Explore the Lesser-Known Benefits of Digital Power

Nov 1, 2006 12:00 PM
By Marty Pandola, Product Manager, Zilker Labs Inc., Austin, Texas

Although the use of digital control eases implementation of the basic power management requirements, it also affords numerous secondary benefits that can make POL power designs more compact, flexible and efficient.

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Several semiconductor suppliers offer digital power ICs as improved solutions over traditional analog approaches. To replace an existing analog power design, a digital power solution must perform equal to or better than the existing solution and offer key design advantages. Digital solutions match or improve the efficiency, size and cost of analog implementations, while providing many more benefits.

Digital Power

A digital power solution differs from an analog solution in that the pulse width modulation (PWM), loop control and feedback are implemented digitally. Analog signals are converted to digital using analog-to-digital converters. Once the signals are digital, microcontrollers, digital-signal processors or simple state machines control the PWM and the feedback loop.

First and foremost is power conversion. Digital power becomes interesting when the efficiency and cost is equal to or better than a comparable analog power-conversion solution. However, power conversion is only part of the overall system solution. Having the digital controller in a standard CMOS-silicon process allows for the integration of the power management with the power conversion. The integration of the thermal and power management features begins to make the digital power solution compelling.

Integration Reduces Component Count

Key aspects of power management include voltage and current monitoring, voltage sequencing, voltage tracking, fault detection and fault management. Thermal management includes the ability to monitor the temperature throughout the system and respond to overtemperature conditions by controlling fans or shutting down parts of the system. Integrating the power and thermal management with the power conversion removes the need for additional power and thermal management ICs in the system. For example, an analog power system with four power rails may require more than 150 components, whereas the same system using a digital solution would require fewer than 50 components (Fig. 1).

Digital solutions reduce component count not just by integration, but also by improving upon existing features. In analog solutions, RC time constants are used to set delay and ramp times. These times are now part of the configuration of the digital power IC and do not require an external RC to set the time. Similarly, external resistors and capacitors are not required when configuring the loop compensation and output preset on a digital power IC.

Configuring features in the digital domain and the integration of the thermal and power management with the power conversion reduces the overall component count, which lowers system cost and improves system reliability.

Design Flexibility

The power engineer's job is to design a reliable power distribution system of many voltage rails. These voltage rails are required to sequence or track other rails to properly bias ASICs, FPGAs, microprocessors or other digital logic that is present in the system. As the product design moves through the various design phases, changes in the design may occur. These changes could be an addition of a power rail or the need for more current on any given rail, or a transient response requirement may tighten. This would require the power distribution to be redesigned.

A digital power solution provides the flexibility to easily adapt to the requirement changes. A new voltage rail is easily added to the power-management system through the use of the industry-standard System Management Bus (SMBus). The digital power ICs communicate with each other over the SMBus using the Power Management Bus (PMBus) protocol. The addition of a new rail is easily integrated into the monitoring, sequencing, margining and fault detection schemes that have already been designed. The digital power IC for the new rail is given its own SMBus address and is added to the system. There is no need to reprogram or add more stand-alone power-management ICs just because of the additional voltage rail.

Digital power ICs have the ability to configure the load-line impedance, or output-voltage droop. This flexibility helps balance out errors over load when current sharing or to compensate out inherent droop in the load. Adding compensation flattens out the load line. Sometimes droop can be helpful. Intentionally adding droop can improve transient response. A tighter transient requirement generally means adding more capacitors. However, for relatively small improvements, adding more droop in a digital converter will improve the transient response. Fig. 2 shows a transient response with and without output-voltage droop. The load step causes the output to droop down by 30 mV, which allows the output to reach the target voltage faster and thus a faster transient response.

This technique can be used to achieve a smaller ΔV with the same number of components or a similar ΔV with fewer components.

Other requirement changes are also easily adapted, generally without any hardware changes. Delay time, ramp-up time and ramp-down time can be reconfigured during development through external resistor settings or simple PMBus commands. There is no longer the need to recalculate and change RC time constants all over the board in order to adapt the delay and ramp times as the design progresses. Likewise, there may be times where the required voltage for a given rail will change. Again, this change is accommodated simply by using a PMBus command to the power IC without the need for changing external components. Many ASICs or FPGAs require their voltage rails to ramp up and ramp down in a particular order to prevent latch-up in the device. Digital power ICs communicate with each other and can sequence their voltages for the application. If the sequence order changes, the digital power ICs are reconfigured to the new sequence order. This flexibility makes it easy to adapt a design to the new system requirements without having to change the board layout. This greatly reduces the development time of the product.

Power-supply sequencing is controlled through the SMBus as shown in Fig. 3. Once ENABLE goes high, output 1 waits a predetermined delay period and then ramps to its final voltage. The first device sends a power-good signal over the SMBus to the second device. Output 2 waits its delay period and then ramps to its final value.

A flexible design reduces the number of board revisions during the development phase. Reducing or eliminating board revisions reduces product development time as well as product development cost.