required

pulse width may be larger. This will be explained in the next

section.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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