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Controller saves power at every turn

Posted: 07 Jun 2005 ?? ?Print Version ?Bookmark and Share

Keywords:national semiconductor? lm5021? pwm controller?

National Semiconductor Corp.'s LM5021 PWM controller combines all the best power-saving features needed by designers to build energy-efficient single-ended flyback and forward ac-dc converters for telecom, networking, and industrial and consumer power supplies.

"Energy efficiency is more important today than ever, with legislation or advisory standards on standby power developing worldwide. The integrated LM5021 PWM controller offers designers of ac-dc power converters the right combination of performance, space savings, comprehensive protection and lower heat dissipation to reliably meet the new energy standards," said Paul Greenland, director of product marketing for the company's Power Management group. &Quot;The LM5021 also complements National's LM5000 family of high-voltage isolated dc-dc PWM controllers and drivers, enabling our customers to design complete power systems from the AC line cord to the point of load."

The controller integrates a high-frequency current-mode design (for improved transient response and simplified loop compensation) with internal slope compensation (LM5021-1 only, to protect against subharmonic oscillation at duty cycles greater than 50 percent), as well as user-programmable soft-start and a 0.7A MOSFET driver. It includes skip-, burst-, and low-power bias circuitry to ensure improved efficiency over a wide load range. The cycle-skipping burst mode reduces losses during standby (with the chip easily meeting the so-called 1W standby initiative), and during light-load operation. The chip's built-in soft-start timer allows the designer to minimize start-up surge currents that can stress components or saturate the power transformer. Operating current is 25?A in the start-up mode.

Additional features include the chip's under-voltage lockout circuitry, which has wide hysteresis, allowing designers to select a smaller bias supply filter capacitor for cost and real-estate savings. The LM5021 also includes cycle-by-cycle current limiting and a (programmable) hiccup-mode for protection against overload/short-circuits and sustained operation above the power limit. The switching frequency is programmed by a single resistor and the chip can be synchronized to an external clock to reduce interference. Internal leading-edge blanking of the current sense signal reduces external filtering.

The LM5021 is available in two versions, distinguished by the maximum duty cycle of its pulse-width modulator: 80 percent (LM5021-1), and 50 percent (LM5021-2), to cater to flyback, and forward converters, respectively. Available in MSOP-8 and MDIP-8 packages, the LM5021 is priced at 45 cents each in 1,000 quantities. All packages are available Pb-free. A datasheet is available online.

- Vincent Biancomano


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