MC33199 Datasheet, PDF (13/18 Pages) Freescale Semiconductor, Inc – Automotive ISO 9141 Serial Link Driver

FUNCTIONAL DEVICE OPERATION
FUNCTIONAL DEVICE OPERATION
POWER SUPPLIES AND REFERENCE VOLTAGE
The device has two power supplies :
A 5V supply, VCC, normally connected to the MCU supply
voltage. This pin sinks typically 1mA during operation. A
VBAT supply voltage, VS, normally tied to the car battery
voltage. This pin can sustain up to 40V DC. Care should be
taken for reverse battery protection and transient voltages
higher than 40V.
The voltage reference generator is supplied from both
VCC and VBAT. It provides reference voltage for the K and L
lines comparators thresholds. The reference voltage is
dependant on VBAT voltage : it is linear versus VBAT
voltage, for VBAT from 5.6V to 18V. Below 5.6V and over
18V the reference voltage is clamped (see Figure 12). The
reference is connected externally to the device, through REF-
OUT pin. It is available for other needs. It can supplied 50µΑ
max (see Figure 13).
PATH FUNCTIONS BETWEEN MCU, K AND L
LINES
The path function from the MCU to the K line is composed
of a driver interfacing directly with the MCU through the TXD
pin. The TXD pin is CMOS compatible. This driver controls a
power transistor which can be turned ON or OFF. When it is
ON, it pull the DIA pin low. This pin is known as K line in the
ISO 9141 specification. The DIA pin structure is open
collector, without pull up component. This allow the
connection of several MC33199 on the K line and the use of
a single pull up resistor per system (see Figure 25). In order
to protect the DIA pin against short circuits to VBAT, the
device incorporates a current limitation (see Figure 17) and a
thermal shutdown. This current limitation will also act when
the device drives a K line bus exhibiting large parasitic
capacitor value (see Special functionality of I1 pin).
The path from this DIA pin, or K line, to the MCU is done
through a comparator. The comparator threshold voltage is
connected to REF-IN-K pin. It can be tied to the REF-OUT
voltage, if the VBAT dependant threshold is to be achieved.
The second input of this comparator is internally connected
to DIA pin. The output of the comparator is available on RXD
output pin, normally connected to a MCU I/O port. RXD pin
has a 2kOhms internal pull up resistor.
The path from the L line, used during wake-up sequence
of the transmission, to the MCU is done through a second
comparator. The comparator threshold voltage is connected
to REF-IN-L pin. As the REF-IN-K pin, it can be tied to the
REFOUT voltage, if the VBAT dependant threshold need to
be achieved. The second input of this comparator is internally
connected to L pin. The output of the comparator is available
on LO output pin, which is an open collector structure. LO is
normally connected to a MCU I/O port.
The DIA, and L pins can sustain up to 38V DC. Care
should be taken for reverse battery protection and transient
voltages higher than 38V.
The DIA and L pins both have internal pull down current
source of typically 7.5µΑ (see Figure 15). So the L line
exhibits a 10µΑ pull down current. The DIA pin has the same
behavior when it is in OFF state, that is when TXD is at logic
high level.
SPECIAL FUNCTIONALITY OF I1 PIN
The MC33199 has a unique feature which allows the
transmission Baud rate to be up to 200kBaud. In practice, the
K line can be several meters long, and thus can have a large
parasitic capacitor value. This parasitic capacitor value will
slow down the low to high transition of the K line, and indeed
will limit the Baud rate transmission. For the K line to go from
low to high level, the parasitic capacitor need to be charged,
and it can only be charged by the pull up resistor. A low pull
up resistor value would result in fast charge time of the
capacitor, but also in large output current, and large power
dissipation in the driver.
To avoid this problem, the MC33199 incorporates a
dynamic current source, which is temporary activated at the
low to high transition of the TXD pin, that is when the DIA pin
or K line should switch from low to high level (see Figure 6 &
Figure 7).
This current source is available at I1 pin. It has a typical
value of 80mA. It is activated for 4µs (see Figure 22 &
Figure 23) and is automatically disabled after this time.
During that time it will charge the K line parasitic capacitor.
This extra current will quickly rise the K line voltage up to the
Vbat, and will result in reduce rise time on the K line. With this
feature the MC33199 can ensure Baud rate transmission of
up to 200kBaud.
During high to low transition on the K line, the parasitic
capacitor of the bus line will be discharged by the output
transistor of the DIA pin. In this case, the total current may
exceed the internal current limitation of the DIA pin. If so, the
current limitation will act, and discharge current will be limited
to typically 60mA (See Figure 7 & Figure 17).
If a high Baud rate is necessary, the I1 pin need to be
connected to the DIA as shown in the typical application
Figure 24. The I1 pin can also be left open, if the I1
functionality and high Baud rate are not suited in the
application.
Analog Integrated Circuit Device Data
Freescale Semiconductor
33199
13