33199 datasheet(13/18 Pages) FREESCALE

13 / 18 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

33199

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 50

µΑ

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 7.5

µΑ (see Figure 15). So the L line

exhibits a 10

µΑ 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 4

µs (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.

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