Calculating surface temperature of stepup converter

[Wisely]

Hola

I am using a TPS62122 stepup converter. This have junction to ambient thermal resistance set to 114.4°C/W and a max load of 75mA.
Am I correct that on a PCB which have no extra cu for thermal disipation that when this converter is generating a 3.5V form a 12V supply, the temperature may reach as high as 90°C @65mA load when ambient temperature is app. 30°C ?

Calculated as such. T = 30°C + [(12-3.5)*0.065]*114.4 = 93°C

Have used similar methodes for LDO's but are unsure how to calculate an *estimated* surface temperature for stepup converters

Thanks in advance

 

[mtwieg]

Nope, that's the nice thing about switching converters is that they have lower dissipation than linear regulators. Dissipation isn't equal to (Vin-Vout)*Iout, but rather Vin*Iin-Vout*Iout.

 

[Wisely]

Sooooo, then a rough estimate could be done by this formula

T = Ta + (Pin - Pout)*RθJA ???

Where Ta is ambient temperature

 

[mtwieg]

Correct. Though RJA will depend greatly on your PCB layout, enclosure, etc.