Place the protection and filtering circuitry as close to the bus connector, J1, to prevent transients, ESD and noise from propagating onto the board. The layout example provides information on components around the device itself. Transient voltage suppression (TVS) device can be added for extra protection, shown as D1. The production solution can be either a bi-directional TVS diode or a varistor with ratings matching the application requirements. This example also shows optional bus filter capacitors C10 and C11. A series common mode choke (CMC) is placed on the CANH and CANL lines between TCAN4550-Q1 and connector J1.
Design the bus protection components in the direction of the signal path. Do not force the transient current to divert from the signal path to reach the protection device. Use supply and ground planes to provide low inductance.
Note: High-frequency currents follows the path of least impedance and not the path of least resistance.
Use at least two vias for supply and ground
connections of bypass capacitors and protection devices to minimize trace and via
inductance.
- Bypass and bulk capacitors should be placed as
close as possible to the supply terminals of transceiver, examples are C3,
C4 and C5 on the FLTR, VIO, VCCOUT, pins and C6 and C7
on the VSUP supply.
- Bus termination: this layout example shows split
termination. This is where the termination is split into two resistors, R4 and R5, with the center or split
tap of the termination connected to ground via capacitor C9. Split
termination provides common mode filtering for the bus. When bus termination
is placed on the board instead of directly on the bus, additional care must
be taken to ensure the terminating node is not removed from the bus thus
also removing the termination.
- As terminal 8 (nINT) and 9 (GPO2) are open drain an
external resistor to VIO is required. These can have a value
between 2 kΩ and 10 kΩ.
- Terminal 12 (WAKE) is a bi-directional triggered wake-up
input that is usually connected to an external switch. It should be
configured as shown with a 10 nF (C8) to GND where R2 is 33 kΩ and R3 is 3
kΩ.
- Terminal 15 (INH) can be left floating if not used but a
100 kΩ pull-down resistor can be used to discharge the INH to a sufficient
level when the INH output is high-Z.
- Terminal 1 should have a series resistor (R8)
when using a crystal oscillator. More information about sizing this resistor
can be found in the TCAN455x Clock Optimization and Design Guidelines
application note.