UCC3888

ICC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . 8mA
Current into TON Pin . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 1.5mA
Voltage on VOUT Pin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20V
Current into TOFF Pin . . . . . . . . . . . . . . . . . . . . .. . . . . . . 250mA
Storage Temperature . . . . . . . . . . . . . . .  . . . -65°C to +150°C
Note: Unless otherwise indicated, voltages are referenced to ground and currents are positive into, negative out of, the specified terminals.

The UCC3888 controller is optimized for use as an off-line, low power, low voltage, regulated bias supply. The unique circuit topology utilized in this device can be visualized as two cascaded flyback converters, each operating in the discontinuous mode, both driven from a single external power switch. The significant benefit of this approach is the ability to achieve voltage conversion ratios as high as 400V to 2.7V with no transformer and low internal losses.

The control algorithm utilized by the UCC3888 sets the switch on time inversely proportional to the input line voltage and sets the switch off time inversely proportional to the output voltage. This action is automatically controlled by an internal feedback loop and reference. The cascaded configuration allows a voltage conversion from 400V to 2.7V to be achieved with a  zero, the switch off time approaches infinity.

The output voltage is set internally to 5V. It can be programmed for other output voltages with two external resistors. An isolated version can be achieved with this topology as described further in Unitrode Application Note U-149.