Digital Power Management Meets Analog Accuracy

Jun 1, 2006 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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Power management functions such as monitoring, supply sequencing and margining, and fault management are considered by many to be the primary reasons for adopting digital control in power supplies. However, the MAX8688 controller from Maxim Integrated Products demonstrates that it is possible to implement these same functions using analog power supplies, while still having the convenience of PMBus-compliant communications, very accurate regulation of output voltage and low cost.

A complete digital power-supply controller/monitor, the MAX8688 can be used to control/monitor up to a total of 127 point-of-load dc-dc converters (POLs) on a single bus (see the figure). According to the company, the controller provides a level of functionality that, up until now, has only been obtainable in all-digital power supplies.

The MAX8688 is also described as the intelligence behind the company's PowerMind family of products, which will also include multiphase controllers. By keeping the PWM in the analog domain, the PowerMind approach eases the migration to digital control, eliminating requirements for new compensation methods and jittery output, both of which may be associated with digital PWMs.

By tapping into the enable, feedback node and/or reference input of a POL, the MAX8688 provides tracking, sequencing and fine setting of the output voltage down to ±0.2% accuracy over the 0°C to 85°C temperature range. Because the MAX8688 is in full control of the output, tasks like margining up or down and transitioning the output voltage at a controlled rate are simple. The MAX8688 uses its 12-bit ADC to monitor output voltage, current and on-board temperature. Consequently, multiple warning and fault thresholds are set and served with the flexibility of digital control.

In the PowerMind approach, a master microcontroller communicates to the MAX8688 through the PMBus. The system controller facilitates data logging and system-level control. Although the MAX8688 holds all the power supplies' peak temperature, output current and output voltage data, the system controller can poll each power supply at fixed intervals and log the information intelligently for future analysis.

The MAX8688 can also modify the performance of the power supply remotely, enabling programming of the output voltage in 500-µV steps, and/or the sequencing, tracking and fault/warning thresholds. Current sharing among POLs is also possible. By manipulating the output voltage of the slave modules, the output current can be matched to that of the master module, regardless of the physical distance between the power supplies.

A free graphical user interface (GUI) is used to configure the power supply. This GUI requires no knowledge of digital control theory or how to write software/firmware. Using the company's MAXQ2000 microcontroller as a system controller, the GUI writes the necessary firmware, which can be loaded in the flash memory and can run on the microcontroller.

On power-up, the microcontroller loads the necessary register of the MAX8688 to work specifically as intended. This performance is repeatable and independent of component tolerances and lot-to-lot variations.

The MAX8688 requires a 3.3-V ±10% supply voltage, and can control output voltages from 0 V to 5.5 V. Available in a 24-pin, 4-mm × 4-mm TQFN, the controller is priced starting at $1.95 in lots of 10,000.

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