MC33388 Datasheet, PDF (13/28 Pages) Motorola, Inc – Fault Tolerant CAN Interface

MC33388
Electromagnetic Compatibility
Introduction
The device is designed for optimized noise emission (EMI)
and high susceptibility performances (EMC). The source for
both disturbance and susceptibility is primarily coming form the
bus line wires. They are by far the longest connections
compared to the printed circuit board of the application
receiving the MC333388, the microcontroller and the other
components.
CAUTION : The common mode voltage characteristics are
dependant upon immediate device environment, such as bus
capacitor loading, bus wire length and type, etc... In addition, the
symmetry of the CANL and CANH bus lines is key parameter to
optimize common mode glitches. For instance un-symmetry
could result in different parasitic capacitors value between
CANL to GND and CANH to GND and will increase common
mode glitches and degrade overall system performances.
EMI Noise
In order to minimize the HF noise generated by the
complete application, the MC33388 minimizes the common
mode voltage and current glitches occurring at each bus
transition : from dominant to recessive and from recessive to
dominant. This is achieves by excellent matching in signal
transition between CANL and CANH. The common mode
voltage and current glitches are defined as follow :
CmV = (Vcanh+Vcanl)/2,
CmI = (Icanh+Icanl)/2.
The device is optimized for dual wires operations. Under a
fault condition, for instance one CAN bus connection shorted to
fixed voltage, e.g GND, the common mode will be considerably
degraded.
Figure below shows the typical signals for common mode
voltage measured at CANL and CANH pins.
Figure 5. Typical Common Mode Glitch
Measured at CANL CANH
EMC Susceptibility Performances
The MC33388 is optimized for high immunity from external
field disturbances. The bus lines are by far the primary antenna
for external field coupling to the CANL, CANH, Rtl and Rth
connections. The device performances are characterized using
the Bulk Current Injection (BCI) test method, derivated from
specification ISO 11452-4.
Susceptibility evaluation with BCI :
The component is configured according to the electrical
schematic very close to the typical application schematic, figure
4. Main difference is that the microcontroller is replaced by an
external generator and analyzer connected to RX, TX and
NERR through optical link. A network composed of two nodes
equipped with MC33388, one in emitter and one in receiver is
evaluated. The disturbance is applied to both CANL and CANH
twisted pair bus line lines with appropriate coupling clamp.
During test sequences, received bits are compared to
transmit bits. When received bits are different from transmit bits
the device is considered as fail for the particular frequency.
NERR: 5V/div
Common Mode
Glitch
100mV/div
Tx: 5V/div
Results
Figures 7 and 8 below describe the device susceptibility
performances in the frequency range of 1 to 400MHz with target
of injected current of 200mA and 316mA.
When the target current is reached and when no
susceptibility is observed, the next frequency point is analyzed,
until reaching the max frequency, 400Mhz. If a susceptibility is
observed for a particular frequency, the free point is marked.
Figure 7 below shows results with a target susceptibility level
of 316mA or 49dBmA with a 1KHz 80% modulation added to the
injected current. Figure 8 shows the susceptibility levels with a
target susceptibility level of 200mA or 46dBmA without
modulation added to the injected current.
Figure 6. Min. Susceptibility Level With Modulation
Figure 7. Min. Susceptibility Level Without Modulation
60
60
LIMIT OF TEST (49dB) (No Susceptibility)
LIMIT OF TEST (46dB) (No Susceptibility)
50
50
46
40
40
Susceptibility Points
30
NO SUSCEPTIBILITY AREA
Susceptibility Points
30
NO SUSCEPTIBILITY AREA
20
20
10
1.0
10
100
FREQUENCY (MHz)
1000
10
1.0
10
100
1000
FREQUENCY (MHz)
MC33388
MOTOROLA
13