Power Electronics

Smart PWM Controller Benefits Notebook Power Supplies

STMicroelectronics has announced an innovative current-mode primary controller IC designed for single-ended switching power converters. Suited for use in high-end ac-dc adapters and chargers for notebook or laptop PCs, the L6668 also is suitable for power supplies of set-top boxes and other digital consumer equipment. In addition, energy-saving features make the IC an option in low-power auxiliary converters.

The IC includes functions such as the ability to interface with the PFC controller that enables pre-regulator turn off when the load level falls below a certain threshold, saving typically 0.5 W to 1 W of no-load power consumption. This interface easily can be connected to the RUN or PFC_OK pins of ST’s L6563 Power Factor Correction IC.

In addition, the chip’s high-voltage (HV) technology, when combined with a multi-chip approach, makes it possible to integrate a high-voltage start-up system that is connected directly to the rectified mains voltage. The HV generator is physically located on a separate chip made with BCD (Bipolar-CMOS-DMOS) off-line technology and able to withstand 700 V. The HV generator is controlled by a low-voltage chip, where all of the control functions reside. This feature, in addition to burst-mode operation and the very low quiescent current, helps it to be compliant with energy-saving recommendations with minimum usage of external components.

The L6668 achieves its high level of efficiency by operating in three different modes, depending on the load conditions. At heavy load, the IC drives the converter at a programmable fixed frequency to optimize its size and weight. Then, if the load is reduced below a threshold, the switching frequency is adapted to minimize conversion losses. Additionally, the improved Standby function causes a smooth frequency reduction as the load is progressively reduced. Eventually, at very light load, the device enters a special operating mode (burst-mode with fixed, externally programmed peak current) that brings the switching frequency to extremely low values, essentially eliminating all switching losses.

The L6668 also includes programmable soft-start, slope compensation for stable operation at duty cycles greater then 50%, a disable function, current sense leading edge blanking to improve noise immunity, latched disable for overvoltage or overtemperature shutdown, and protective output current limiting in case the secondary diode fails to short circuit.

The new power supply controller is Blue Angel, Energy Star and EU Code of Conduct compliant, and is available now in an SO16 package. Available in production volumes, pricing for the L6668 is $0.40 in quantities of either 1000 or 2500 for tape-and-reel.

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