Frederik Dostal from Analog Devices tells PSD how it is now possible to have compact digital power supplies with very low noise.
Digital power has been a growing trend in power supplies for well over a decade, and its benefits are undeniable. For example, digital power is highly precise, being easily controlled and configured by software, which leads to better dynamic performance and efficiency. Digital power is also more robust, because of the integrated diagnostics. However, up until now, these benefits were mostly restricted to larger power supplies as they are more noise resistant.
Frederik Dostal, Subject Matter Expert for Power Management at Analog Devices explains, “There are many applications that could benefit from new power solutions. For example, optical transceivers need a power supply with very low noise, very accurate voltage regulation, and relatively high power. We often think of optical electronics as low power, but you often have lots of optical transceivers running together at enormous speeds, and that consumes a lot of power. FPGAs, ASICs and SoC point-of-load regulators require a small form factor, fast transient response, very accurate voltage regulation, and very small ripple voltages on the output for loads. Requirements for industrial systems include low EMI, small solution size, high efficiency and high switching frequencies for power dense solutions. Many communication systems, and their building blocks require very high load currents in a small form factor with small output ripple and low noise. Digital control would be an ideal technology for all of these applications if the solution was small enough.”
Finding a solution that is digital, compact and low noise has proven to be very difficult, as smaller devices would be required to combine the sensitive analog components on the same silicon as the digital design. Digital systems need a clock, which generates EMI that cannot easily be isolated from the analog circuitry. This noise is then coupled onto the output voltage.
In an attempt to overcome this barrier, Analog Devices has integrated its Silent Switcher technology into a monolithic digital power supply. Silent Switcher technology offers extremely low noise operation through opposing current loops creating a field cancellation effect. The resulting LT7176/LT7176-1 devices are single-channel buck regulators. The difference between the two devices is that the LT7176-1 variant splits the output current into two phases, with two inductors to reduce the output ripple voltage. This technique enables the use of inductors of lower height, which is important for some applications. The low EMI operation enables higher efficiency through very fast switching transitions, with very low switching losses. Higher switching frequencies also allow for even smaller inductors, saving cost and board space.
LT7176/LT7176-1 have an input voltage range of up to 4V. They are normally used to step the supply voltage down to the core voltage, with an output voltage between 0.3V and 3.4V and an output current of up to 24A. A complete power solution only requires power inductors, an output capacitor and an input capacitor to be added.
Dostal highlights other notable features of the device by saying, “A PMBus is included to control the device and to take care of telemetry. For accurate output voltages, differential output voltage sensing is used. In many of the applications for which the devices are intended, such as powering FPGAs, SoCs and optical transceivers, a well regulated output voltage is essential. Providing Kelvin sensing capability provides greater accuracy. Even systems that don't require require super high accuracy still benefit after a load change, as having an accurate DC output provides more headroom to allow for transients. The device comes with a guaranteed +/- 0.25% output voltage accuracy up to 125oC from 0.6V to 1.375V output, and up to 150oC, its +/- 0.5%, which is still very good. The part, since is often used in data centres and other applications that can run hot, is specified right up to 150oC. It uses current mode control, with a 25ns typical minimum on-time for switching at very high switching frequencies. Having very short on-time helps to get very good regulation, especially fast regulation after transients, so that the output voltage doesn't overshoot or undershoot much at all.”
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