UC3849B

LINEAR INTEGRATED CIRCUIT

UNISONIC TECHNOLOGIES CO., LTD

5 of 10

www.unisonic.com.tw

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OPERAT I ON DESCRI PT I ON

The UC3849B devices integrate many useful designs into one controller for low-power switch-mode power

supplies. The following descriptions highlight some of the features of the UC3849B series.

Start-up Current

The start-up current is only 2.5

μA. Low start-up current allows a start-up resistor with a high resistance and a

low-wattage to supply the start-up power for the controller. For AC/DC adaptor with universal input range design, a

2.5~3M

dissipation solution.

Power-Saving Mode Operation

The proprietary Power-Saving Mode function provides linearly decreasing the switching frequency under

light-load conditions for higher efficiency. The feedback voltage, which is sampled from the voltage feedback loop, is

taken as the reference. Once the feedback voltage dropped below the threshold voltage, the switching frequency

starts to decrease. This Power-Saving Mode function dramatically reduces power consumption under light-load

conditions. The 20KHz minimum frequency control also eliminates the audio noise at any loading conditions.

At zero load condition, the magnitude of power loss is in proportion to the number of switching events within a

fixed period of time. Reducing switching events leads to the reduction on the power loss and thus conserves the

energy. The UC3849B enter burst mode at standby condition to minimize the switching loss and reduces the

standby power consumption. Power supplies using the UC3849B can easily meet even the strictest regulations

regarding standby power consumption.

Switch Frequency Set

operating. At no load or light load condition, most of the power dissipation in a switching mode power supply is from

switching loss on the MOSFET transistor, the core loss of the transformer and the loss on the snubber circuit. The

magnitude of power loss is in proportion to the number of switching events within a fixed period of time. So lower

switch frequency at lower load, which more and more improve IC’s efficiency at light load. At from no load to light