DP83848QSQX Pull up resistors Power stress

Hello,

Please, In the application below and for the component DP83848QSQX, how to calculate the power stress of the pull-up resistance R (in RED)

And how to calculate the power stress of the transformer.

Please have a look at the following link: e2e.ti.com/.../501825

Thank you for replaying,

BR,

Abdelmouneim,

I believe your post below was appended to the wrong E2E thread. I have split it and joined it here to keep the content together. I will respond shortly with an answer to your question.

Patrick

In 100M mode, the single-ended signal swing of each pin will be +/- 0.5V, i.e. each 50 Ohm resistor will see 0.5V across it. Based on this, the current will be 10mA (0.5V / 50 Ohms) and the wattage will be 5mW (10mA * 0.5V).

For most applications, we typically focus on 10M operation since the signal swings are larger. In that case, the single-ended signal swing of each pin will be +/- 1.25V, i.e. each 50 Ohm resistor will see 1.25V across it. Based on this, the current will be 25mA (1.25V / 50 Ohms) and the wattage will be 31.25mW (25mA * 1.25V).

For our evaluation boards, we often use 1/16 W or 1/10 W termination resistors, but based on the actual calculations, this level of power rating is not actually required.

Patrick

Hi Patrick,

Thank you for answering, we just need to understand how did you calculate the voltage across the resistor (0.5V at 100M, and 1.25V at 10M). With the knowledge that we have a pull up supply of 3.3V, if we substract 1.25V from the 3.3V we get 2.05V (3.3v-1.25v=2.05v) and if we substract - (-1.25v) for the 3.3V this will lead to a 4.55V across the resistor ( 3.3v - (-1.25v))=4.55v ). !!!

And What is the DC value of the +/- 1.25V swinging?

Could you please attach your answer with a formula or a kind of explanation.

Thank you,
Abdelmouneim,

The common mode voltage of the signaling is set by the transformer center tap and will therefore be 3.3V. Relative to ground, the signals will swing between 2.05V and 4.55V as you have noted. However, the voltage across the resistor will only be 1.25V in either case.

Patrick