Feature: Power
One important characteristic of a power supply's performance is its recovery time – the time it takes for its output voltage to return to its regulated value after a load transient has occurred
to support many features in various applications. And in high- speed RF systems like 5G transceivers and beamformers, the power requirements are even stricter when it comes to bandwidth and RF noise levels. A frequently encountered solution here is the traditional two-
stage (buck + LDO) set-up, but its high output current makes it inefficient and bulky, since it also requires more heatsinks. Equally, it is uneconomic to use a single buck regulator to power
Building a power supply for processors
By Erik Lamp, Product Applications Engineer, and Xinyu Liang, Applications Engineering Manager, Analog Devices
demanding loads, which brings us to multiphase buck regulators, which deliver high performance, are scaleable and offer ripple interleaving. However, for fast transient and ultra-low RF noise performance in high-speed RF ASICs, a multiphase buck regulator requires many output capacitors and multiple LC filter stages, components that add to board space and costs. ADI’s Silent Switcher 3 architecture features ultra-low noise – from 10Hz-100kHz – ultra-low EMI emissions
typically 4μVrms
and ultra-fast transient response with a high-gain error amplifier. In this product family, the LT8627SP features the highest current rating (16A), making it a perfect candidate for a multiphase buck configuration in any high-current, noise-sensitive application. Due to the low supply voltage nature of all ASIC loads (< 1V) and the widely-used 12V power distribution system, a multiphase buck is very sensitive to the minimum ON time. Te innovative Silent Switcher 3 technology offers the smallest ON time (15ns), which allows the LT8627SP to easily operate at a switching frequency above 1MHz, meeting the ripple, size, noise and bandwidth requirements.
T
his article will discuss a multiphase monolithic buck solution to solve high-current, fast-transient requirements in building a power supply for a processing unit. With its ultrafast transient feature, the novel low-output-noise architecture by Analog Devices called Silent Switcher 3 is an ideal
solution for multiphase operations. Te article will also discuss the architecture as well as detailed design tips and considerations to help with their designs.
Growing power consumption Processing devices like CPUs, FPGAs and ASICs use a lot of power, which continues to grow as their computing capabilities increase
Important characteristics One important characteristic of a power supply’s performance is its recovery time – the time it takes for its output voltage to return to its regulated value aſter a load transient has occurred. Every power supply has a limit to how fast it recovers, which is
typically related to its control loop bandwidth. A higher control loop bandwidth means the inductor current can ramp up or down faster during transients, compensating for the charge change on the output capacitors, which yields a shorter recovery time. One example of using a four-phase LT8627SP is to power a RF digital load with a maximum load current of 50A; see
1.8VOUT
Figure 1. Tis power supply outputs 1.8V with a 2MHz switching frequency. To shorten the charge compensation time during a transient, low ESR ceramic capacitors are used, but not high ESR polymer capacitors or electrolytic capacitors. Interleaving PWM technology (90° per phase) is applied to increase the equivalent
www.electronicsworld.co.uk November 2023 21
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