El Segundo, Calif. - International Rectifier's IR3081 control IC/IR3086 phase IC chip set, which uses the company's XPhase architecture for multiphase interleaved buck dc/dc converters, enables major scaling of an n-phase design. The chip set provides a solution that exceeds VRM 10.0 power requirements for the advanced low-voltage, high-current microprocessors used in servers and high-end desktop computers today. It also offers easy scaling for future multiphase requirements.
The architecture is based on a single-control IC that communicates with multiple-phase ICs via a five-wire analog control bus. The bus carries voltage identification (VID), pulse-width modulation (PWM) control, phase-timing, bias voltage and current-sensing information. With that scheme, phase ICs can be simply added or removed without changing the fundamental topology.
The chip set's architecture is in marked contrast to three types of configurations presently implemented: PWM controller ICs that have integrated MOSFET gate drivers; designs that have a separate controller and gate drivers; and designs that use a dual-phase controller with integrated gate drivers and built-in synchronization and current-sharing functions.
The XPhase architecture overcomes the drawbacks specific to each configuration, according to George Schuellein, director of International Rectifier's Computing IC Design Center (Providence, R.I.). These drawbacks include long pc-board traces that ultimately increase noise and place limits on switching frequency due to increased parasitic inductance and limitations on the practical number or phases that can be achieved.
The IR3081 control IC features a 6-bit VID with 0.5 percent overall system accuracy, programmable dynamic VID slew rate and programmable hiccup overcurrent protection with delay to prevent false triggering. It implements feed-forward voltage-mode control and trailing-edge modulation.
The architecture allows the controller to adapt to variations in the input voltage at each phase and will reduce voltage variation during load transients to minimize power loss under heavy load. The control IC can be tailored to attain a loop response of 200 nano-seconds or better. In addition, it senses the average-value inductor current to ensure very accurate current sharing. The devices operate from a 12-volt input, with undervoltage lockout at 9.5 V.
The IR3086 phase IC features a gate-drive current of 2.5 amps (average) and supports 1-MHz-per-phase operation. Both its control and synchronous-FET drivers are compatible with either 5- or 12-V operation. The chip features so-called body-braking (i.e., reference to body diode in a synchronous MOSFET) control to offer a nearly 2x improvement in inductor slew rate.
In typical operation using the chip set and the company's DirectFET MOSFETs (IRF-6602 control FET and IRF-6601 synchronous FET), a 100-A, six-phase converter yields an efficiency of 84 percent with less than 4 percent variation in phase current and average current per phase.
The IR3081 Control IC, in a 28-lead MLPQ, is priced at $1.65 each in 10,000-piece quantities. The IR3086 Phase IC, in a 20-lead MLPQ, is priced at $1.25 each. Both are sampling and are expected to reach production in April.
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