FEATURES

· Transformerless Off-line

Applications

· Ideal Primary-side Bias Supply

· Efficient BiCMOS Design

· Wide Input Range

· Fixed or Adjustable

Low Voltage Output

· Uses Low Cost SMD Inductors

· Short Circuit Protected

· Optional Isolation Capability

DESCRIPTION

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 discontinuous

mode, and both driven from a single external power switch. The significant

benefit of this approach is the ability to achieve voltage conversion ratios of 400V

to 12V with no transformer and low internal losses.

The control algorithm utilized by the UCC1889 is to force the switch on time to be

inversely proportional to the input line voltage while the switch off time is made 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 12V to be achieved with a switch duty cycle greater

than 10%. This topology also offers inherent short circuit protection since as the

output voltage falls to zero, the switch off time approaches infinity.

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

With reference to the application diagram below, when input voltage is first applied,

the RON current into TON is directed to VCC where it charges the external capacitor,

C3, connected to VCC. As voltage builds on VCC, an internal undervoltage lockout

holds the circuit off and the output at DRIVE low until VCC reaches 8.4V. At this

time, DRIVE goes high turning on the power switch, Q1, and redirecting the current

into TON to the timing capacitor, CT. CT charges to a fixed threshold with a current

ICHG=0.8 · (VIN - 4.5V)/RON. Since DRIVE will only be high for as long as CT

charges, the power switch on time will be inversely proportional to line voltage.

This provides a constant line voltage-switch on time product.