The file is the preliminary description of UCC1858 UCC2858 UCC3858, which provides all the necessary control functions for active power factor corrected preregulators that require high efficiency at low power operation. The controller achieves near unity power factor by shaping the AC input line current waveform to correspond to the AC input line voltage using average current mode control. The operation of the UCC3858 closely resembles that of previously designed Unitrode PFC parts with additional features to allow higher efficiency boost converter operation at light loads. This is accomplished by linearly scaling back the PWM frequency when the output of the voltage error amplifier drops below a predetermined user programmable level indicating a light load condition. The frequency is scaled back by reducing the charging current for the CT ramp (in proportion to the output power), and increasing the dead time. There is also an instantaneous reset input to pull the IC out of foldback mode quickly when the load comes back up. The PWM technique used in the UCC3858 is leading edge modulation. When combined with the more conventional trailing edge modulation on the downstream converter, this scheme offers the benefit of reduced rip- ple current on the bulk storage capacitor. The oscillator is designed for easy synchronization to the downstream converter. A simple synchroni- zation scheme can be implemented by connecting the PWM output of the downstream converter to the SYNC pin.
The Unitrode UCC1858, UCC2858, and UCC3858 provide all control functions necessary for active power factor corrected preregulators requiring high efficiency at low power operation. The controller achieves near unity power factor by shaping the AC input line current waveform to correspond to the AC input line voltage using average current mode control.
| Supply Voltage | 12V to 18V Operation |
|---|---|
| Storage Temperature | -55°C to +150°C |
| Junction Temperature | -55°C to +150°C |
| Oscillator Initial Accuracy | 90 to 110 kHz |
| Voltage Reference Output | 7.5V |
| Open Loop Gain (Voltage Amplifier) | 45 to 63 dB |
The device supports 12V to 18V operation.
The PWM frequency is programmable and can be synchronized to a downstream converter.
The FBL pin selects the level of the voltage error amplifier output at which frequency foldback begins.