MCP2551-ESLASHP datasheet(3/24 Pages) MICROCHIP

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DS21667F-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

protocol controller and the physical bus. The MCP2551

device 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

Recessive. A Dominant state occurs when the

differential voltage 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 voltage (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

connected

(given

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 low-

ered due to the decreased power dissipation in the

transmitter 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

drivers for the CANH and CANL signals are internally

regulated 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

connected from pin 8 (RS) to ground. The slope must

be proportional to the current output at RS, which will

further reduce EMI emissions.

1.4.1

HIGH-SPEED

High-Speed mode is selected by connecting the RS pin

to VSS. In this mode, the transmitter output drivers 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

achieved

applying

specific

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 the RS pin. 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

operation via the RS pin (at higher bus rates, the first

CAN message may be lost).

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