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參數(shù)資料
| 型號： | MCP2551 |
| 廠商： | Microchip Technology Inc. |
| 元件分類： | CAN |
| 英文描述： | High-Speed CAN Transceiver |
| 中文描述： | 高速CAN收發(fā)器 |
| 文件頁數(shù)： | 3/20頁 |
| 文件大?。?/td> | 364K |
| 代理商： | MCP2551 |
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
,
allowing a maximum of 112 nodes to be con-
nected (given a minimum differential input resistance of
20 k
and a nominal termination resistor value of
120
).
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 R
S
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 (R
S
) to ground, with the slope propor-
tional to the current output at R
S
, further reducing EMI
emissions.
1.4.1
HIGH-SPEED
The High-Speed mode is selected by connecting the
R
S
pin to V
SS
. 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 (R
EXT
) between R
S
and V
OL
(usually ground).
The slope is proportional to the current output at the R
S
pin. Since the current is primarily determined by the
slope-control resistance value R
EXT
, 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 R
S
. 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 R
S
pin (at higher bus rates the first CAN
message may be lost).
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