32.4 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 Thermal Resistance Data
- θJC = Package thermal resistance, Junction-to-case thermal resistance (°C/W), see Thermal Resistance Data
- 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 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:
Icp ≈ VDD * Cload * fsw
Where Cload = pin load capacitance and fsw = average switching frequency of I/O pin.
| Pin Count | Package | TA Range | θJA [°C/W] | θJC [°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 |