UC3800

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 10

www.unisonic.com.tw

QW-R103-083.a



OPERATION DESCRIPTION

The UTC UC3800 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 UTC UC3800 series.

HV Start-up

UTC UC3800 integrated HV start circuit, and provide about 1mA current to charge VDD pin during power on state

from HV pin. When VDD cap voltage is higher than UVLO (OFF), the charge current is switched off. At this moment,

the VDD capacitor provides current to UTC UC3800 until the auxiliary winding of the main transformer starts to

provide the operation current.

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 22KHz 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 UTC UC3800 enter burst mode at standby condition to minimize the switching loss and reduces the

standby power consumption. Power supplies using the UTC UC3800 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