Editor’s note : This month we have a new TI author, John Griffith, a systems engineer for the Interface group at TI where he has supported CAN transceivers in both applications and systems engineering roles. John received his Bachelor’s and Master’s Degrees in Electrical Engineering from Rochester Institute of Technology (RIT), Rochester, New York. John is a member of the ISO 11898 CAN Task Force developing the latest CAN physical layer standard and has one shared patent.
Over the last few decades, controller area network (CAN) applications have shifted from using primarily 5V protocol controllers to incorporating largely 3.3V controllers. However, the use of 5V CAN transceivers is still prevalent, so it is common to find CAN transceiver applications that pair a 3.3V controller with a 5V transceiver. This setup presents a number of challenges that can be solved by utilizing a 3.3V CAN transceiver in certain applications.
Mixing controller and transceiver supply voltages in one application necessitates having at least one regulated supply rail for each of the voltages. In some cases, this adds cost, board space, and complexity to the overall design solely to support the 5V transceiver. For these applications, switching the CAN transceiver to the 3.3V supply rail can alleviate this issue.
Figure 1 shows a block diagram of a two-rail application utilizing a 5V transceiver and a 3.3V controller. Figure 2 shows the simpler setup feasible if both the CAN transceiver and controller operate on 3.3V.
One typical concern is that the common-mode bias voltages for 5V and 3.3V CAN transceivers are not the same and could cause communication issues. Fortunately, this was addressed by the ISO18898-2 standard, which requires that the receiver portion of CAN-compliant transceivers be able to handle common-mode ranges of –2V to +7V. This requirement was further extended to a ±12V range with the ISO11898-5 release.
Figure 3 shows the common-mode bias voltage range of 3.3V transceivers. (typically biased to 0.7*Vcc to comply with both 2V and 3V CAN standard requirements), 5V transceivers, and common-mode input ranges required by the two CAN standards. There is still plenty of margin for larger common-mode offsets in the system in addition to those caused by mixing 5V and 3.3V CAN transceivers. This makes interoperability due to common-mode offset a non-issue.
One caveat to setting the common-mode bias voltage of 3.3V CAN transceivers to anything other than 0.5*Vcc is that a common-mode shift occurs when the device transmits. This shift occurs because the high-side and low-side drivers are sized to roughly the same strength, centering the resulting dominant bit to Vcc/2. A common-mode shift from 0.7*Vcc to 0.5*Vcc occurs every time the 3.3V transceiver transitions between recessive and dominant bits (Figure 4).
This shift in common-mode voltages can cause unwanted conducted and radiated emissions on the bus. To counteract this, a split termination can be placed on the bus to filter common-mode noise (Figure 5 ). This filter can be tuned by varying the value of the capacitor.
Another typical concern around adding 3.3V CAN transceivers is that they have a smaller output differential voltage than their 5V counterparts and, thus, have lower noise immunity. The CAN standard addresses this by requiring that all transceivers be capable of driving a minimum of 1.5V output differential voltage, and that receivers have the input threshold set at 0.9V differential. Therefore, as long as the 3.3V transceiver is capable of driving a minimum of 1.5V, there is 0.6V of margin built into the system for line loss and noise margin. To compensate for having less voltage headroom, the driver typically needs to be sized larger in 3.3V versus 5V transceivers.
In conclusion, common misconceptions that 3.3V and 5V CAN transceivers cannot be mixed in the same network or that you cannot build a robust 3.3V CAN network are simply not true. More than 10 years after their introduction, 3.3V CAN transceivers are still gaining popularity in applications such as white goods, server back planes, smart grid, and battery-powered devices because of their ability to simplify end product designs. With these points in mind, and an eye out for common-mode emissions, 3.3V CAN transceivers can find their place in many more applications moving forward.
Join us next time as we explain how RF interference affects linear circuits such as op amps, and show how device specifications can be used to choose devices with better immunity to RF interference.
Here’s more information about CAN.