In the Absolute Maximum Ratings box in the data sheet, there is a line which reads "Continuous Pdiss (T = 85 °C) 4.25 W (derate 47.2 mW/° C above 85 °C)". Nominal bias of 5V@96 mA results in only 0.48 watts. How do you even approach 4.25 W power dissipation with this device?

Hi there,

Yes, this is confusing and unfortunately plagues many HMC datasheets; I'll try to help you make sense of it.

If you take the inverse of 47.2mW/ °C you get a thermal resistance of 21.2°C/W. At the nominal 480mW of power dissipation the temp rise is a meager 10.2°C which results in a 95.2°C junction temperature when operating at +85°C max.

The 4.5W is a 'theoretical' value of the maximum allowable power dissipation for an MTTF of 1M hours. The value in the datasheet appears to be wrong as this is typically derived as follows: (Max Tj - Max Operating temp) / Thermal Resistance = (135°C - 85°C) / 21.2°C = 2.36W not 4.5W.

The only practical use of this value is as a measure of stress (or de-rating if you prefer). The HMC439 dissipates 480mW of the 2.36W that theoretically could be dissipated before reaching the maximum junction temperature for an MTTF of 1M hours. This results in a stress level of 20% from (0.48W / 2.36W).

Bet Regards,

Marty