Introduction

A power supply delivers its specified efficiency and noise only when the design around it respects the switching loop, the thermal path and the filtering. The regulator itself is well specified, but the layout and the component choice decide whether the rail is clean and the board stays cool. This application note explains the practical rules for applying an ADI buck regulator and a low-noise LDO in a real multi-rail system.

Building the Power Tree

Start from the loads and the noise target rather than from a favorite part. A multichannel regulator such as the ADP5054 provides several rails from one input in a compact package, which shrinks the board and reduces the part count, while a single synchronous buck such as the ADP2370 serves a design that needs only one efficient rail. Phase-shift the channels so the switching noise spreads across time instead of stacking at one frequency, which lowers the input ripple and the conducted emissions and eases the filtering.

Sequencing and Power-Good

Many devices require their rails in a defined order, so use the sequencing and the power-good signals of the regulator to meet the requirement without extra logic. Confirm the order and the timing against the load datasheets.

Efficiency and the Thermal Path

The efficiency sets the heat the board must remove, so compare it at the working point rather than at the full scale, and confirm the derating at the real ambient. Keep the switching loop small, because the loop area sets the radiated and the conducted noise, and give the exposed pad a good thermal path to the board. Place the input capacitor close to the regulator, keep the inductor and the output capacitor close and use a short, wide return.

The Switching Loop

The high-di/dt loop that runs through the input capacitor, the switch and the ground is the source of most of the emissions, so make that loop as small as possible and keep it away from the sensitive analog area. This single rule does more for the emissions than any downstream filter.

Cleaning the Analog Rail

A converter, a reference or a precision amplifier sees the supply ripple directly, so a low-noise LDO such as the ADP7118 placed behind the switcher removes it and delivers a clean rail with a high power-supply rejection. Place the LDO close to the analog load and keep its output and noise-reduction capacitors close, because the high-frequency residual depends on the layout as much as the part. A small, well-placed capacitor near the load often does more than a much larger one far away.

Getting It Right

A power design succeeds when the efficiency, the noise and the layout are managed together: build the tree from the loads, keep every switching loop small, clean the sensitive rails with a low-noise LDO and sequence the rails correctly. Confirm the result with the real load and the enclosure rather than on an open bench, because the thermal and the EMC behavior change once the board is in a case. BeiLuo supplies the ADI regulators from stock and will review the power tree and the layout with you before the design is released.