ZHCS120B March 2011 – July 2015 TPS7A4001
PRODUCTION DATA.
10 Layout
10.1 Layout Guidelines
10.1.1 Board Layout Recommendations to Improve PSRR and Noise Performance
To improve AC performance such as PSRR, output noise, and transient response, TI recommends designing the board with separate ground planes for IN and OUT, with each ground plane connected only at the GND pin of the device. In addition, the ground connection for the output capacitor should connect directly to the GND pin of the device.
Equivalent series inductance (ESL) and ESR must be minimized to maximize performance and ensure stability. Every capacitor (CIN, COUT, CBYP) 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 they may impact system performance negatively and even cause instability.
If possible, and to ensure the maximum performance denoted in this product data sheet, use the same layout pattern used for the TPS7A40 evaluation board, available at www.ti.com.
10.2 Layout Example
Figure 17. Recommended Layout Example
10.3 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 may 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 should 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 should trigger at least 45°C above the maximum expected ambient condition of the 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 TPS7A4001 has been designed to protect against overload conditions. It was not intended to replace proper heatsinking. Continuously running the TPS7A4001 device into thermal shutdown degrades device reliability.
10.4 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. 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 (PD) is equal to the product of the output current times the voltage drop across the output pass element, as shown in Equation 2:
