Primary-side controller advances digital power

Los Gatos, Calif.&#8212Proclaiming a paradigm shift in power supply control techniques with its iW1688 PWM chip, iWatt says it's the first to secure significant benefits of digital control as applied to the primary side of low-power AC/DC adapters and chargers. Using what the company says is a unique combination of sampling techniques and an efficient digital engine, the iW1688 is designed to achieve tight regulation and a smaller, less expensive and more efficient implementation for the 2- to 5-watt adapters used in cellular and cordless phones, digital music players, and still cameras.

“In an environment of increasing regulatory pressures on AC/DC supplies for low power applications, the iW1688 is a game changing product. It significantly
simplifies meeting tighter no-load power requirements while eliminating external components, such as optocouplers, that add both cost and reliability issues,” said Doyle Slack, VP of marketing.

Traditional solutions using analog (DC feedback) methods in the primary loop create problems with regulation, notes George Hill, director of marketing. Indeed, up to this time securing tight supply regulation required secondary feedback loops that add both components and create a concern as to the overall reliability of the optocoupler, he notes. High regulation with an analog solution and DC feedback becomes more difficult when the feedback signal is too filtered&#8212the feedback loop's cycle-by-cycle response is lost. iWatt's solution, on the other hand, uses AC feedback from the primary side to restore cycle-by-cycle response and high regulation&#8212there are no regulation errors (which become particularly significant during the transition from light-load to full-load conditions) associated with the sampling process. The chip tightly regulates output voltage in the constant-voltage mode to less than &plusmn5 percent; and current in the constant-current mode to less than &plusmn10 percent&#8212best in-class performance for primary-feedback systems, according to the company.

The iW1688's digital approach, moreover, allows design-in of some valuable functionality, including robust over- and under-voltage shutdown. Its control loop also provides internal compensation, thus doing away with several more external components.

In moving the action to the primary side, the iW1688 does away with the optocoupler and associated components traditionally used for coordinating secondary-side feedback. There are other alternatives to the optocoupler approach, notes Hill. One solution uses an auxiliary winding in a traditional topology to replace the optocoupler. The drawback to that scheme, says Hill, is that operating power for the PWM circuitry also goes over the same winding as the feedback information, thus creating an additional regulation issue.

Click here for the device datasheet. The iW1688 is fully qualified and in volume production now. The chip, packaged in a RoHS-compliant 5-lead SOT-23 package and available in 3,000 piece reels, is priced at 31 cents each in 10k pieces.

iWatt , 1-408-374-4200,

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