• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 33 subscribers
  • Views 1332 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AM26C32: How to calculate termination resistor power rating

Shinichi Inoue
Guru 20696 points
Part Number: AM26C32
Other Parts Discussed in Thread: AM26C31

Dear Specialists,

My customer is considering AM26C32 and has a question about termination resistor power rating.

I would be grateful if you could advise.

---

What power rating should I use for the RS485 terminating resistor?

Could you please let me know the how to calculate as well.

We'd like to use a 1/10W rated resistor, is that possible?

I looked for the specs for the differential voltage amplitude in the driver side AM26C31 data sheet, but I couldn't find it.
Where is it written in the data sheet and what V is it?

If the differential voltage is 5V full swing, the resistance power will be 5*5/100=0.25w, which will require a fairly large chip resistor.

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    Guru 20696 points

    Dear Specialists,

    I also attached application circuit listed below. 

    Best regards,

    Shinichi

  • +1 in reply to Shinichi Inoue
    Guru 284830 points

    The RS-422 specification requires enough drive strength to drop at least 2 V over the 100 Ω termination resistor. But the specification has no upper limit.

    The AM26C31 typically drops 3.1 V (see the VOD specification), resulting in 0.096 W. But if you require protection against shorts, you might need more.

  • +1 in reply to Shinichi Inoue
    TI__Mastermind 47007 points

    Hi Shinichi,

    So under normal conditions the driver will not output greater than VCC between its differential outputs - in reality it will be lower than VCC as the driver architecture includes voltage drop due to diode and output impedance is non-zero,  for typical RS-422 loading it is typically 3.1V for this device as Clemens mentioned. 

    With that being said, if you design for VCC the resistor would be 1/4W (VCC = 5V; R = 100Ω -->  P = 250mW = 1/4W ) 

    If you design for typical - then you are looking at 96.1mW - so a 1/8W or 1/10W would cover typical - but wouldn't leave much  margin for VOD's over typical so I wouldn't go this low generally. 

    Generally - in the interest of a robust design using 1/4W termination resistors will keep termination safe up to 5V being placed across it - which in most applications will never get to 5V - unless there is some type of short to VCC on the power rail. So 1/4W is fine for normal operation - but if there a chance that the system could experience > 5V across the 100 ohms (in terms of faults/ shorts to supply especially if there is a ground shift  at receiver) then it may be warranted to use a higher rating to prevent damage during "expected" faults. 

    Please let me know if you have any other questions. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Guru 20696 points

    Hi Clemens and Parker,

    Thank you for your reply.

    I understand termination resistor generally uses 1/4W rating.

    I'll share this with the customer.

    When they have an additional question, I consult you again.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi