11.1 Layout Guidelines

Layout is a critical part of good power-supply design. There are several signal paths that conduct fast-changing currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise or degrade the power-supply performance. To help eliminate these problems, the IN pin must be bypassed to ground with a low ESR ceramic bypass capacitor with an X5R or X7R dielectric.

The GND pin must be tied directly to the PowerPAD under the device. The PowerPAD must be connected to any internal PCB ground planes using multiple vias directly under the device.

Equivalent series inductance (ESL) and equivalent series resistance (ESR) must be minimized to maximize performance and ensure stability. Every capacitor (C_IN, C_OUT, C_NR/SS, and C_FF) must be placed as close as possible to the device and on the same side of the PCB as the regulator itself.

Do not place any of the capacitors on the opposite side of the PCB from where the regulator is installed. The use of vias and long traces is strongly discouraged because these circuits can negatively affect system performance, and can even cause instability.

11.2 Layout Examples

Figure 38. PCB Layout Example

Figure 39. PCB Layout Example Schematic

NOTE: C_IN and C_OUT are size 1206 capacitors, and C_NR, R1, and R2 are size 0402.

Figure 40. PCB Layout Example

11.3 Thermal Considerations

Thermal protection disables the output when the junction temperature rises to approximately 170°C, allowing the device to cool. When the junction temperature cools to approximately 150°C, the output circuitry is enabled. Depending on power dissipation, thermal resistance, and ambient temperature, the thermal protection circuit can cycle on and off. This cycling limits the dissipation of the regulator, protecting it from damage as a result of overheating.

Any tendency to activate the thermal protection circuit indicates excessive power dissipation or an inadequate heatsink. For reliable operation, junction temperature must be limited to a maximum of 125°C. 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 must trigger at least 45°C above the maximum expected ambient condition of any particular 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 TPS7A30 is designed to protect against overload conditions. This circuitry is not intended to replace proper heatsinking. Continuously running the TPS7A30 into thermal shutdown degrades device reliability.

11.4 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 or JEDEC low- and high-K boards are provided in the Thermal Information table. Using heavier copper increases the effectiveness in removing heat from the device. The addition of plated through-holes to heat dissipating layers also improves the heatsink effectiveness.

Power dissipation depends on input voltage and load conditions. Power dissipation (P_D) can be approximated by the product of the output current times the voltage drop across the output pass element, as shown in Equation 9.

Equation 9.

Estimating the junction temperature can be done by using the thermal metrics Ψ_JT and Ψ_JB, as discussed in the Thermal Information table. These metrics are a more accurate representation of the heat transfer characteristics of the die and the package than R_θJA. The junction temperature can be estimated with Equation 10.

Equation 10.

where

• P_D is the power dissipation given by Equation 9,
• T_T is the temperature at the center-top of the device package, and
• T_B is the PCB temperature measured 1 mm away from the device package on the PCB surface.

For more information about measuring T_T and T_B, see the application note Using New Thermal Metrics (SBVA025), available for download at www.ti.com.