12.4 Thermal And Reliability Information

The TPS8269x output current may need to be de-rated if it is required to operate in a high ambient temperature or deliver a large amount of continuous power. The amount of current de-rating is dependent upon the input voltage, output power and environmental thermal conditions. Care should especially be taken in applications where the localized PWB temperature exceeds 65°C.

The TPS8269x die and inductor temperature should be kept lower than the maximum rating of 125°C, so care should be taken in the circuit layout to ensure good heat sinking. Sufficient cooling should be provided to ensure reliable operation.

Three basic approaches for enhancing thermal performance are listed below:

• Improve the power dissipation capability of the PCB design.
• Improve the thermal coupling of the component to the PCB.
• Introduce airflow into the system.

To estimate the junction temperature, approximate the power dissipation within the TPS8269x by applying the typical efficiency stated in this datasheet to the desired output power; or, by taking a power measurement if you have an actual TPS8269x device or a TPS8269x evaluation module. Then calculate the internal temperature rise of the TPS8269x above the surface of the printed circuit board by multiplying the TPS8269x power dissipation by the thermal resistance.

The thermal resistance numbers listed in the Thermal Information table are based on modeling the MicroSIP™ package mounted on a high-K test board specified per JEDEC standard. For increased accuracy and fidelity to the actual application, it is recommended to run a thermal image analysis of the actual system. Figure 46 and Figure 47 are thermal images of TI’s evaluation board with readings of the temperatures at specific locations on the device.

Figure 46. V=3.6v, V=2.85v, I=400ma 80mw Power Dissipation At Room Temp._INOUTOUT

Figure 47. V=3.6v, V=2.85v, I=800ma 330mw Power Dissipation At Room Temp._INOUTOUT

The TPS8269x is equipped with a thermal shutdown that will inhibit power switching at high junction temperatures. The activation threshold of this function, however, is above 125°C to avoid interfering with normal operation. Thus, it follows that prolonged or repetitive operation under a condition in which the thermal shutdown activates necessarily means that the components internal to the MicroSIP package are subjected to high temperatures for prolonged or repetitive intervals, which may damage or impair the reliability of the device.

MLCC capacitor reliability/lifetime is depending on temperature and applied voltage conditions. At higher temperatures, MLCC capacitors are subject to stronger stress. On the basis of frequently evaluated failure rates determined at standardized test conditions, the reliability of all MLCC capacitors can be calculated for their actual operating temperature and voltage.

Figure 48. Capacitor Lifetime vs Capacitor Case Temperature

Figure 49. Capacitor B1 Lifetime vs Capacitor Case Temperature

Failures caused by systematic degradation can be described by the Arrhenius model. The most critical parameter (IR) is the Insulation Resistance (i.e. leakage current). The drop of IR below a lower limit (e.g., 1 MΩ) is used as the failure criterion, see Figure 48. Figure 49 (B1 life) defines the capacitor lifetime based on a failure rate reaching 1%. Note that the wear-out mechanisms occurring in the MLCC capacitors are not reversible but cumulative over time.