Power Considerations

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

Equation 1: T_J = T_A + (P_D x θ_JA)
Equation 2: T_J = T_A + (P_D x (θ_HEATSINK + θ_JC))

where:

θ_JA = Package thermal resistance, Junction-to-ambient (°C/W), see Thermal Resistance Data
θ_JC = Package thermal resistance, Junction-to-case thermal resistance (°C/W), see Thermal Resistance Data
θ_HEATSINK = Thermal resistance (°C/W) specification of the external cooling device
P_D = Device power consumption (W)
T_A = 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 T_J in °C.

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.

Table 1. Thermal Resistance Data
Pin Count	Package	T_A Range	θ_JA [°C/W]	θ_JC [°C/W]
32	VQFN	-40°C to 125°C	41	16
32	TQFP	-40°C to 125°C	69	20
48	VQFN	-40°C to 125°C	36	14.4
48	TQFP	-40°C to 125°C	68	17