Why do CAN tranceivers need so much current when transmitting dominant bits?

• Why do CAN tranceivers need so much current when transmitting dominant bits?
  Posted by AndersJ on 31 Mar, 2017 02:05
• Investigating current demand and bulk capacitance when transmitting,
  I noticed there is significantly higher current flowing IN to the tranceiver
  than what goes out to the bus termination.
  The datasheet confirms that several tens of mA are used within the tranceiver.
  
  Why does the tranceiver need so much current internally when sending a dominant bit?
  
• #1 Reply
  Posted by 406dany on 31 Mar, 2017 03:26
• I have see the same effect on a MCP2551
  i have a big spike on VDD when transmitting bit on CAN bus (or VAN bus)
  I think it is the result of CAN bus differentiel resistor as i have several Equipment on the bus (
  but not sure ...
  
• #2 Reply
  Posted by Niklas on 31 Mar, 2017 09:05
• Have you checked the current consumption with and without 60 ohms (120//120) bus termination? A 2V differential signal over a 60 ohm load will give you approx 30 mA of current. In addition to that you also have the capacitive load, <=100pF/node, and the wire harness. Also the topology of the output stage might draw some bias and control current.
• #3 Reply
  Posted by AndersJ on 31 Mar, 2017 09:54
• I have checked current consumption with a large termination resistance, 250 Ohms.
  I have also done this statically, or at least extremely slowly, near DC.
  
  My original post is still valid:
      there is significantly higher current flowing IN to the tranceiver
      than what goes out to the bus termination.
  
  Also I have checked the device with a thermal camera.
  When the tranceiver is toggled on/off manually the temperature rises about 10 degrees C within a few seconds.
  
  The question remains:
  Why does it need so much power internally?
  If there was a heavy bus load or a short, then so be it,
  but why does this happen when there is practically no load?
  
  Its no big deal, expecially since other devices seem to be similar.
  I'm just curious.
  
• #4 Reply
  Posted by Niklas on 31 Mar, 2017 14:25
• I would guess that it is related to the biasing and base current to the output drivers. The dominant state is not Vcc or 0V and the outputs also have slope control. More analog than digital CMOS inverter type of output.
• #5 Reply
  Posted by T3sl4co1l on 31 Mar, 2017 18:10
• Some internal current mirror logic and control amp sort of stuff would be reasonable, but that shouldn't account for more than a few mA in a competent design.
  
  Unfortunately this isn't a question you'll find an answer to, short of a pile of NDAs and an awful lot of schmoozing the manufacturer, or becoming an employee at the manufacturer (again under similar constraints). 
  
  Tim