2000 Jan 13

4

Philips Semiconductors

Product specification

CAN transceiver for 24 V systems

PCA82C251

FUNCTIONAL DESCRIPTION

The PCA82C251 is the interface between the CAN

protocol controller and the physical bus. It is primarily

intended for applications up to 1 Mbaud in trucks and

buses. The device provides differential transmit capability

to the bus and differential receive capability to the CAN

controller. It is fully compatible with the

“ISO 11898-24 V”

standard.

A current limiting circuit protects the transmitter output

stage against short-circuit to positive and negative battery

voltage. Although the power dissipation is increased

during this fault condition, this feature will prevent

destruction of the transmitter output stage.

If the junction temperature exceeds a value of

approximately 160

°C, the limiting current of both

transmitter outputs is decreased. Because the transmitter

is responsible for the major part of the power dissipation,

this will result in a reduced power dissipation and hence a

lower chip temperature. All other parts of the IC will remain

operating. The thermal protection is particularly needed

when a bus line is short-circuited.

The CANH and CANL lines are also protected against

electrical transients which may occur in an automotive

environment.

Pin 8 (Rs) allows three different modes of operation to be

selected: high-speed, slope control or standby.

For high-speed operation, the transmitter output

transistors are simply switched on and off as fast as

possible. In this mode, no measures are taken to limit the

rise and fall slope. Use of a shielded cable is

recommended to avoid RFI problems. The high-speed

mode is selected by connecting pin 8 to ground.

The slope control mode allows the use of an unshielded

twisted pair or a parallel pair of wires as bus lines.

To reduce RFI, the rise and fall slope should be limited.

The rise and fall slope can be programmed with a resistor

connected from pin 8 to ground. The slope is proportional

to the current output at pin 8.

If a HIGH level is applied to pin 8, the circuit enters a low

current standby mode. In this mode, the transmitter is

switched off and the receiver is switched to a low current.

If dominant bits are detected (differential bus voltage

>0.9 V), RXD will be switched to a LOW level.

The microcontroller should react to this condition by

switching the transceiver back to normal operation

(via pin 8). Because the receiver is slower in standby

mode, the first message will be lost at higher bit rates.

Table 1

Truth table of the CAN transceiver

Notes

1. X = don’t care.

2. If another bus node is transmitting a dominant bit, then RXD is logic 0.

Table 2

Pin Rs summary

TXD

CANH

CANL

BUS STATE

RXD

4.5 to 5.5 V

0

HIGH

LOW

dominant

0

4.5 to 5.5 V

1 (or floating)

floating

floating

recessive

floating if

floating if

floating

floating

floating

floating

floating

CONDITION FORCED AT PIN Rs

MODE

RESULTING VOLTAGE OR CURRENT AT PIN Rs

standby

10

slope control

high-speed