Power dissipation (PD) means maximum power consumption to maintain its performance.

Here is the formula to calculate PD:

[PD] = (Maximum Power Consumption [Tjmax] - Ambient Temperature [Ta]) / Junction-to-Ambient Thermal Resistance [θja]

For example, described on its datasheet as:

Tjmax = 125°C, Ta = 85°C, θja = 40°C/W

PD = (125-85) / 40 = 1.0 W

Therefore, the PD of this example is 1.0 W.

When the Ta is 25°C, its PD becomes 2.5 W by the formula.
In this case, supposed that the output current is 1.5 A, the maximum dropout voltage can be estimated as:

2.5 W/1.5 A = 1.68 V

Therefore, supposed that the output voltage is fixed at 1.2 V, the input voltage must be kept under 2.88 V.

Some datasheets refer to θjc instead of θja. Replace the θja in the formula with the θjc and calculate PD like this:

[PD] = ([Tjmax] - Package Surface Temperature [Tc]) / Junction-to-Case Thermal Resistance [θjc]

Attention: These values are derived from measurements* under certain conditions. These values depend on the actual PCB.

• * The measurements are conducted according to the conditions regulated by JEDEC (ex. the chip mounted in the center of the 3 inch x 3 inch four-layer board).