33975 datasheet(23/32 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33975

TYPICAL APPLICATIONS

OPERATIONAL MODES

Figure 13. Analog Ratiometric Conversion

To read a potentiometer sensor, the wiper should be

grounded and brought back to the module ground, as

illustrated in Figure 13. With the wiper changing the

impedance of the sensor, the analog voltage on the input will

represent the position of the sensor.

Using the Analog feature to provide 4.0 mA of pull-up

current to an analog sensor may induce error due to the

accuracy of the current source. For this reason, a ratiometric

conversion must be considered. Using two current sources

(one for the sensor and one to set the reference voltage to the

A/D converter) will yield a maximum error (owing to the

33975) of 4%.

Higher accuracy may be achieved through module level

calibration. In this example, we use the resistor values from

Figure 13 and assume the current sources are 4% from each

other. The user may use the module end-of-line tester to

calculate the error in the A/D conversion. By placing a

1.0 kΩ, 0.1% resistor in the end-of-line test equipment and

assuming a perfect 4.0 mA current source from the 33975, a

calculated A/D conversion may be obtained.

Using the equation yields the

following:

The ADC value of 213 counts is the value with 0% error

(neglecting the resistor tolerance and AMUX input offset

voltage). Now we can calculate the count value induced by

the mismatch in current sources. From a sample device the

maximum current source was measured at 3.979 mA and

minimum current source was measured at 3.933 mA. This

yields 1.16% error in A/D conversion due to the current

source mismatch. The A/D measurement will be as follows:

This A/D conversion is 1.16% low in value. The error

correction factor of 1.0115 may be used to correct the value:

An error correction factor may then be stored in E2

memory and used in the A/D calculation for the specific input.

Each input used as analog measurement will have a

dedicated calibrated error correction factor.

POWER MOSFET/LED DRIVER AND MONITOR

Because of the flexible programming of the 33975 device,

it may be used to drive small loads like LEDs or MOSFET

gates. It was specifically designed to power up in the Normal

mode with the inputs tri-state. This was done to ensure the

LEDs or MOSFETs connected to the 33975 power up in the

off-state. The Switch Programmable (SP0–SP7) inputs have

a source-and-sink capability, providing effective MOSFET

gate control. To complete the circuit, a pull-down resistor

should be used to keep the gate from floating during the

Sleep modes. Figure 14, page 24, shows an application

where the SG0 input is used to monitor the drain-to-source

voltage of the external MOSFET. The 750 Ω resistor is used

to set the drain-to-source trip voltage. With the 4.0 mA

current source enabled, an interrupt will be generated when

the drain-to-source voltage is approximately 1.0 V.

VDD

VPWR

VDD

SCLK

INT

AMUX

MOSI

MISO

AN0

VBAT

SP0

SP1

SP7

SG1

SG0

SG12

SG13

WAKE

VBAT

SCLK

INT

4.0

VPWR

4.0 mA

1.21 kΩ

0.1%

4.36 V to 5.32 V

Analog

Ports

VREF(H)

VREF(L)

Analog Sensor

or Analog Switch

VPWR

4.0

VPWR

33975

MCU

ADC =

I1 x R1

I2 x R2

x 225

ADC =

4.0mA x 1.0kΩ

x225

4.0 mA x 1.21 kΩ

ADC = 210 counts

ADC =

3.933 mA x 1.0 kΩ

x225

3.979 mA x 1.21 kΩ

ADC = 208 counts

ADC = 208 counts x 1.0116

ADC = 210 counts

33975 Datasheet(PDF) 23 Page - Freescale Semiconductor, Inc

33975 Datasheet(HTML) 23 Page - Freescale Semiconductor, Inc

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