40.4.2 Junction Temperature

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 40-2
  • θJC = Package thermal resistance, Junction-to-case thermal resistance (°C/W), see Table 40-2
  • 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 needs to be fitted on the chip, the second equation must be used to compute the resulting average chip-junction temperature TJ in °C.