33.4 Power Considerations

The average chip-junction temperature, TJ, in °C, can be obtained from the following equations:

  • Equation 1: TJ = TA + (PD x θJA)
  • Equation 2: TJ = TA + (PD x (θHEATSINK + θJC))
where:
  • θJA = Package thermal resistance, Junction-to-ambient (°C/W), see table below
  • θJC = Package thermal resistance, Junction-to-case thermal resistance (°C/W), see table below
  • θHEATSINK = Thermal resistance (°C/W) specification of the external cooling device
  • PD = Device power consumption (W)
  • TA = Ambient temperature (°C)

From the first equation, the user can derive the estimated lifetime of the chip and decide whether a cooling device is necessary or not. If a cooling device has to be fitted on the chip, the second equation must be used to compute the resulting average chip-junction temperature TJ in °C.

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

Icp ≈ VDD × Cload × fsw

Where Cload = pin load capacitance and fsw = average switching frequency of I/O pin.

Table 33-4. Power Dissipation and Junction Temperature vs. Temperature
Pin CountPackage TypeθJA (°C/W)θJC (°C/W)
14SOIC5826
14TSSOP9520
20SOIC4421
20SSOP60.625
20VQFN79.736
24VQFN60.625