UCC1889
UCC2889
UCC3889 Off-line Power Supply Controller
FEATURES DESCRIPTION Transformerless Off-line
Applications The UCC1889 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 discontinuIdeal Primary-side Bias Supply
ous mode, and both driven from a single external power switch. The significant
Efficient BiCMOS Design
benefit of this approach is the ability to achieve voltage conversion ratios of 400V
to 12V with no transformer and low internal losses.
Wide Input Range
The control algorithm utilized by the UCC1889 is to force the switch on time to be
Fixed or Adjustable
inversely proportional to the input line voltage while the switch off time is made inLow Voltage Output
versely proportional to the output voltage. This action is automatically controlled by
Uses Low Cost SMD Inductors an internal feedback loop and reference. The cascaded configuration allows a voltage conversion from 400V to 12V to be achieved with a switch duty cycle greater
Short Circuit Protected
than 10%. This topology also offers inherent short circuit protection since as the
output voltage falls to zero, the switch off time approaches infinity.
Optional Isolation Capability
The output voltage can be easily set to 12V or 18V. Moreover, it can be programmed for other output voltages less than 18V with a few additional components. An isolated version can be achieved with this topology as described further
in Unitrode Application Note U-149. OPERATION …
