11.3 Power Dissipation

The total power dissipation inside TPS65251 should not to exceed the maximum allowable junction temperature of 125°C. The maximum allowable power dissipation is a function of the thermal resistance of the package (R_JA) and ambient temperature.

To calculate the temperature inside the device under continuous loading use the following procedure.

1. Define the set voltage for each converter.
2. Define the continuous loading on each converter. Make sure do not exceed the converter maximum loading.
3. Determine from the graphs below the expected losses (Y axis) in watts per converter inside the device. The losses depend on the input supply, the selected switching frequency, the output voltage and the converter chosen.
4. To calculate the maximum temperature inside the IC use the following formula:
Equation 27.
where
• T_A is the ambient temperature
• P_DIS is the sum of losses in all converters
• θ_JA is the junction to ambient thermal impedance of the device and it is heavily dependant on board layout

VO (from top to bottom) = 5 V , 3.3 V, 2.5 V, 1.8 V, 1.2 V

Figure 36. Buck 1 Losses (W) vs Output Current
V_IN = 12 V, ƒ_SW = 500 kHz

VO (from top to bottom) = 5 V , 3.3 V, 2.5 V, 1.8 V, 1.2 V

Figure 38. Buck 2 and 3 Losses (W) vs Output Current
V_IN = 12 V, ƒ_SW = 500 kHz

VO (from top to bottom) = 5 V , 3.3 V, 2.5 V, 1.8 V, 1.2 V

Figure 37. Buck 1 Losses (W) vs Output Current
V_IN = 12 V, ƒ_SW = 1.1 MHz

VO (from top to bottom) = 5 V , 3.3 V, 2.5 V, 1.8 V, 1.2 V

Figure 39. Buck 2 and 3 Losses (W) vs Output Current
V_IN = 12 V, ƒ_SW = 1.1 MHz