PCA82C251 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 04 — 25 August 2011 3 of 17
NXP Semiconductors
PCA82C251
CAN transceiver for 24 V systems
7.2 Pin description
8. Functional description
The PCA82C251 is the interface between a CAN protocol controller and the physical bus.
It is primarily intended for applications up to 1 MBd 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-circuits to
positive and negative battery voltage. Although power dissipation will increase as a result
of a short circuit fault condition, this feature will prevent destruction of the transmitter
output stage.
If the junction temperature exceeds approximately 160 C, the limiting current of both
transmitter outputs is decreased. Because the transmitter is responsible for most of the
power dissipated, this will result in reduced power dissipation and hence a lower chip
temperature. All other parts of the IC will remain operational. The thermal protection is
needed, in particular, 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 and 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 slopes. A
shielded cable is recommended to avoid RFI problems. High-speed mode is selected by
connecting pin 8 to ground.
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 slopes should be limited. The rise and fall
slopes 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
Table 3. Pin description
Symbol Pin Description
TXD 1 transmit data input
GND 2 ground
V
CC
3 supply voltage
RXD 4 receive data output
V
ref
5 reference voltage output
CANL 6 LOW-level CAN voltage input/output
CANH 7 HIGH-level CAN voltage input/output
Rs 8 slope resistor input
