Introduction

A power supply is only as good as its layout. A converter that meets its datasheet on a reference board can radiate EMI, oscillate or overheat in a real product if the layout is poor, and the sensitive analog circuits it powers can suffer even when the converter itself is fine. This application note explains how to design and lay out a TI step-down converter and LDO, using the TPS54331 and TPS7A4700 as references, so that efficiency, ripple and thermal performance meet specification.

The Switching Loop

In a step-down converter, the current is switched through a loop that includes the input capacitor, the high-side switch, the inductor and the catch diode. That loop carries a fast-changing current, so it radiates EMI and carries parasitic inductance that raises voltage spikes at the switching transitions. The single most effective layout rule is to make the loop as small as possible: place the input capacitor close to the converter, keep the loop compact, and return the current directly beneath the trace so the enclosed area is minimized. A tight loop reduces EMI and voltage stress at the same time.

Input and Output Capacitors

The input capacitor supplies the switched current, so it must be close to the converter and have a low-impedance path. The output capacitor filters the ripple and must be placed close to the inductor output and the load. The ground returns of both should connect to a solid ground plane rather than through long traces.

Clean Analog Rails

The switching that makes a converter efficient also makes it noisy, and that noise is unacceptable on an analog or RF rail. The solution is to post-regulate the sensitive rail with a low-noise LDO such as the TPS7A4700, which delivers up to 1 amp with only a few microvolts of output noise. Its power-supply rejection ratio determines how much of the converter's ripple reaches the output, so the LDO's placement and decoupling matter: keep it close to the load, and give it the input and output capacitors the datasheet requires.

Setting the Output Voltage

The TPS7A4700 sets its output without an external feedback divider, using the ANY-OUT configuration, which removes a source of noise and error. Confirm the pin configuration for the target voltage, because the setting is made in hardware rather than by a resistor choice.

Thermal Design

Both a converter and an LDO dissipate power, and the package must carry that heat to the board. The TPS54331 uses a PowerPAD package whose thermal pad must connect to a copper area, and the LDO dissipates the input-to-output voltage drop times the current. Calculate the junction temperature from the dissipation and the thermal resistance, and keep it below the rated maximum with margin. A large voltage drop at high current in an LDO produces significant heat, which is why a switcher is preferred where the drop is large.

Bench Validation

Before production, measure the converter's efficiency across the load range, measure the output ripple and the LDO's output noise, and confirm the temperatures under sustained load. BeiLuo's analog lab can perform these measurements and supply samples for validation, so the design is confirmed before the board is committed.

Conclusion

Most power-supply problems trace back to the layout or the thermal design rather than to the parts. Keep the switching loop tight, post-regulate the sensitive rail with a low-noise LDO, and plan the thermal path from the die to the board; do those things and the supply will meet its specification in the product, not only on the bench.