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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
ORDERING INFORMATION
Device

Name
Temperature Range (T
A
)
Package
Drawing
Package
MC33794EK/R2
-40°C to 85°C
1390-02
54 SOICW-EP
ELECTRIC FIELD
IMAGING DEVICE
MC33794
EK SUFFIX
54-LEAD SOICW-EP
CASE 1390-02 Sensors
2
Freescale Semiconductor
MC33794
INTERNAL BLOCK DIAGRAM
Figure 1. Simplified Functional Block Diagram
CONTROL
LOGIC
MUX
OUT
OSC
MUX
IN
RECT
LPF
GAIN AND
OFFSET
ATTN
LAMP CKT
ISO-9141
POR/
WD
V
CC
REG
V
DD
REG
CLK
R_OSC
SHIELD_EN
SHIELD
LP_CAP
LEVEL
SIGNAL
LAMP_SENSE
LAMP_MON
LAMP_OUT
ISO_IN
ISO_OUT
LAMP_CTRL
LAMP_GND
V
DD
_
MON
PWR_MON
GND and HEAT SINK
AGND
V
PWR
WD_IN
RST
V
CC
V
DD
E1E9
REF_A*, REF_B*
TEST
A,B,C,D
4
150 39 k 10 nF
22 k (Nominal)
ISO-9141
2.8 k 700 *
2.8 k * REF_A and REF_B are
not switched to ground
when not selected.
300 700 *
(Note: All Resistor Values are Nominal) Sensors
Freescale Semiconductor
3
MC33794
SOICW-EP TERMINAL CONNECTIONS
Figure 2. SOICW-EP Terminal Connections
Table 1. SOICW-EP TERMINAL FUNCTION DESCRIPTION
Terminal
Terminal
Name
Formal Name
Definition
1
RST
Reset
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
CC
REG is
out-of-range high or low, and a watchdog timer that will force a reset if WD_IN
is not asserted at regular intervals. Timing is derived from the oscillator and will
change with changes in the resistor attached to R_OSC.
2
WD_IN
Watchdog In
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 doesnt 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.
3, 5, 7,
2023, 31,
33, 34,
4850, 52
NC
No connect
These terminals may be used at some future date and should be left open.
4
LAMP_GND
Lamp Ground
This is the ground for the current from the lamp. The current into LAMP_OUT
flows out through this terminal.
6
LAMP_OUT
Lamp Driver
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.
LAMP_OUT
1
PWR_MON
LP_CAP
R_OSC
NC
NC
NC
NC
CLK
V
DD
_
MON
V
DD
V
PWR
LEVEL
11
12
13
14
15
16
17
18
19
20
9
10
21
22
23
24
25
26
27
6
7
8
4
5
SHIELD_EN
D
C
B
A
SIGNAL
NC
LAMP_SENSE
LAMP_MON
NC
LAMP_GND
NC
2
3
54
44
43
42
41
40
39
38
37
36
35
46
45
34
33
32
31
30
29
28
49
48
47
51
50
53
52
RST
WD_IN
NC
E3
E2
E1
TEST
NC
NC
GND
NC
SHIELD
AGND
V
CC
E4
REF_A
E9
E8
E7
E6
E5
NC
ISO_OUT
REF_B
NC
ISO_IN
NC
LAMP_CTRL
ISO-9414 Sensors
4
Freescale Semiconductor
MC33794
8
LAMP_SENSE
Lamp Sense
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.
9
LAMP_MON
Lamp Monitor
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.
10
SHIELD_EN
Shield Driver
This terminal is used to enable the shield signal. The shield is disabled when
SHIELD_EN is a logic low (ground)
1114
A, B, C, D
Selector Inputs
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.
15
SIGNAL
Undetected Signal
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.
16
LEVEL
Detected Level
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.
17
PWR_MON
Power Monitor
This is connected through a voltage divider to V
PWR
. It allows reduction of the
voltage so it will fall within the range of the ADC on the MCU.
18
LP_CAP
Low-Pass Filter Capacitor
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.
19
R_OSC
Oscillator Resistor
A resistor from this terminal to circuit ground determines the operating frequency
of the oscillator. The MC33794 is optimized for operation around 120 kHz.
24
CLK
Clock
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.
25
V
DD
_
MON
V
DD
Monitor
This is connected through an internal voltage divider to V
DD
REG. It allows
reduction of the voltage so it will fall within the range of the ADC on the MCU.
26
V
DD
V
DD
Capacitor
A capacitor is connected to this terminal to filter the internal analog regulated
supply. This supply is derived from V
PWR
through internal V
DD
REG.
27
V
PWR
Positive Power Supply
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
CC
REG) and
8.5 V (internal V
DD
REG) to power the MCU and external devices.
28
V
CC
5.0 V Regulator Output
This output terminal requires a 47 µF capacitor and internal V
CC
REG provides
a regulated 5.0 V for the MCU and for internal needs of the MC33794.
29
AGND
Analog Ground
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.
30
SHIELD
Shield Driver
This terminal connects to cable shields to cancel cable capacitance.
Table 1. SOICW-EP TERMINAL FUNCTION DESCRIPTION (continued)
Terminal
Terminal
Name
Formal Name
Definition Sensors
Freescale Semiconductor
5
MC33794
32
GND
Ground
This terminal and metal backing is the IC power return and thermal radiator/
conductor.
35
TEST
Test Mode Control
This terminal is normally connected to circuit ground. There are special
operating modes associated with this terminal when it is not at ground.
3644
E1E9
Electrode Connections
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.
45, 46
REF_A,
REF_B
(E10, E11)
Reference Connections
(Or as additional electrodes)
These terminals can be individually selected to measure a known capacitance
value. Unlike E1-E9, these two inputs are not grounded when not selected.
47
ISO_OUT
ISO-9141 Output
This terminal translates ISO-9141 receive levels to 5.0 V logic levels for the
MCU.
51
ISO_IN
ISO-9141 Input
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.
53
ISO-9141
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 level