TCAN1042-Q1: CANH waveform is abnormal

Hi Sherry,

Can you please check VCC as well?  CANH is pulled to VCC during dominant output periods, and I wonder if the higher supply current that is required in this state may be causing the supply to droop for some time after the transition.

Regards,
Max

Hi Max,

Thanks for your quick reply!

Yes, the customer said at the VCC pin, there is also another circuit. They will give me the VCC waveform tomorrow. If the waveform of VCC also droop, what should we do? Adding capacitor at the VCC pin? How large is the value?

Thanks & Best Regards,

Sherry

Sherry,

There are a few things you could look into:

 - The amount of current pulled from VCC during a dominant state depends on the loading of the CAN bus.  This should be close to 60 Ohms (i.e., since there are typically two 120-Ohm termination resistances placed in parallel at opposite ends of the bus).  So, you should be careful to not go significantly lower in resistance from this (for example, if you have more than two terminating nodes on a network).

 - The amount of supply droop for a given load current would then depend on a few factors:

- The load regulation ability of the regulator (which determines its transient response) and the maximum output current (which should not be exceeded).  These could be addressed by modifying the regulator circuitry.

- The effective impedance of the power distribution network on the PCB.  For example, longer or thinner power traces from the regulator output to the transceiver could introduce both additional resistance and parasitic inductance which could degrade the transient response.  The decoupling capacitor will help to compensate for this.  So, improvements could be made either by revising power routing or (more easily) via adjusting the decoupling capacitance.  In terms of value, typically 100 nF is suitable for most applications.  If droop is observed with this value, though, you could try placing additional higher capacitances in parallel.

Regards,
Max

Hi Sherry,

Were you able to debug this issue any further?  Please let me know if there is any further help I can provide.

Regards,
Max

Hi Max,

Thanks for your explanation.

In the datasheet, the termination resistor is 120ohm. But you said there are typically two 120-Ohm termination resistances placed in parallel at opposite ends of the bus. I'm confused.

Thanks & Best Regards,

Sherry