# 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 1. Power Dissipation and Junction Temperature vs. Temperature
Pin Count Package Type θJA (°C/W) θJC (°C/W)
14 SOIC 58 26
14 TSSOP 95 20
20 SOIC 44 21
20 SSOP 60.6 25
20 VQFN 79.7 36
24 VQFN 60.6 25