# Is this way of maximum current calculation correct for this voltage regulator?

Regarding using the LD39050PU33R voltage regulator which is 5V to 3.3V fixed voltage regulator without any heatsink, its maximum current is given as 500mA. I was trying to figure out how they calculated this 500mA. So I tried to calculate the maximum current by myself by using the thermal resistances and maximum junction temperature as follows:

From the thermal data below: The junction to case thermal resistance R_jc is sum of those above; so R_jc = 65 °C/W.

For 5V input the voltage drop across the regulator will be Vd = 5V-3.3V = 1.7V.

Max junction temperature is 125°C. And in the worse case if we have 55°C ambient temperature the ΔT = 70°C.

That means maximum power dissipated Pd can be found from:

ΔT = Pd * R_jc

Pd = 70 / 65 = 1.077W

This makes the max current I = Pd / Vd then I = 1.077 / 1.7 = 633mA.

In my calculation I took max ambient temperature 55°C. To obtain 500mA, the ambient temperature would be taken as around 70°C. Is my conclusion correct?