ISL72028SEH

FN8764 Rev 3.00

Page 10 of 18

August 16, 2016

Functional Description

Overview

The Intersil ISL72028SEH is a 3.3V radiation tolerant CAN

transceiver that is compatible with the ISO11898-2 standard for

use in CAN (Controller Area Network) serial communication

systems.

The device performs transmit and receive functions between the

CAN controller and the CAN differential bus. It can transmit and

receive at bus speeds of up to 5Mbps. It is designed to operate

over a common-mode range of -7V to +12V with a maximum of

120 nodes. The device is capable of withstanding ±20V on the

CANH and CANL bus pins outside of ion beam and ±16V under

ion beam.

Slope Adjustment

The output driver rise and fall time has three distinct selections

that may be chosen by using a resistor from the RS pin to GND.

Connecting the RS pin directly to GND results in output switching

times that are the fastest, limited only by the drive capability of

the output stage. RS = 10kΩ provides for a typical slew rate of

8V/µs and RS = 50kΩ provides for a typical slew rate of 4V/µs.

Putting a high logic level to the RS pin places the device in a low

power shutdown mode. The protocol controller uses this mode to

switch between low power shutdown mode and normal transmit

mode.

Cable Length

The device can work per ISO11898 specification with a 40m

cable and stub length of 0.3m and 60 nodes at 1Mbps. This is

greater than the ISO requirement of 30 nodes. The cable type

specified is twisted pair (shielded or unshielded) with a

characteristic impedance of 120Ω. Resistors equal to this are to

be terminated at both ends of the cable. Stubs should be kept as

short as possible to prevent reflections.

Cold Spare

High reliability system designers implementing data

communications have to be sensitive to the potential for single

point failures. To mitigate the risk of a failure, they will use

redundant bus transceivers in parallel. In this arrangement, both

active and quiescent devices can be present simultaneously on

the bus. The quiescent devices are powered down for cold spare

and do not affect the communication of the other active nodes.

To achieve this, a powered down transceiver (VCC < 200mV) has

a resistance between the VREF pin or the CANH pin or CANL pin

and the VCC supply rail of >480kΩ (max) with a typical resistance

>2MΩ. The resistance between CANH and CANL of a

powered-down transceiver has a typical resistance of 80kΩ.

Low Power Shutdown Mode

When a high level is applied to the RS pin, the device enters the

low power shutdown mode in which the driver and receiver are

switched off to conserve power. The bus pins are at High Z and R

pin will be at logic high. In low power shutdown the transceiver

draws 50µA (max) of current.

A low level on the RS pin brings the device back to operation.

Using 3.3V Devices in 5V Systems

Looking at the differential voltage of both the 3.3V and 5V

devices, the differential voltage is the same, the recessive

common-mode output is the same. The dominant

common-mode output voltage is slightly lower than the 5V

counterparts. The receiver specs are also the same. Though the

electrical parameters appear compatible, it is advised that

necessary system testing be performed to verify interchangeable

operation.

Split Mode Termination

The VREF pin provides a VCC/2 output voltage for split mode

termination. The VREF pin has the same ESD protection, short

circuit protection and common-mode operating range as the bus

pins.

The split mode termination technique is shown in Figure 19.

It is used to stabilize the bus voltage at VCC/2 and prevent it from

drifting to a high common-mode voltage during periods of

inactivity. The technique improves the electromagnetic

compatibility of a network. The split mode termination is put at

each end of the bus.

The CL capacitor between the two 60Ω resistors, filters unwanted

high frequency noise to ground. The resistors should have a

tolerance of 1% or better and the two resistors should be

carefully matched to provide the most effective EMI immunity. A

typical value of CL for a high speed CAN network is 4.7nF, which

generates a 3dB point at 1.1Mbps. The capacitance value used is

dependent on the signaling rate of the network.

FIGURE 19. SPLIT TERMINATION

NODE

#1

NODE

#n

CANH

CANL

VREF

VREF

CL

CL

NODE

#2

60Ω

60Ω

60Ω

60Ω