Detailed Description
The MAX33040E/MAX33041E are fault-protected CAN transceivers designed for industrial applications with a number of integrated robust protection feature set. These devices provide a link between the CAN protocol controller and the physical wires of the bus lines in a CAN. They can be used for DeviceNet™ applications as well.
These CAN transceivers are fault-protected on CANH and CANL up to ±40V, making it suitable for applications where overvoltage protection is required. These devices are rated up to a high ±40kV HBM ESD on CANH and CANL, suitable for protection during the manufacturing process, and even in the field where there is human interface for installation and maintenance. In addition, a common-mode voltage of ±25V enables communication in noisy environments where there are ground plane differences between different systems due to close proximity of heavy equipment machinery or operation from different transformers. The devices' dominant timeout prevents the bus from being blocked by a hung-up microcontroller, and the outputs CANH and CANL are short-circuit, current-limited, and are protected against excessive power dissipation by thermal shutdown circuitry that places the driver outputs in a high-impedance state.
These devices can operate up to 5Mbps with a standby mode where it shuts off the transmitter and reduces the current to 30μA (typ).
These devices feature ±40V of fault protection. CANH and CANL data lines are capable of withstanding a short from -40V to +40V. This extended overvoltage range makes it suitable for applications where accidental shorts to power supply lines are possible due to human intervention.
Transmitter
The transmitter converts a single-ended input signal (TXD) from the local CAN controller to differential outputs for the bus lines CANH and CANL. The truth table for the transmitter and receiver is provided in
Table 1.
The MAX33040E/MAX33041E protects the transmitter output stage against a short-circuit to a positive or negative voltage by limiting the driver current. Thermal shutdown further protects the devices from excessive temperatures that may result from a short or high ambient temperature. The transmitter returns to normal operation once the temperature is lowered below the threshold.
Transmitter-Dominant Timeout
The devices feature a transmitter dominant timeout (t
DOM) that prevents erroneous CAN controllers from clamping the bus to a dominant level by maintaining a continuous low TXD signal. When TXD remains in the dominant state (low) for greater than 2.5ms typical t
DOM, the transmitter is disabled, releasing the bus to a recessive state (
Figure 4). After a dominant timeout fault, the transmitter is re-enabled when receiving a rising edge at TXD. The transmitter dominant timeout limits the minimum possible data rate to 9kbps for standard CAN protocol.
The receiver reads the differential input from the bus line CANH and CANL and transfers this data as a single-ended output RXD to the CAN controller. It consists of a comparator that senses the difference VDIFF = (CANH-CANL), with respect to an internal threshold of 0.7V. If VDIFF > 0.9V, a logic-low is present on RXD. If VDIFF < 0.5V, a logic-high is present. The CANH and CANL common-mode range is ±25V. RXD is a logic-high when CANH and CANL are shorted or terminated and undriven.
Standby Mode
Drive STBY pin high for standby mode, which switches the transmitter off and the receiver to a low current and low-speed state. The supply current is reduced during standby mode. The bus line is monitored by a low differential comparator to detect and recognize a wakeup event on the bus line. Once the comparator detects a dominant bus level greater than 2.3μs typical tWAKE, RXD pulls low. Drive the STBY low for normal operation.
Slow Slew Rate
Connect a 39.2kΩ resistor between ground and the STBY pin to reduce the slew rate on the transmitter output. The STBY pin voltage should be between 0.1V to 0.6V to remain in slow slew rate. This will change the MAX33040E with a slow slew rate of 15V/μs for rising edge compared with normal mode at 120V/μs. For falling edge, the slow slew rate is 20V/μs compared with normal mode at 80V/μs.
Table 1. Transmitter and Receiver Truth Table (When Not Connected to the Bus)
| MODE |
TXD |
TXD LOW TIME |
CANH |
CANL |
BUS STATE |
RXD |
| Normal (STBY = LOW) |
LOW |
< tDOM |
HIGH |
LOW |
DOMINANT |
LOW |
| Normal (STBY = LOW) |
LOW |
> tDOM |
VDD/2 |
VDD/2 |
RECESSIVE |
HIGH |
| Normal (STBY = LOW) |
HIGH |
X |
VDD/2 |
VDD/2 |
RECESSIVE |
HIGH |
| Standby (STBY = HIGH) |
X |
X |
HIGH |
LOW |
DOMINANT |
LOW |
| Standby (STBY = HIGH) |
X |
X |
VDD/2 |
VDD/2 |
RECESSIVE |
HIGH |
| Shutdown (SHDN = STBY = HIGH) |
X |
X |
VDD/2 |
VDD/2 |
RECESSIVE |
HIGH |
X = Don't care
Drive SHDN pin high for shutdown mode, which switches the transmitter and receiver off. The supply current is reduced to maximum of 5μA during shutdown mode. Drive the SHDN pin low for normal operation.