33975 datasheet(24/34 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33975

TYPICAL APPLICATIONS

OPERATIONAL MODES

TYPICAL APPLICATIONS

The 33975’s primary function is the detection of open or

closed switch contacts. However, there are many features

that allow the device to be used in a variety of applications.

The following is a list of applications to consider for the IC:

• Sensor power supply

• Switch monitor for metallic or elastomeric switches

• Analog sensor inputs (Ratiometric)

• Power MOSFET/LED driver and monitor

• Multiple 33975 devices in a module system

The following paragraphs describe the applications in

detail.

SENSOR POWER SUPPLY

Each input may be used to supply current to sensors

external to a module. Many sensors such as Hall effect,

pressure sensors, and temperature sensors require a supply

voltage to power the sensor, and provide an open collector or

analog output. Figure 13 shows how the 33975 may be used

to supply power and interface to these types of sensors. In an

application where the input makes continuous transitions,

consider using the wake-up/interrupt command to disable

the interrupt for the particular input.

Figure 13. Sensor Power Supply

METALLIC/ELASTOMERIC SWITCH

Metallic switch contacts often develop higher contact

resistance over time owing to contact corrosion. The

corrosion is induced by humidity, salt, and other elements

that exist in the environment. For this reason the 33975

provides two settings for contacts. When programmed for

metallic switches, the device provides higher wetting current

to keep switch contacts free of oxides. The higher current

occurs for the first 20ms of switch closure. Where longer

duration of wetting current is desired, the user may send the

wetting current timer command and disable the timer. Wetting

current will be continuous to the closed switch. After the time

period set by the MCU, the wetting current timer command

may be sent again to enable the timer. The user must

consider power dissipation on the device when disabling the

timer. (Refer to the paragraph entitled Temperature Monitor,

page 23.)

To increase the amount of wetting current for a switch

contact, the user has two options. Higher wetting current to a

switch may be achieved by paralleling SGn or SPn inputs.

This will increase wetting current by 32mA for each input

added to the switch-to-ground contact and 16mA for switch-

to-battery contacts. The second option is to simply add an

external resistor pull-up to the VPWR supply for switch-to-

ground inputs or a resistor to ground for a switch-to-battery

input. Adding an external resistor has no effect on the

operation of the device.

Elastomeric switch contacts are made of carbon and have

a high contact resistance. Resistance of 1.0 k

Ω is common.

In applications with elastomeric switches, the pull-up and

pull-down currents must be reduced to prevent excessive

power dissipation at the contact. Programming for a lower

current settings is provided in the Functional Device

Operation Section beginning on page 16 under Table 8,

Metallic Command.

ANALOG SENSOR INPUTS (RATIOMETRIC)

The 33975 features a 22-to-1 analog multiplexer. Setting

the binary code for a specific input in the analog command

allows the microcontroller to perform analog to digital

conversion on any of the 22 inputs. On rising edge of the CS,

the multiplexer connects a requested input to the AMUX pin.

The AMUX pin is clamped to max of VDD volts regardless of

the higher voltages present on the input pin. After an input

has been selected as the analog, the corresponding bit in the

next SO data stream will be logic [0].

The input pin, when selected as analog, may be

configured as analog with high-impedance, analog with

4.0mA pull-up, or analog with 32mA pull-up. Figure 14,

page 25, shows how the 33975 may be used to provide a

ratiometric reading of variable resistive input.

VDD

VPWR

VDD

SCLK

INT

MOSI

MISO

VBAT

SP0

SP1

SP7

SG1

SG0

SG12

SG13

WAKE

33975

SCLK

INT

MCU

4.0

VPWR VPWR

32 mA

IOC[7:0]

Reg

Input Capture

Timer Port

Hall-Effect

Sensor

4.0

VPWR VPWR

VBAT

AMUX

VPWR

SG13

VDD

AMUX

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