OPERATIONAL MODES Figure 14. Analog Ratiometric Con" />
參數(shù)資料
型號: MC33975TEKR2
廠商: Freescale Semiconductor
文件頁數(shù): 17/32頁
文件大?。?/td> 0K
描述: IC SWITCH DETECT MULT 32-SOIC
標(biāo)準(zhǔn)包裝: 1,000
應(yīng)用: 開關(guān)監(jiān)控
接口: SPI 串行
封裝/外殼: 32-SSOP(0.295",7.50mm 寬)裸露焊盤
供應(yīng)商設(shè)備封裝: 32-SOICW 裸露焊盤
包裝: 帶卷 (TR)
安裝類型: 表面貼裝
Analog Integrated Circuit Device Data
24
Freescale Semiconductor
33975
TYPICAL APPLICATIONS
OPERATIONAL MODES
Figure 14. Analog Ratiometric Conversion
To read a potentiometer sensor, the wiper should be
grounded and brought back to the module ground, as
illustrated in Figure 14. 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 14 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 k
, 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 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 15, page 25, shows an application
where the SG0 input is used to monitor the drain-to-source
voltage of the external MOSFET. The 750
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
SI
SO
SCLK
INT
CS
AMUX
MOSI
MISO
AN0
VBAT
SP0
SP1
SP7
SG1
SG0
SG12
SG13
WAKE
VBAT
SCLK
CS
INT
32
4.0
VPWR
4.0mA
1.21k
0.1%
4.36V to 5.32V
Analog
Ports
VREF(H)
VREF(L)
Analog Sensor
or Analog Switch
R2
I2
I1
R1
VPWR
mA
4.0
VPWR
mA
32
mA
33975
MCU
ADC =
I1 x R1
I2 x R2
x255
ADC =
4.0mA x 1.0k
x 255
4.0mA x 1.21k
ADC = 210 counts
ADC =
3.933 mA x 1.0k
x255
3.979 mA x 1.21k
ADC = 208 counts
ADC = 208 counts x 1.0116
ADC = 210 counts
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