UC3875A

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 10

www.unisonic.com.tw

QW-R103-127.E

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OPERATION DESCRIPTION

The UTC UC3875A 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 UC3875A series.

Start-up Current

The start-up current is only 5μA. 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

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 22 KHz 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 UC3875A enter burst mode at standby condition to minimize the switching loss and reduces the

standby power consumption. Power supplies using the UTC UC3875A can easily meet even the strictest regulations

regarding standby power consumption.

Switch Frequency Set

The maximum switch frequency is set through the 100KΩ RI-pin resistor to 65KHz. Switch frequency is

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 load condition, The IC will operate at from Burst mode to Reducing Frequency