10.1 Layout Guidelines

To improve AC performance such as PSRR, output noise, and transient response, it is recommended that the PCB be designed with separate ground planes for V_IN and V_OUT, with each ground plane connected only at the ground pin (GND) of the device. In addition, the ground connection for the bypass capacitor should connect directly to the GND pin of the device.

Solder pad footprint recommendations for the TPS731xx are presented in Application Bulletin Solder Pad Recommendations for Surface-Mount Devices (SBFA015), available from the TI website at www.ti.com.

10.2 Layout Example

Figure 35. Example Layout (DBV Package)

10.3 Thermal Considerations

Thermal protection disables the output when the junction temperature rises to approximately 160°C, allowing the device to cool. When the junction temperature cools to approximately 140°C, the output circuitry is again enabled. Depending on power dissipation, thermal resistance, and ambient temperature, the thermal protection circuit may cycle on and off. This limits the dissipation of the regulator, protecting it from damage due to overheating.

Any tendency to activate the thermal protection circuit indicates excessive power dissipation or an inadequate heatsink. For reliable operation, junction temperature should be limited to 125°C maximum. To estimate the margin of safety in a complete design (including heatsink), increase the ambient temperature until the thermal protection is triggered; use worst-case loads and signal conditions. For good reliability, thermal protection should trigger at least 35°C above the maximum expected ambient condition of your application. This 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 TPS731xx has been designed to protect against overload conditions. It was not intended to replace proper heatsinking. Continuously running the TPS731xx into thermal shutdown degrades device reliability.

10.3.1 Power Dissipation

The ability to remove heat from the die is different for each package type, presenting different considerations in the PCB layout. The PCB area around the device that is free of other components moves the heat from the device to the ambient air. Performance data for JEDEC low- and high-K boards are shown in the Thermal Information table. Using heavier copper will increase the effectiveness in removing heat from the device.

Power dissipation depends on input voltage and load conditions. Power dissipation (P_D) is equal to the product of the output current times the voltage drop across the output pass element (V_IN to V_OUT):

Equation 6.

Power dissipation can be minimized by using the lowest possible input voltage necessary to assure the required output voltage.