MCP2551ESLASHP datasheet(3/20 Pages) MICROCHIP

3 / 20 page

 2002 Microchip Technology Inc.

Preliminary

DS21667C-page 3

MCP2551

1.0

DEVICE OVERVIEW

The MCP2551 is a high-speed CAN, fault-tolerant

device that serves as the interface between a CAN pro-

tocol controller and the physical bus. The MCP2551

provides differential transmit and receive capability for

the CAN protocol controller and is fully compatible with

the ISO-11898 standard, including 24V requirements. It

will operate at speeds of up to 1 Mb/s.

Typically, each node in a CAN system must have a

device to convert the digital signals generated by a CAN

controller to signals suitable for transmission over the

bus cabling (differential output). It also provides a buffer

between the CAN controller and the high-voltage spikes

that can be generated on the CAN bus by outside

sources (EMI, ESD, electrical transients, etc.).

1.1

Transmitter Function

The CAN bus has two states: Dominant and Reces-

sive. A dominant state occurs when the differential volt-

age between CANH and CANL is greater than a

defined voltage (e.g.,1.2V). A recessive state occurs

when the differential voltage is less than a defined volt-

age (typically 0V). The dominant and recessive states

correspond to the low and high state of the TXD input

pin, respectively. However, a dominant state initiated

by another CAN node will override a recessive state on

the CAN bus.

1.1.1

MAXIMUM NUMBER OF NODES

The MCP2551 CAN outputs will drive a minimum load

of 45

Ω, allowing a maximum of 112 nodes to be con-

nected (given a minimum differential input resistance of

20 k

Ω and a nominal termination resistor value of

120

Ω).

1.2

Receiver Function

The RXD output pin reflects the differential bus voltage

between CANH and CANL. The low and high states of

the RXD output pin correspond to the Dominant and

Recessive states of the CAN bus, respectively.

1.3

Internal Protection

CANH and CANL are protected against battery short-

circuits and electrical transients that can occur on the

CAN bus. This feature prevents destruction of the

transmitter output stage during such a fault condition.

The device is further protected from excessive current

loading by thermal shutdown circuitry that disables the

output drivers when the junction temperature exceeds

a nominal limit of 165°C. All other parts of the chip

remain operational and the chip temperature is lowered

due to the decreased power dissipation in the transmit-

ter outputs. This protection is essential to protect

against bus line short-circuit induced damage.

1.4

Operating Modes

The RS pin allows three modes of operation to be

selected:

• High-Speed

• Slope-Control

• Standby

These modes are summarized in Table 1-1.

When in High-Speed or Slope-Control mode, the driv-

ers for the CANH and CANL signals are internally regu-

lated to provide controlled symmetry in order to

minimize EMI emissions.

Additionally, the slope of the signal transitions on

CANH and CANL can be controlled with a resistor con-

nected from pin 8 (RS) to ground, with the slope propor-

tional to the current output at RS, further reducing EMI

emissions.

1.4.1

HIGH-SPEED

The High-Speed mode is selected by connecting the

RS pin to VSS. In this mode, the transmitter output driv-

ers have fast output rise and fall times to support high-

speed CAN bus rates.

1.4.2

SLOPE-CONTROL

Slope-Control mode further reduces EMI by limiting the

rise and fall times of CANH and CANL. The slope, or

slew rate (SR), is controlled by connecting an external

resistor (REXT) between RS and VOL (usually ground).

The slope is proportional to the current output at the RS

pin. Since the current is primarily determined by the

slope-control resistance value REXT, a certain slew rate

is achieved by applying a respective resistance.

Figure 1-1 illustrates typical slew rate values as a

function of the slope-control resistance value.

1.4.3

STANDBY MODE

The device may be placed in standby or “SLEEP” mode

by applying a high-level to RS. In SLEEP mode, the

transmitter is switched off and the receiver operates at

a lower current. The receive pin on the controller side

(RXD) is still functional but will operate at a slower rate.

The attached microcontroller can monitor RXD for CAN

bus activity and place the transceiver into normal oper-

ation via the RS pin (at higher bus rates the first CAN

message may be lost).

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