SBVS204F June 2012 – September 2014
For best overall performance, all circuit components are recommended to be located on the same side of the circuit board and as near as practical to the respective LDO pin connections. Ground return connections to the input and output capacitor, and to the LDO ground pin, must also be as close to each other as possible and connected by a wide, component-side, copper surface. The use of vias and long traces to create LDO circuit connections is strongly discouraged and negatively affects system performance. This grounding and layout scheme minimizes inductive parasitics and thereby reduces load-current transients, minimizes noise, and increases circuit stability.
A ground reference plane is also recommended. This reference plane serves to assure accuracy of the output voltage, shield noise, and behaves similar to a thermal plane to spread (or sink) heat from the LDO device when connected to the PowerPAD™. In most applications, this ground plane is necessary to meet thermal requirements.
The TPS7A470x contains a thermal shutdown protection circuit to turn off the output current when excessive heat is dissipated in the LDO. Thermal shutdown occurs when the thermal junction temperature (TJ) of the main pass-FET exceeds 170°C (typical). Thermal shutdown hysteresis assures that the LDO again resets (turns on) when the temperature falls to 150°C (typical). Because the TPS7A470x is capable of supporting high input voltages, a great deal of power can be expected to be dissipated across the device at low output voltages, which causes a thermal shutdown. The thermal time-constant of the semiconductor die is fairly short, and thus the output oscillates on and off at a high rate when thermal shutdown is reached until power dissipation is reduced.
For reliable operation, the junction temperature must be limited to a maximum of 125°C. To estimate the thermal margin in a given layout, increase the ambient temperature until the thermal protection shutdown is triggered using worst-case load and highest input voltage conditions. For good reliability, thermal shutdown must be designed to occur at least 45°C above the maximum expected ambient temperature condition for the application. This configuration produces a worst-case junction temperature of 125°C at the highest expected ambient temperature and worst-case load.
The internal protection circuitry of the TPS7A470x is designed to protect against thermal overload conditions. The circuitry is not intended to replace proper heat sinking. Continuously running the TPS7A470x into thermal shutdown degrades device reliability.
JEDEC standards now recommend the use of PSI thermal metrics to estimate the junction temperatures of the LDO while in-circuit on a typical PCB board application. These metrics are not strictly speaking thermal resistances, but rather offer practical and relative means of estimating junction temperatures. These PSI metrics are determined to be significantly independent of copper-spreading area. The key thermal metrics (ΨJT and ΨJB) are given in the Thermal Information table and are used in accordance with Equation 9.