Controllers Build Flexible Power Designs for Next-gen CPUs

Nov 23, 2005 11:44 AM
By David Morrison, Editor, PETech

Power IC vendors continue to develop multiphase power supply controllers that can satisfy a range of requirements for next-generation microprocessors. These chips provide high levels of accuracy and precision, while meeting demands for small size and design flexibility such as the ability to scale output current. These trends are exemplified by the MAX8809A/MAX8810A single-chip multiphase controllers with integrated dual-phase MOSFET drivers from Maxim Integrated Products. These ICs implement 2-, 3- or 4-phase synchronous, buck converter designs that fully comply with Intel’s VRD 11/VRD10 and AMD’s K8 Rev F specifications.

The MAX8809A operates as a single-chip, 2-phase solution with on-chip drivers, making it suitable for power requirements up to 65 W. The controller also includes an additional PWM output on chip, so that 2-phase designs may be scaled up to three phases simply by adding the MAX8552 driver chip. This 3-phase option enables the supply to deliver up to 90 A.

The MAX8810A also integrates a dual driver but adds two PWM outputs. Thus, the MAX8810A can be paired with the MAX8523 dual driver to achieve a low-cost 2-chip solution for 4-phase designs up to 150 A (see the figure).

A selection pin (SEL) allows the designer to program the controller to support three different processors, including Intel VRD10.1 and next-generation CPUs from both Intel and AMD. Included is support for VID codes, VID signal thresholds, proper startup sequencing and overvoltage protection thresholds. This makes it easy for designers to transport a design to new applications.

The MAX8809A/MAX8810A use an extremely low input bias current (0.1 µA, typ) differential amplifier to support remote output-voltage sensing at the processor die. This eliminates the effects of trace impedance in the output and return paths. Initial output voltage accuracy is ±0.4% due to a high-accuracy DAC combined with precision current-sense amplifiers. The precision error amplifier ensures high voltage-positioning accuracy over the full load range.

The MAX8809A/MAX8810A use peak current-mode control to provide fast transient response and inherent current sharing. Applications can use resistor current sensing or take advantage of the inductor dc resistance to save cost and improve efficiency. Well-matched current-sense amplifiers ensure that current-sharing mismatch is less than 5% between phases.

Figure. When combined with a dual MOSFET driver chip, the MAX8810A controller implements a 4-phase stepdown converter with up to 150 A of output.

Current-sharing accuracy is further enhanced via a proprietary Rapid Active Averaging* (RA²) scheme, which eliminates errors due to current-sense component tolerances. Current-sense information is fully temperature compensated, which results in accurate voltage positioning and current-limit operation over the full -40°C to +85°C operating range.

Overcurrent protection is extremely robust; current limiting is cycle-by-cycle and based on temperature-corrected average current. The MAX8809A/MAX8810A fold back current to 50% of the maximum value during an overcurrent event, reducing stress on input and output components. Other protection mechanisms include overtemperature protection and overvoltage protection. Additional features include a wide input-supply range up to 26 V, an enable control pin and power-good output signal.

In addition, soft-start time is externally programmable with a resistor. Turn-off uses a unique “soft-stop” feature, whereby the output voltage is regulated down to 0 V. This feature prevents negative voltage spikes on the output at turn-off, eliminating the need for an external Schottky clamp diode.

The MAX8809A is available in a 5-mm × 5-mm, 40-pin TQFN, while the MAX8810A is offered in a 6-mm × 6-mm, 48-pin TQFN. Unit pricing for the MAX8809A and MAX8810A starts at $2.40 and $2.50, respectively, in quantities of 1000. For more information on the MAX8809A/10A, visit: www.maxim-ic.com/desktop-cpu-power.

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