The MAX33011E/MAX33012E and MAX33014E/MAX33015E are a family of fault-protected CAN transceivers with fault detection and fault reporting. They are designed for applications where expeditious troubleshooting is important to increase the up time of important control systems, addressing common faults like overcurrent, overvoltage, and transmission failure. These devices are ideal for harsh industrial applications with a number of integrated robust protection feature set that improve the reliability of end equipment. 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™, CAN Kingdom, and CANOpen™ applications as well.
All CAN transceivers in the family are fault protected up to ±65V, making them suitable for applications where overvoltage protection is required. These devices are rated up to a high ±45kV ESD of human body model (HBM), suitable for protection during the manufacturing process, and even in the field where there is a human interface for installation and maintenance. In addition, a common-mode voltage range of ±25V enables communication in noisy environments where there are ground plane differences between different systems due to the close proximity of heavy equipment machinery or operation from different transformers. 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 protected against excessive power dissipation by thermal shutdown circuitry that places the driver outputs in a high-impedance state.
The MAX33011E/MAX33012E and MAX33014E/MAX33015E can operate up to 5Mbps. This family has the option to slow the slew rate to 8V/μs to minimize EMI, enabling the use of unshielded-twisted or parallel cable. The MAX33011E/MAX33012E are packaged in an industry-standard 8-pin SO, while the MAX33014E/MAX33015E are packaged in a 10-pin TDFN. This family of robust CAN transceivers has an operating temperature from -40°C to +125°C.
±65V Fault Protection
These devices feature ±65V of fault protection. The CANH and CANL data lines are capable of withstanding a short from -65V to +65V. 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.
Table 1. Transmitter and Receiver Truth Table (When Not Connected to the Bus)
STBY
TXD
TXD LOW
TIME
CANH
CANL
BUS STATE
RXD
LOW
LOW
< tDOM
HIGH
LOW
DOMINANT
LOW
LOW
LOW
> tDOM
VDD/2
VDD/2
RECESSIVE
HIGH
LOW
HIGH
X
VDD/2
VDD/2
RECESSIVE
HIGH
X = Don't care
Transmitter Output Protection
This family of CAN transceivers protects the transmitter output stage against a short-circuit to a positive or negative voltage by limiting the driver current. See the CANH and CANL short-circuit current TOC graphs. Thermal shutdown further protects the devices from excessive temperatures that may result from a short. The transmitter returns to normal operation once the short is removed.
Transmitter-Dominant Timeout
These devices feature a transmitter-dominant timeout (tDOM) 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 tDOM, 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.
Receiver
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 in normal mode and ±12V in standby mode. RXD is a logic-high when CANH and CANL are shorted or terminated and undriven.
Fault Detection and Reporting
This family of devices has fault detection for overcurrent, overvoltage, and transmission failure in normal mode operation. The detection of faults and reporting them out to the local CAN controller provide additional information that benefits the troubleshooting of a given problem in a CAN bus system, reducing down time, improving equipment efficiencies, and keeping service costs down.
To enable fault detection upon power-up, 100 low-to-high transitions need to pass through TXD, which is typically 1 or 2 CAN frame messages depending on data payload size (classic or extended format) and which protocol is used. Fault detection is not enabled in standby and silent mode. After the 100 low-to-high transitions on TXD, if a fault is detected, then another 16 low-to-high transitions on TXD are required to shift out the fault code shown in Table 2. In addition, 10 more pulses are needed to clear the fault.
Table 2. Fault Detection and Reporting
FAULT
CONDITION (FAULT DETECTION ENABLED)
FAULT CODE
POSSIBLE CAUSE
Overcurrent
>85mA
101010
CANH shorted to CANL
CANH connected to GND and CANL connected to VDD
Overvoltage
CANH > +29V or CANL < -29V
101100
CMR fault
Transmission Failure
RXD unchanged for 10 consecutive pulses, recommended minimum frequency = 200kHz
110010
Open load (both termination resistors missing) on CANH and CANL
Exceeds driver's common-mode range
CANH and/or CANL connected to a fixed voltage source
Overvoltage Detection
Overvoltage detection is triggered when CANH is above approximately +29V or CANL is below approximately -29V. This indicates that the CAN bus has likely violated the CMR range or that a short fault on CANH and/or CANL has occurred and is beyond the ±29V threshold. Once overvoltage detection is triggered, the FAULT pin transitions from low to high and the fault code is clocked out of RXD through TXD.
Overcurrent Detection
Overcurrent detection is triggered when there is a high current path from VDD to GND through a short from CANH to CANL. In addition, shorts far away from the CAN node may not be detected due to high cable impedance. See Figure 7 for overcurrent detection maximum operating frequency versus cable length as a reference. A Cat5e copper-clad aluminum cable is used. The maximum frequency will vary with the type of cable.
Figure 7. Overcurrent Detection Operating Frequency vs. Cable Length
Transmission Failure Detection
Transmission failure detection is triggered when the signal on RXD does not match TXD for 10 consecutive cycles after fault detection is enabled. This can occur when both termination resistors are missing. Other scenarios include, but are not limited to, shorting CANH to GND or CANL to VDD resulting in the differential signal not meeting the receiver’s VIH and VIL specification.
Fault Reporting
When a fault occurs and the FAULT pin goes high, the CANH and CANL lines are placed in high impedance, and a 6-bit fault code of the first detected fault condition is stored in an internal register. 16 low-to-high transitions need to go through TXD to shift the fault code out through RXD.
An overcurrent fault timing diagram is shown in Figure 8.
The fault condition can be cleared only after the 6-bit fault code is reported through the RXD pin of the chip. Soon after the fault code is reported, send 10 clock pulses through the TXD pin, which deasserts the FAULT pin. The FAULT pin cannot be cleared in standby or silent mode. Fault detection is disabled after fault is cleared and another 100 low-to-high transitions are required on TXD to re-enable fault detection.
Standby Mode (MAX33012E, MAX33014E)
Drive the 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 to 60μA 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 longer than tWAKE, RXD is pulled low. Drive the STBY pin low for normal operation. Fault detection is disabled in standby mode.
Slow Slew Rate Mode
Connect a 26.1kΩ resistor between ground and the STBY/S pin. This will put the device in slow slew rate mode where the typical rising slew rate is 10V/s and the typical falling slew rate is 18V/s, enabling the use of unshielded-twisted or parallel cable, compared with normal mode at 140V/μs falling and 180V/μs rising slew rate. The STBY pin voltage should be between 0.2V to 0.6V to remain in slow slew rate mode. Slow slew rate mode is recommended for transmitter frequencies which have a data rate that is less than 1Mbps.
Silent Mode (MAX33011E, MAX33015E)
Drive S high to place the device in silent mode. This disables the transmitter regardless of the voltage level at TXD. However, RXD is still active and monitors activity on the bus line. Make sure that the FAULT pin is cleared before entering into silent mode. Fault detection is disabled in silent mode.
Loopback Mode (MAX33014E, MAX33015E)
The LPBK pin enables the local CAN controller to perform self-diagnostics. A logic-high to LPBK places the transceiver in a high impedance state to the bus. This allows data to pass internally from the driver to receiver in a loopback test mode without disturbing the bus. When LPBK is connected to the ground, the transceiver operates in normal mode.
Logic Compatibility (MAX33014E, MAX33015E)
A separate input VL allows the devices to communicate with logic systems down to 1.62V while operating from a +5V supply. This provides a reduced input voltage threshold to the TXD, STBY, S, and LPBK inputs, and provides a logic-high output at RXD that is compatible with the microcontroller's supply rail. The logic compatibility eliminates an external logic level translator and longer propagation delay. Connect VL to VDD to operate with +5V logic systems.
