LM5576-Q1: How to calculate the derating according to different ambient tempratures?

Hello

It is somewhat tricky to calculate the thermal performance.  You could try to use webench, if that feature

is enabled for this device.

Also, the data sheet gives advice about what kind of thetaJA you can expect for the size of copper area

used in the EVM for this device.

Otherwise, this is how you should proceed:

First determine the estimated efficiency from either curves in the data sheet or from webench.

Then calculate the power dissipation = Pd = output power*(1-n)/n  where n is the efficiency.

Next calculate the required thermal resistance = thetaJA = (Tj-Ta)/Pd

For Tj I would use a maximum of 125C, but set it lower to be conservative.  Ta=85C.

Finally, you would use this extremely approximate equation to estimate the required copper area = A = 500/(thetaJA-3)

The area is in square cm.  

The LM43603 has a similar package and the data sheet has a curve of thetaJA vs. copper area that you may

be able to use to check you analysis.

Thanks