TCAN4551-Q1: Oscillation related to VCCFLTR

Hello Ethan,

This momentary disruption is due to the sudden change in the load that comes from the CAN bus being connected to the LDO when the transceiver starts to drive a dominant state.  This is a complex load that has Resistive, Inductive and Capacitive elements to it from the bus which come together to create the overall bus impedance.  Enabling the driver during message transmission causes both a change in current from the LDO, and a change to the bus impedance which requires the LDO's feedback loop to detect and make the appropriate compensation, as well as a ringing on the bus from the change in bus impedance at this location.

The LDO's feedback loop needs some amount of time to adjust to the new load requirements that results in the voltage variance.  An additional load at the start of a message transmission results in a small momentary drop in the voltage level, and an reduction in load results in an increase in the voltage level.  Typically decoupling capacitors are used to help source or sink the current necessary to stabilize the voltage during changing load conditions and allow the LDO time to adjust.

Adjusting the decoupling capacitance on the VCCFLTR pin has some impact on waveform but it can be eliminated with an adjustment to the decoupling capacitors.

Decoupling capacitors are most effective when they are placed between the LDO and the Load.  However, the issue is that there is no way to add additional decoupling capacitors between the internal LDO and CAN transceiver.  Adjusting the caps on the external VCCFLTR pin help a little but because they are connected to the same LDO output, they are not in the direct current path and can only help the LDO voltage recover from the sudden CAN bus load requirements.

Also the amount of ringing can vary due to the properties of the CAN bus used during the testing.

Regards,

Jonathan