Power Considerations

The average die junction temperature, T_J (in °C) is given from the formula:

T_J = T_A + P_D * R_θJA

where P_D is the total power dissipation.

The total thermal resistance of a package (R_θJA) can be separated into two components, R_θJC and R_θCA, representing the barrier to heat flow from the semiconductor junction to the package (case) surface (R_θJC) and from the case to the outside ambient air (R_θCA). These terms are related by the equation:

R_θJA = R_θJC + R_θCA.

R_θJC is device-related and cannot be influenced by the user. However, R_θCA is user-dependent and can be minimized by thermal management techniques such as PCB thermal design, heat sinks, and thermal convection. Thus, good thermal management on the part of the user can significantly reduce R_θCA so that R_θJA approximately equals R_θJC.

Power usage can be calculated by adding together system power consumption and I/O module power consumption. The current drawn from pins with a capacitive load may be estimated (for one pin) as follows:

I_cp ≈ V_DD * C_load * f_sw

Where C_load = pin load capacitance and f_sw = average switching frequency of I/O pin.