Datasheet

UCC28063

SLUSAO7 – SEPTEMBER 2011

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Phase Management and Light-Load Operation

Under light-load conditions, switching losses may dominate over conduction losses and efficiency may be

improved if one phase (channel) is turned off. At a certain power level, the reduction of switching losses is

greater than the increase in conduction losses. Turning off one phase at light load is especially valuable for

meeting light-load efficiency standards. This is one of the major benefits of interleaved PFC and it is especially

valuable for meeting 80+ design requirements.

The PHB input can be used to force the UCC28063 to operate in single-phase mode. When PHB is driven below

0.8 V, channel B will stop switching and channel A on-time will automatically double to compensate. The device

will resume dual-phase mode when PHB is raised above 1.0 V. For customized phase management, an external

circuit can detect the conditions for switching to single-phase operation and drive PHB accordingly. To operate

continuously in two-phase mode (normal mode) when phase management is not desired, simply connect PHB to

VREF.

As load current decreases, the error amplifier commands less ac-line input current by lowering COMP voltage. In

applications where the ac-line is limited to the low-voltage range only, it may be advantageous to connect PHB

directly to COMP to allow automatic selection of single-phase operation without additional external circuitry.

External Disable

The UCC28063 can be externally disabled by purposefully grounding the VSENSE pin with an open-drain or

open-collector driver. When disabled, the device supply current drops significantly and COMP is actively pulled

low. This disable method forces the device into standby mode and minimizes its power consumption. This is

particularly useful when standby power is a key design aspect. When VSENSE is released, the device enters

soft-start mode.

Product Folder Link(s): UCC28063