One can determine the required copper area by following a few basic guidelines:
- Determine the value of the circuit’s power dissipation, PD
- Specify a maximum operating ambient temperature, TA(MAX). Note that when specifying this parameter, it must be kept in mind that, because of internal temperature rise due to power dissipation, the die temperature, TJ, will be higher than TA by an amount that is dependent on the thermal resistance from junction to ambient, θJA. Therefore, TA must be specified such that TJ does not exceed the absolute maximum die temperature of 150°C.
- Specify a maximum allowable junction temperature, TJ(MAX), which is the temperature of the chip at maximum operating current. Although no strict rules exist, typically one should design for a maximum continuous junction temperature of 100°C to 130°C, but no higher than 150°C which is the absolute maximum rating for the part.
- Calculate the value of junction to ambient thermal resistance, θJA
- Choose a copper area that will ensure the specified TJ(MAX) for the calculated θJA. θJA as a function of copper area in square inches is shown in Figure 6-4.
The maximum value of thermal resistance, junction to ambient θJA, is defined as:
Equation 6. θJA = (TJ(MAX) - TA(MAX) )/ PD(MAX)
where
- TJ(MAX) = the maximum recommended junction temperature
- TA(MAX) = the maximum ambient temperature in the user’s environment
- PD(MAX) = the maximum recommended power dissipation
Note: The allowable thermal resistance is determined by the maximum allowable heat rise , TRISE = TJ(MAX) - TA(MAX) = (θJA) (PD(MAX)). Thus, if ambient temperature extremes force TRISE to exceed the design maximum, the part must be de-rated by either decreasing PD to a safe level, reducing θJA, further, or, if available, using a larger copper area.