MAX3051
+3.3V, 1Mbps, Low-Supply-Current
CAN Transceiver
9
Maxim Integrated
Detailed Description
The MAX3051 interfaces between the CAN protocol
controller and the physical wires of the bus lines in a
CAN. It provides differential transmit capability to the
bus and differential receive capability to the CAN con-
troller. It is primarily intended for +3.3V single-supply
applications that do not require the stringent fault pro-
tection specified by the automotive industry (ISO 11898)
The MAX3051 features four different modes of opera-
tion: high-speed, slope-control, standby, and shutdown
mode. High-speed mode allows data rates up to
1Mbps. The slope-control mode can be used to pro-
gram the slew rate of the transmitter for data rates of up
to 500kbps. This reduces the effects of EMI, thus allow-
ing the use of unshielded twisted or parallel cable. In
standby mode, the transmitter is shut off and the
receiver is pulled high, placing the MAX3051 in low-
current mode. In shutdown mode, the transmitter and
receiver are switched off.
The MAX3051 input common-mode range is from -7V to
+12V, exceeding the ISO 11898 specification of -2V to
+7V. These features, and the programmable slew-rate
limiting, make the part ideal for nonautomotive, harsh
environments.
The transceivers operate from a single +3.3V supply
and draw 35µA of supply current in dominant state and
2µA in recessive state. In standby mode, supply cur-
rent is reduced to 8µA. In shutdown mode, supply cur-
rent is less than 1µA.
CANH and CANL are output short-circuit current limited
and are protected against excessive power dissipation
by thermal-shutdown circuitry that places the driver
outputs into a high-impedance state.
Transmitter
The transmitter converts a single-ended input (TXD)
from the CAN controller to differential outputs for the
bus lines (CANH, CANL). The truth table for the trans-
mitter and receiver is given in Table 1.
Receiver
The receiver reads differential inputs from the bus lines
(CANH, CANL) and transfers this data as a single-
ended output (RXD) to the CAN controller. It consists of
a comparator that senses the difference V
DIFF
= (CANH
- CANL) with respect to an internal threshold of +0.75V.
If this V
DIFF
is greater than 0.75, a logic-low is present at
RXD. If V
DIFF
is less than 0.75V, a logic-high is present.
The receiver always echoes the CAN BUS data.
The CANH and CANL common-mode range is -7V to
+12V. RXD is logic-high when CANH and CANL are
shorted or terminated and undriven.
Mode Selection
High-Speed Mode
Connect RS to ground to set the MAX3051 to high-
speed mode. When operating in high-speed mode, the
MAX3051 can achieve transmission rates of up to
1Mbps. In high-speed mode, use shielded twisted pair
cable to avoid EMI problems.
Slope-Control Mode
Connect a resistor from RS to ground to select slope-
control mode (Table 2). In slope-control mode, CANH
and CANL slew rates are controlled by the resistor con-
nected to the RS pin. Maximum transmission speeds
are controlled by R
RS
and range from 40kbps to
500kbps. Controlling the rise and fall slopes reduces
EMI and allows the use of an unshielded twisted pair or
a parallel pair of wires as bus lines. The equation for
selecting the resistor value is given by:
R
RS
(kΩ) ≈ 12000 / (maximum speed in kbps)
See the Slew Rate vs. RRS graph in the
Typical
Operating Characteristics
.
Standby Mode
If a logic-high is applied to RS, the MAX3051 enters a
low-current standby mode. In this mode, the transmitter
