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Power Electronics

Digital Power Is Mostly Analog

Digital power is quite the buzz these days. Let's take a moment to examine the term. The simplest definition of “digital power” is the control of a switching regulator via a digital interface. This could include, for instance, controlling the output voltage, switching frequency or sequencing of a multiple-channel supply via an I2C or similar digital bus. Start-up, margining, power-up and power-down sequencing can all be controlled via a digital signal or signals. This is quite feasible with today's technology, and many power management ICs today operate in this manner: analog switching regulators controlled via a digital interface.

A second, more complicated meaning adds so-called “digital telemetry” to the first definition. This adds a provision for monitoring switching power-supply attributes such as temperature, output current, input current, input voltage and output voltage. Other information such as an ID tag, fault condition information or even time-stamped events can be stored and reported using on-chip nonvolatile memory. High-end systems with expensive digital ICs are a target market for this kind of digital power supply, while lower cost consumer products do not require this functionality.

A third and most ambitious meaning of digital power is the complete replacement of all analog circuitry inside the switching regulator with digital circuitry. Some argue that such replacement will make switching regulators easier to design, configure, stabilize, adjust and sell. It is further suggested that by writing a few simple lines of code, a core digital power IC can be configured into a boost regulator, buck regulator, inverter, SEPIC, flyback or forward converter.

This definition of digital power is particularly troublesome since power supplies are analog. Even a digital switching regulator, which replaces the error amplifier and pulse width modulator with an ADC and DSP, still requires a voltage reference, current-sense circuitry, and switches or FET drivers. These components are inherently analog and cannot be eliminated from a switching regulator in any implementation. In fact, an ADC is more analog than it is digital. And even in digital implementations, the inductor or transformer and capacitors do not disappear.

Like most choices in life, digital power comes down to a question of economics: Why use a complex process to do both dense logic and high voltage when it is possible to use a simple process, which offers only high-voltage devices for the entire switcher? Vendors such as Linear Technology sell many switching regulator products capable of operating from 25-V, 36-V, 40-V or 60-V inputs, and others capable of generating up to 60-V outputs. These products are mostly analog in design, and they fulfill customers' requirements. Although a more complicated process with dense logic and high voltage could be used to produce such products, how does that help the customer?

Millions of dc-dc converter ICs are sold into huge, and diverse, markets each year. Power levels for just monolithic dc-dc ICs (where the power switch or switches are on the same die as the control circuitry) range from milliwatts to 50 W. Output currents range from a few mA to 10 A, and output voltages from less than 1 V to 330 V. There are thousands of ICs from which to choose from dozens of vendors.

Dc-dc products sell into markets ranging from mobile phones to automobiles to LCD TVs to base stations to Big Iron servers and routers and many more. The requirements for these markets vary, and while every customer in every market expects solutions that work, few of them care how the IC accomplishes the task. Many “digital power” implementations are interesting technical achievements, but modern analog implementations actually perform better, in a smaller footprint.

Customers specify switching regulator products because of their performance, and they don't really care if they are analog or digital inside. Although the digital content in Linear Technology's power products increases modestly each year, we expect to maintain our leadership position in the marketplace, for even digital power is mostly analog.

Steve Pietkiewicz joined Linear Technology in 1987. Since then he has served as a design engineer, as design engineering manager for switching regulators and, more recently, as general manager of Power Products. Pietkiewicz holds a BSEE degree from the University of California at Berkeley.

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