Figure 2: The enhanced analog controller design retains the fundamental closed-loop design, but allows for digital setting of some parameters under external control via a digital port such as PMBus, I2C, SPI or other.
flexibility for real-time setting of operational parameters. For example, the Intel/Xilinx VR13 standard requires the ability to direct the supply to change its nominal output voltage from 1.2 to 0.9 V and back “on the fly,” which an all-analog supply cannot do. This adaptive voltage scaling (AVS) adjusts the supply output voltage to the minimum required by the processor, depending on its clock speed and workload, and also compensates automatically for process and temperature variations within the processor. To do all this requires a fully programmable, sophisticated, firmware-controlled converter. It is possible to implement some of the desired changes via an I/O port on the supply coupled with digital parameter-setting circuitry. This results in a hybrid supply that has an inner analog-control loop but overall digital supervision and some reporting of supply status, Figure 2. The all-digital supply uses a very different internal
architecture. Rather than implement the control loop using analog circuitry, even with some digital oversight, the digital supply uses analog/digital (A/D) converters to digitize critical internal voltages and currents. The converted values are used by a dedicated, embedded processor (DSP, FPGA) that executes code for closed-loop algorithms. Finally, the algorithms’ outcomes are converted back to analog signals via a digital/analog (D/A) converter, adjusting the voltages and currents as needed, Figure 3. The control algorithm is firmware-based rather than built as a hardwired analog circuit, so the control strategy can be fairly complicated and sophisticated. Even better, a single processor (if powerful enough) can control two or more independent output rails, and coordinate these rails to manage factors such as output levels, ramp rates and relative power on/off timing between these rails. It can also
January 2017
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Figure 3: The all-digital control approach immediately digitizes key voltages and currents, then uses a firmware-
driven processor and algorithms to initiate control action, and so can implement complicated control strategies as well as dynamically adjust those as circumstances demand.
provide detailed reports and historical data on the supply’s status, conditions and changes, so likely failures can be anticipated rather than just reported after they occur. Two examples will show how digital supplies are now
able to serve applications at lower current and power needs than those of data center racks. The NDM2Z-50 from CUI, Inc. (Figure 4) is an all-digital DC/DC point-of- load (PoL) converter for a 4.5- to 14-V input range and 0.6- to 3.3-V programmable output, providing up to 50 A (165 W maximum). It includes an SMBus interface and is PMBus™ compatible. Despite its small package (30.85 x 20.0 x 8.2mm for the horizontal-mount version), it provides features such as voltage tracking, voltage margining, active current sharing, parametric capture, voltage/current/ temperature monitoring, and programmable soft start and soft stop. Its data sheet (Reference 1) includes dozen of graphs showing all aspects of static and dynamic performance.
Summary The power needs of many of today’s electronic systems can no longer be satisfied by even leading-edge analog supplies, but instead require a new form of power-supply architecture for control. The fully digital power-supply implementation has significant and tangible benefits
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