Datasheet MC33794 Datasheet (Freescale)

Freescale Semiconductor

Technical Data

Electric Field Imaging Device

Document Number: MC33794

Rev 9, 11/2006

The MC33794 is intended for applications where noncontact sensing of
objects is desired. When connected to external electrodes, an electric field is
created.,The MC33794 is intended for use in detecting objects in this electric
field. The IC generates a low-frequency sine wave. The frequency is adjustable
by using an external resistor and is optimized for 120 kHz. The sine wave has
very low harmonic content to reduce harmonic interference.

The MC33794 also contains support circuits for a microcontroller unit (MCU)
to allow the construction of a two-chip E-field system.

Features

• Supports up to 9 Electrodes and 2 References or Electrodes

• Shield Driver for Driving Remote Electrodes Through Coaxial Cables

• +5.0 V Regulator to Power External Circuit

• ISO-9141 Physical Layer Interface

• Lamp Driver Output

• Watchdog and Power-ON Reset Timer

• Critical Internal Nodes Scaled and Selectable for Measurement

• High-Purity Sine Wave Generator Tunable with External Resistor

Typical Applications

• Occupant Detection Systems

• Appliance Control Panels and Touch Sensors

• Linear and Rotational Sliders

• Spill Over Flow Sensing Measurement

• Refrigeration Frost Sensing

• Industrial Control and Safety Systems Security

• Proximity Detection for Wake-Up Features

• Touch Screens

• Garage Door Safety Sensing

• Liquid Level Sensing

MC33794

ELECTRIC FIELD

IMAGING DEVICE

EK SUFFIX

54-LEAD SOICW-EP

CASE 1390-02

ORDERING INFORMATION

Device Name Temperature Range (TA)

MC33794EK/R2 -40°C to 85°C 1390-02 54 SOICW-EP

© Freescale Semiconductor, Inc., 2006. All rights reserved.

Package
Drawing

Package

INTERNAL BLOCK DIAGRAM

A,B,C,D

TEST

E1–E9

REF_A*, REF_B*

* REF_A and REF_B are
not switched to ground
when not selected.

WD_IN

V

DD

V

CC

RST

2.8 kΩ

2.8 kΩ

4

700 Ω*

700 Ω*

POR/

WD

MUX
OUT

MUX

IN

CONTROL

LOGIC

22 k

Ω (Nominal)

OSC

RECT

LPF

GAIN AND

OFFSET

150

300 Ω

CLK
R_OSC

39 kΩ

SHIELD_EN

Ω

SHIELD

LP_CAP

10 nF

LEVEL

V

V

PWR

AGND

GND and HEAT SINK

PWR_MON

MON

V

DD

_

LAMP_GND

LAMP_CTRL

ISO_OUT

ISO_IN

CC

REG

V

DD

REG

ATTN

LAMP CKT

ISO-9141

SIGNAL

LAMP_SENSE

LAMP_MON

LAMP_OUT

ISO-9141

(Note: All Resistor Values are Nominal)

Figure 1. Simplified Functional Block Diagram

MC33794

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SOICW-EP TERMINAL CONNECTIONS

RST

WD_IN

NC

LAMP_GND

NC

LAMP_OUT

NC

LAMP_SENSE

LAMP_MON
SHIELD_EN

SIGNAL

LEVEL

PWR_MON

LP_CAP

R_OSC

NC
NC
NC
NC

CLK

MON

V

DD

_

V

DD

V

PWR

1

2

3

4

5

6

7

8

9

10

11

D

12

C

13

B

14

A

15

16

17

18

19

20

21

22

23

24

25

26

27

Figure 2. SOICW-EP Terminal Connections

Table 1. SOICW-EP TERMINAL FUNCTION DESCRIPTION

54

53

52

51

50

49

48

47

46

45

44

43

42

41

40

39

38

37

36

35

34

33

32

31

30

29

28

LAMP_CTRL
ISO-9414
NC
ISO_IN
NC
NC
NC
ISO_OUT
REF_B
REF_A
E9
E8
E7
E6
E5
E4
E3
E2
E1
TEST
NC
NC
GND
NC
SHIELD
AGND
V

CC

Terminal

Terminal

Name

Formal Name Definition

1 RST Reset

2 WD_IN Watchdog In

3, 5, 7,

NC No connect

20–23, 31,

33, 34,

48– 50, 52

4 LAMP_GND Lamp Ground

6 LAMP_OUT Lamp Driver

This output is intended to generate the reset function of a typical MCU. It has a
delay for Power-ON Reset, level detectors to force a reset when V
out-of-range high or low, and a watchdog timer that will force a reset if WD_IN

REG is

CC

is not asserted at regular intervals. Timing is derived from the oscillator and will
change with changes in the resistor attached to R_OSC.

This terminal must be asserted and deserted at regular interval in order to
prevent

RST from being asserted. By having the MCU program perform this

operation more often the allowed time, a check that the MCU is running and
executing its program is assured. If this doesn’t occur, the MCU will be reset. If
the watchdog function is not desired, this terminal may be connected to CLK to
prevent a reset from being issued.

These terminals may be used at some future date and should be left open.

This is the ground for the current from the lamp. The current into LAMP_OUT
flows out through this terminal.

This is an active low output capable of sinking current of a typical indicator lamp.
One end of the lamp should be connected to a positive supply (for example,
battery voltage) and the other side to this terminal. The current is limited to
prevent damage to the IC in the case of a short or surge during lamp turn-on or
burn-out.

MC33794

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Freescale Semiconductor 3

Table 1. SOICW-EP TERMINAL FUNCTION DESCRIPTION (continued)

Terminal

Terminal

Name

Formal Name Definition

8 LAMP_SENSE Lamp Sense

9 LAMP_MON Lamp Monitor

10 SHIELD_EN Shield Driver

11–14 A, B, C, D Selector Inputs

15 SIGNAL Undetected Signal

16 LEVEL Detected Level

17 PWR_MON Power Monitor

This terminal is normally connected to the LAMP_OUT terminal. The voltage at
this terminal is reduced and sent to LAMP_MON so the voltage at the lamp
terminal is brought into the range of the analog-to-digital converter (ADC) in the
MCU.

This terminal is connected through a voltage divider to the LAMP_SENSE
terminal. The voltage divider scales the voltage at this terminal so that battery
voltage present when the lamp is off is scaled to the range of the MCU ADC.
With the lamp off, this terminal will be very close to battery voltage if the lamp is
not burned out and the terminal is not shorted to ground. This is useful as a lamp
check.

This terminal is used to enable the shield signal. The shield is disabled when
SHIELD_EN is a logic low (ground)

These input terminals control which electrode or reference is active. Selection
values are shown in Table 5, Electrode Selection, page 10

. These are logic level

inputs.

This is the undetected signal being applied to the detector. It has a DC level with
the low radio frequency signal superimposed on it. Care must be taken to
minimize DC loading of this signal. A shift of DC will change the center point of
the signal and adversely affect the detection of the signal.

This is the detected, amplified, and offset representation of the signal voltage on
the selected electrode. Filtering of the rectified signal is performed by a capacitor
attached to LP_CAP.

This is connected through a voltage divider to V
voltage so it will fall within the range of the ADC on the MCU.

. It allows reduction of the

PWR

18 LP_CAP Low-Pass Filter Capacitor

19 R_OSC Oscillator Resistor

24 CLK Clock

25 V

26 V

27 V

28 V

MON VDDMonitor

DD

_

DD

PWR

CC

VDD Capacitor

Positive Power Supply

5.0 V Regulator Output

29 AGND Analog Ground

30 SHIELD Shield Driver

A capacitor on this terminal forms a low pass filter with the internal series
resistance from the detector to this terminal. This terminal can be used to
determine the detected level before amplification or offset is applied. A 10 nF
capacitor connected to this terminal will smooth the rectified signal. More
capacitance will increase the response time.

A resistor from this terminal to circuit ground determines the operating frequency
of the oscillator. The MC33794 is optimized for operation around 120 kHz.

This terminal provides a square wave output at the same frequency as the
internal oscillator. The edges of the square wave coincide with the peaks
(positive and negative) of the sine wave.

This is connected through an internal voltage divider to V
reduction of the voltage so it will fall within the range of the ADC on the MCU.

REG. It allows

DD

A capacitor is connected to this terminal to filter the internal analog regulated
supply. This supply is derived from V

through internal V

PWR

DD

REG.

12 V power applied to this terminal will be converted to the regulated voltages
needed to operate the part. It is also converted to 5.0 V (internal V

8.5 V (internal V

REG) to power the MCU and external devices.

DD

This output terminal requires a 47 µF capacitor and internal V
a regulated 5.0 V for the MCU and for internal needs of the MC33794.

REG) and

CC

REG provides

CC

This terminal is connected to the ground return of the analog circuitry. This
ground should be kept free of transient electrical noise like that from logic
switching. Its path to the electrical current return point should be kept separate
from the return for GND.

This terminal connects to cable shields to cancel cable capacitance.

MC33794

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Table 1. SOICW-EP TERMINAL FUNCTION DESCRIPTION (continued)

Terminal

32 GND Ground

35 TEST Test Mode Control

36–44 E1–E9 Electrode Connections

45, 46 REF_A,

47 ISO_OUT ISO-9141 Output

51 ISO_IN ISO-9141 Input

53 ISO-9141 ISO-9141 Bus

54 LAMP_CTRL Lamp Control

Terminal

Name

REF_B

(E10, E11)

Formal Name Definition

Reference Connections

(Or as additional electrodes)

This terminal and metal backing is the IC power return and thermal radiator /
conductor.

This terminal is normally connected to circuit ground. There are special
operating modes associated with this terminal when it is not at ground.

These are the electrode terminals. They are connected either directly or through
coaxial cables to the electrodes for measurements. When not selected, these
outputs are grounded through the internal resistance.

These terminals can be individually selected to measure a known capacitance
value. Unlike E1-E9, these two inputs are not grounded when not selected.

This terminal translates ISO-9141 receive levels to 5.0 V logic levels for the
MCU.

This terminal accepts data from the MCU to be sent over the ISO-9141
communications interface. It translates the 5.0 V logic levels from the MCU to
transmit levels on the ISO-9141 bus.

This terminal connects to the ISO-9141 bus. It provides the drive and detects
signaling on the bus and translates it from the bus level to logic levels for the
MCU.

This signal is used to control the lamp driver. A high logic level turns on the lamp.

MC33794

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Freescale Semiconductor 5

MAXIMUM RATINGS

Table 2. Maximum Ratings

All voltages are with respect to ground unless otherwise noted. Exceeding these ratings may cause a malfunction or

permanent damage to the device.

Rating Symbol Value Unit

Peak VPWR Voltage

Double Battery

1 Minute Maximum T

= 30°C

A

V

PWRPK

V

DBLBAT

40 V

26.5

ESD Voltage

Human Body Model

Machine Model

(1)

(2)

Storage Temperature

Operating Ambient Temperature

Operating Junction Temperature

Thermal Resistance

Junction-to-Ambient

Junction-to-Case

Junction-to-Board

(3)

(4)

(5)

Lead Soldering Temperature (for 10 Seconds)

V
V

T

ESD1

ESD2

STG

T

A

±2000

±200

-55 to 150 °C

-40 to 85 °C

TJ -40 to 150 °C

R

θ

R

θ

R

θ

T

SOLDER

JA

JC

JB

260 °C

41

0.2

3.0

°C/W

Notes

1. ESD1 performed in accordance with the Human Body Model (C

2. ESD2 performed in accordance with the Machine Model (C

ZAP

= 200 pF, R

ZAP

= 100 pF, R

ZAP

= 1500 Ω).

ZAP

= 0 Ω).

3. Junction temperature is a function of on-chip power dissipation, package thermal resistance, mounting site (board) temperature,
ambient temperature, air flow, power dissipation of other components on the board, and board thermal resistance. In accordance with
SEMI G38-87 and JEDEC JESD51-2 with the single layer board horizontal.

4. Indicates the average thermal resistance between the die and the case top surface as measured by the cold plate method
(MILSPEC 883 Method 1012.1) with the cold plate temperature used for the case temperature.

5. Thermal resistance between the die and the printed circuit board per JEDEC JESD51-8. Board temperature is measured on the top
surface of the board near the package.

V

V

MC33794

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STATIC ELECTRICAL CHARACTERISTICS

Table 3. Static Electrical Characteristics

Characteristics noted under conditions 9.0 V ≤ VPWR ≤ 18 V, -40°C ≤ TA ≤ 85°C unless otherwise noted. Typical values noted
reflect the approximate parameter means at T
to GND unless otherwise noted.

Characteristic Symbol Min Typ Max Unit

Voltage Regulators

= 25°C under normal conditions unless otherwise noted. Voltages are relative

A

5.0 V Regulator Voltage

7.0 V ≤ V

≤ 18 V, 1.0 mA ≤ IL ≤ 75 mA, C

PWR

FILT

= 47 µF

Analog Regulator Voltage

9.0 V ≤ V

PWR

≤ 18 V, C

FILT

= 47 µF

Out-of-Range Voltage Detector (Terminal name VCC)

5.0 V Low Voltage Detector

5.0 V High Voltage Detector

5.0 V Out-of-Range Voltage Detector Hysteresis

ISO-9141 Communications Interface

Input Low Level

Input High Level

Input Hysteresis

Output Low

Output High

(6)

(6)

(6)

(6)

(6)

Output Breakdown

= 20 mA

I

OUT

Output Resistance

I

= 40 mA

OUT

V

CC

4.75 5.0 5.25

V

ANALOG

8.075 8.5 8.925

V

4.0 4.52 4.72 V

LV5

V

5.26 5.55 5.83 V

HV5

VIF

VIF

VIF

VIF

VIF

V

VIF

RIF

HYS5

INLO

INHI

INHYS

OLO

OHI

Z

ON

– 0.05 – V

0.30 0.33 – V/V

– 0.53 0.7 V/V

– 0.2 – V/V

– – 0.2 V/V

0.8 – – V/V

40

–

– 58 –

V

V

V

–

Ω

Current Limit

Sinking Current with V

Output Propagation Delay

Out to ISO-9141, C

LOAD

OUT

= 20 pF

< 0.3 V

PWR

IN

IIF

TIF

LIM

DLY

60 90

120

– – 8.0

mA

µs

ISO In

Logic Output Low

I

= 1.0 mA

SINK

Logic Output Pull-Up Current

V

= 0 V

OUT

Input to Output Propagation Delay

ISO-9141 to ISO_IN, R
18 V

= 10 kΩ, CL = 470 pF, 7.0 V ≤ VPWR ≤

L

VIF

TIF

IIF

OLO

PU

DLY

V

– – 1.0

µA

100 – –

µs

– – 5.4

Notes

6. Ratio to V

PWR

MC33794

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Freescale Semiconductor 7

Table 3. Static Electrical Characteristics (continued)

Characteristics noted under conditions 9.0 V ≤ VPWR ≤ 18 V, -40°C ≤ TA ≤ 85°C unless otherwise noted. Typical values noted
reflect the approximate parameter means at T

= 25°C under normal conditions unless otherwise noted. Voltages are relative

A

to GND unless otherwise noted.

Characteristic Symbol Min Typ Max Unit

Electrode Signals

Total Variance Between Electrode Measurements

All C

LOAD

= 15 pF

Electrode Maximum Harmonic Level Below Fundamental

5.0 pF ≤ C

LOAD

≤ 100 pF

(7)

(8)

ELV

EL

VAR

HARM

–

– 3.0

–

-20

–

%

dB

Electrode Transmit Output Range

5.0 pF ≤ C

LOAD

≤ 100 pF

Receive Input Voltage Range

Grounding Switch on Voltage

I

= 1.0 mA

SW

EL

TXV

1.0 – 8.0

RXV 0 – 9.0 V

SW

VON

– – 5.0

Shield Driver

Shield Driver Output Level

0pF ≤ C

LOAD

≤ 500 pF

Shield Driver Input Range

Grounding Switch on Voltage

(9)

SD

TXV

1.0 – 8.0

SDIN 0 – 9.0 V

SW

– – 1.5 V

VON

Logic I/O

CMOS Logic Input Low Threshold

Logic Input High Threshold

Voltage Hysteresis

Input Current

V

= V

CC

IN

VIN = 0 V

V

0.3 – – V

THL

V

THH

V

– 0.06 – V

HYS

– – 0.7 V

IIN

10

-5.0

–

–

Signal Detector

V

V

V

CC

CC

CC

µA

50

5.0

Detector Output Resistance

LP_CAP to LEVEL Gain

LP_CAP to LEVEL Offset

DET

A

REC

V

RECOFF

RO

– 50 – kΩ

3.6 4.0 4.4 A

-3.3 -3.0 -2.7 V

V

Notes

7. Verified by design. Not tested in production.

8. Verified by design and characterization. Not tested in production.

9. Current into grounded terminal under test = 1.0 mA.

MC33794

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8 Freescale Semiconductor

Table 3. Static Electrical Characteristics (continued)

Characteristics noted under conditions 9.0 V ≤ VPWR ≤ 18 V, -40°C ≤ TA ≤ 85°C unless otherwise noted. Typical values noted
reflect the approximate parameter means at T

= 25°C under normal conditions unless otherwise noted. Voltages are relative

A

to GND unless otherwise noted.

Characteristic Symbol Min Typ Max Unit

Lamp Driver

On Resistance

I

= 400 mA

IN

Current Limit

V

= 1.0 V

OUT

On-Voltage

I

= 400 mA

OUT

Breakdown Voltage

I

= 100 µA, Lamp Off

OUT

Voltage Monitors

LAMP_MON to LAMP_SENSE Ratio

PWR_MON to V

V

MON to VDD Ratio

DD

_

PWR

Ratio

Supply

Quiescent supply current

= 14 V

(10)

V

PWR

(11)

Notes

10. Verified by design and characterization. Not tested in production.

11. No external devices connected to internal voltage regulators.

RLD

DSON

ILD

VLDON

VLDZ

LMP

PWR

V

DD_MON

I

pwr

– 1.75 3.5

LIM

0.7 – 1.7

– – 1.4

40

0.1950 0.20524 0.2155 V/ V

MON

0.2200 0.2444 0.2688 V/V

MON

–

–

0.45 0.5 0.55 V/ V

_ 7.0 _ mA

Ω

A

V

V

MC33794

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Freescale Semiconductor 9

DYNAMIC ELECTRICAL CHARACTERISTICS

Table 4. Dynamic Electrical Characteristics

Characteristics noted under conditions 9.0 V ≤ VPWR ≤ 18 V, -40°C ≤ TA ≤ 85°C unless otherwise noted. Typical values noted
reflect the approximate parameter means at T
to GND unless otherwise noted.

Characteristic Symbol Min Typ Max Unit

OSC

(12), (13)

OSC Frequency Stability

OSC Center Frequency

R_OSC = 39 kΩ

Harmonic Content

(12)

2nd through 4th Harmonic Level

5th and Higher

Shield Driver

Shield Driver Maximum Harmonic level below Fundamental

10 pF ≤ C

LOAD

≤ 500 pF

Shield Driver Gain Bandwidth Product

Measured at 120 kHz

POR

(12)

= 25°C under normal conditions unless otherwise noted. Voltages are relative

A

f

– – 10 %

STAB

f

OSC

– 120 –

(12)

OSCH

SD

ARM

HARM

–

–

–

–

-20

-60

– -20 –

SD

GBW

–4.5 –

kHz

dB

dB

MHz

POR Time-Out Period

Watchdog

Watchdog Time-Out Period

Watchdog Reset Hold Time

Lamp Driver

Short Circuit to Battery Survival Time

Notes

12. Verified by design and characterization. Not tested in production.

13. Does not include errors in external reference parts.

ELECTRODE SELECTION

Table 5. Electrode Selection

TERMINAL/SIGNAL D C B A

Source (internal)

E1

E2

E3

E4

E5

E6

E7

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

t

PER

t

WDPER

t

WDHLD

3.0 – – ms

t

SCB

9.0 – 50 ms

50 68 250 ms

9.0 – 50 ms

Table 5. Electrode Selection (continued)

TERMINAL/SIGNAL D C B A

E8

E9

REF_A

REF_B

Internal OSC

Internal OSC after 22 kΩ

Internal Ground

Reserved

1 0 0 0

1 0 0 1

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

MC33794

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10 Freescale Semiconductor

FUNCTIONAL DESCRIPTION

INTRODUCTION

The MC33794 is intended for use in detecting objects
using an electric field. The IC generates a low radio
frequency sine wave. The frequency is set by an external
resistor and is optimized for 120 kHz. The sine wave has very
low harmonic content to reduce potential interference at
higher harmonically related frequencies. The internal
generator produces a nominal 5.0 V peak-to-peak output that
is passed through an internal resistor of about 22 kΩ. An
internal multiplexer routes the signal to one of 11 terminals
under control of the ABCD input terminals. A receiver
multiplexer simultaneously connected to the selected
electrode routes its signal to a detector, which converts the
sine wave to a DC level. This DC level is filtered by an
external capacitor and is multiplied and offset to increase
sensitivity. All of the unselected electrode outputs are
grounded by the device. The current flowing between the
selected electrode and the other grounded electrodes plus
other grounded objects around the electrode causes a

BLOCK DIAGRAM COMPONENTS

Refer to Figure 1, MC33794 Internal Block Diagram,

, for a graphic representation of the block diagram

page 2
information in this section.

OSC

This section generates a high purity sine wave. The center
frequency is controlled by a resistor attached to R_OSC. The
normal operating frequency is around 120 kHz. A square
wave version of the frequency output is available at CLK.
Timing for the Power-ON Reset and watchdog (POR/WD)
circuit are derived from this oscillator’s frequency.

MUX OUT

This circuit directs the output of the sine wave to one of
nine possible electrode outputs or two reference terminals.
All unused terminals are automatically grounded (except the
two reference terminals). The selected output is controlled by
the ABCD inputs.

ELECTRODES E1-E9

These are the electrode terminals. They are connected
either directly or through coaxial cables to the electrodes for
measurements. Every electrode has a 2.8K (
in series with the external pad and the internal electronics.
Only one of these electrodes can be selected at a time for
capacitance measurement. All of the other unselected
electrodes are switched to ground by an internal switch that
has an internal on-resistance of approximately 700
signal at the selected electrode terminal is routed to the
shield driver amplifier by an internal switch. All of the coaxial
cable shields should be isolated from ground and connected
SHIELD.

± 20%) resistor

Ω. The

voltage drop across the internal resistance. Objects brought
into or out of the electric field change the current and resulting
voltage at the IC terminal, which in turn reduces the voltage
at LP_CAP and LEVEL.

A shield driver is included to minimize the effect of
capacitance caused by using coaxial cables to connect to
remote electrodes. By driving the coax shield with this signal,
the shield voltage follows that of the center conductor,
significantly reducing the effective capacitance of the coax
and maintaining sensitivity to the capacitance at the
electrode.

The MC33794 is made to work with and support a
microcontroller. It provides two voltage regulators, a Power­ON-reset/out-of-range voltage detector, watchdog circuit,
lamp driver and sense circuit, and a physical layer ISO-9141
communications interface.

REF_A & REF_B ELECTRODES

These terminals can be individually selected like E1
through E9. Unlike E1 through E9, these terminals are not
grounded when not selected. Both terminals have a 2.8K

± 20%) resistor in series with the external pad and the

(
internal electronics. The purpose of these terminals is to
allow known capacitors to be measured. By using capacitors
at the low and high end of the expected range, absolute
values for the capacitance on the electrodes can be
computed. These terminals can be used for electrodes E10
and E11 with the only difference is that these two electrodes
will not be grounded when not selected.

SHIELD DRIVE

This circuit provides a buffered version of the returned AC
signal from the electrode. Since it nearly has the same
amplitude and phase as the electrode signal, there is little or
no potential difference between the two signals thereby
cancelling out any electric field. In effect, the shield drives
and isolates the electrode signal from external virtual
grounds. A common application is to connect the Shield Drive
to the shield of a coax cable used to connect an electrode to
the corresponding electrode terminal. Another typical use is
to drive a ground plane that is used behind an array of touch
sensor electrodes in order to cancel out any virtual grounds
that could attenuate the AC signal.

MUX IN

This circuit connects the selected electrode, reference, or
one of two internal nodes to an amplifier/detector. The
selection is controlled by the ABCD inputs and follows the
driven electrode/reference when one is selected.

MC33794

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Freescale Semiconductor 11

RECT

The rectifier circuit detects the level from MUX IN by
offsetting the midpoint of the sine wave to zero volts and
inverting the waveform when it is below the midpoint. It is
important to avoid DC loading of the signal, which would
cause a shift in the midpoint voltage of the signal.

LPF

The rectified sine wave is filtered by a low pass function
formed by an internal resistance and an external capacitance
attached to LP_CAP. The nominal value of the internal
resistance is 50 k
selected to provide filtering of noise while still allowing the
desired settling time for the detector output. A 10 nF
capacitor would allow 99% settling in less than 5.0 ms. In
practice, it is recommended you wait at least 1.5 ms after
selecting an electrode before reading LEVEL.

Ω. The value of the external capacitor is

GAIN AND OFFSET

This circuit multiplies the detected and filtered signal by a
gain and offsets the result by a DC level. This results in an
output range that covers 1.0 V to 4.0 V for capacitive loading
of the field in the range of 10 pF to 100 pF. This allows higher
sensitivity for a digital-to-analog converter with a 0 V-to-5.0 V
input range.

ATTN

This circuit passes the undetected signal to SIGNAL for
external use.

SHIELD_EN

A logic low on this input disables the shield drive. The
purpose of doing this is to be able to detect that the shield
signal is not working or the connection to the coax shields is
broken. If either of these conditions exists, there will be little
or no change in the capacitance measured when the
SHIELD_EN is changed. If the SHIELD output is working and
properly connected, the capacitance of the coax will not be
cancelled when this terminal is asserted and the measured
capacitance will appear to change by approximately the
capacitance between the center conductor and the shield in
the coax.

(with appropriate current setting resistor) is connected to a
positive voltage source and the other is connected to
LAMP_OUT, and LAMP_GND is connected to ground, the
lamp will light. This circuit provides current limiting to prevent
damage to itself in the case of a shorted lamp or during a
high-surge condition typical of an incandescent lamp
burnout.

LAMP_GND should always be connected to ground even

if the lamp circuit is not used.

ISO-9141

This circuit connects to an ISO-9141 bus to allow remote
communications. ISO_IN is data from the bus to the MCU
and ISO_OUT is data to drive onto the bus from the MCU.

POR/WD

This circuit is a combined Power-ON Reset and watchdog
timer. The RST output is held low until a certain amount of
time after the V
minimum operating threshold. If VCC falls below the level at
any time, RST is pulled low again and held until the required
time after V
also included, which will force a reset if VCC rises above a
maximum voltage. The watchdog function also can force RST
low if too long an interval is allowed to pass between positive
transitions on WD_IN.

REG output (VCC) has remained above a

CC

has returned high. An over voltage circuit is

CC

INTERNAL VCC REGULATOR

This circuit converts an unregulated voltage from VIN to a
regulated 5.0 V source, which is used internally and available
for other components requiring a regulated voltage source.

INTERNAL VDD REGULATOR

This is a regulator for analog devices that require more
than 5.0 V. This is used by the device and some current is
available to operate op-amps and other devices. By having
this higher voltage available, some applications can avoid the
need for a rail-to-rail output amplifier and still achieve the 0 V­to-5.0 V output for a digital-to-analog converter input.
V

DD_MON

allows a 0.0 V-to-5.0 V ADC to measure VDD. Normal value
for V

is a divided output from internal VDD REG, which

is 8.5 Volts.

DD

LAMP CKT

This section controls the operation of the LAMP_OUT
terminal. When LAMP_CTRL is asserted, LAMP_OUT is
pulled to LAMP_GND. If one side of an indicator lamp or LED

CONTROL LOGIC

This contains the logic that decodes and controls the

MUXes and some of the test modes

APPLICATION INFORMATION

The MC33794 is intended to be used where an object’s
size and proximity are to be determined. This is done by
placing electrodes in the area where the object will be. The
proximity of an object to an electrode can be determined by
the increase in effective capacitance as the object gets closer
to the electrode and modifies the electric field between the

MC33794

12 Freescale Semiconductor

electrode and surrounding electrically common objects. The
shape and size of an object can be determined by using
multiple electrodes over an area and observing the
capacitance change on each of the electrodes. Those that
don’t change have nothing near them, and those that do
change have part of the object near them.

Sensors

A “capacitor” can be formed between the driving electrode
and the object, each forming a “plate” that holds the electric
charge. Capacitance is directly proportional to the area of the
electrode plates. Doubling the area doubles the capacitance.
Capacitance is also directly proportional to the dielectric
constant of the material between the plates. Air typically has
a dielectric constant of 1 (unity) whereas water can have a
dielectic constant of 80 (which means the capacitance is
roughly 80 times larger). Plastics and glass that are
commonly used in touch panel applications have dielectric
constants greater than unity. A third consideration is that
capacitance is inversely proportional to the distance between
the plates. Doubling the distance between the two plates will
reduce capacitance by four. This property can be exploited in
cases where small distances need to be measured.

From the above, it can be seen that increased detection
sensitivity is a function of the plate size, the dielectric
constant of the material between the plates, and the distance
between them.

The voltage measured at LEVEL is an inverse function of
the capacitance between the electrode being measured and
the surrounding electrodes and other objects in the electric
field surrounding the electrode. Increasing capacitance
results in decreasing voltage. The value of series resistance

Ω) was chosen to provide a nearly linear relationship at

(22 k
120 kHz over a range of 10 pF to 100 pF.

The measured value may change with any change in
frequency, series resistance, driving voltage, the dielectric
constant of the capacitor, or detector sensitivity. These can
change with temperature and time. There are several ways to
compensate for these changes. One method uses the
REF_A and REF_B capacitors. Another method may use
long term averages to set a baseline value.

Using REF_A and REF_B, a typical measurement
algorithm would start by measuring the voltage for two known
value capacitors (attached to REF_A and REF_B). The value
of these capacitors would be chosen to be near the minimum
and maximum values of capacitance expected to be seen at
the electrodes. These reference voltages and the known
capacitance values are then used with the electrode

measurement voltage to determine the capacitance seen by
the electrode. This method can be used to detect short- and
long-term changes due to objects in the electric field and
significantly reduce the effect of temperature-and time­induced changes.

Another approach is to run long term averaging of the
electrode values. Long term, in this case, may mean several
seconds. These long term averages are then used as a set
point so that short term changes in the field intensity can be
reliably determined. This is typically the method used for
touch panel applications.

The MC33794 does not contain an ADC. It is intended to
be used with an MCU that contains one. Offset and gain have
been added to the MC33794 to maximize the sensitivity over
the range of 0 pF to 100 pF. An 8-bit ADC can resolve around

0.4 pF of change and a 10-bit converter around 0.1 pF.
Higher resolution results in more distant detection of smaller
objects. Due to the relatively slow data access requirements
(approximately 2 ms per electrode), digital over-sampling
techniques can be used to extend the resolution of 8- or 10­bit converters by 2 or 3 bits.

DC loading on the electrodes should be avoided. A typical
situation where this might occur is if moisture gets in direct
contact between electrodes, or between an electrode and
ground or shield drive. The signal is generated with a DC
offset that is more than half the peak-to-peak level. This
keeps the signal positive above ground at all times. The
detector uses this voltage level as the midpoint for detection.
All signals below this level are inverted and added to all
signals above this level. Loading of the DC level will cause
some of the positive half of the signal to be inverted and
added and will change the measurement.

If it is not possible to assure that the electrodes will always
have a high DC resistance to ground source, a series
capacitor of about 10 nF should be connected between the IC
electrode terminals and the electrodes. This capacitor will
block DC bias voltages to the detector. Note that it is also
advisable to add a DC blocking capacitor in series with the
Shield Driver output as well.

MC33794

Sensors
Freescale Semiconductor 13

EXAMPLE APPLICATION DIAGRAM

+9 to +18 V

47

VCC

Analog_IN

Analog_IN

Analog_IN

Analog_IN

MCU

ISO_Tx

ISO_Rx

Watchdog

Reset

GPx

Electrode Select

Shield Disable

µF

10 nF

Indicator

Lamp

(Optional)

(Optional)

(Optional)

0.1

µF

LAMP_OUT

LP_CAP

LEVEL

VDD

PWR_MON
LAMP_MON

LAMP_SENSE

ISO_IN

ISO_OUT

WD_IN

RST

LAMP_CTRL

LAMP_GND

TEST

4

A, B, C, D

SHIELD_EN

R_OSC AGND

V

MON

_

PWR

33794

VCC

VDD

ISO-9141

SIGNAL

REF_A

REF_B

SHIELD

GND

E1

E2

E9

47 µF

Monitor (Optional)

100 pF

10 k

1

2

9

Ω

10 pF

ISO-9141 Bus

Field
Electrodes

39 k

Ω

Figure 3. Example Application Diagram

MC33794

Sensors

14 Freescale Semiconductor

PACKAGE DIMENSIONS

PAGE 1 OF 3

EK SUFFIX

CASE 1390-02

ISSUE C

MC33794

Sensors
Freescale Semiconductor 15

PACKAGE DIMENSIONS

PAGE 2 OF 3

EK SUFFIX

CASE 1390-02

ISSUE C

PAGE 2 OF 3

MC33794

Sensors
Freescale Semiconductor 16

PACKAGE DIMENSIONS

PAGE 1 OF 3

EK SUFFIX

CASE 1390-02

ISSUE C

PAGE 3 OF 3

PAGE 3 OF 3

MC33794

Sensors
Freescale Semiconductor 17

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© Freescale Semiconductor, Inc. 2006. All rights reserved.

MC33794
Rev 9
11/2006

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characteristics of their non-RoHS-compliant and/or non-Pb-free counterparts. For further
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