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UC3875CG-S08-R Datasheet(PDF) 5 Page - Unisonic Technologies |
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UC3875CG-S08-R Datasheet(HTML) 5 Page - Unisonic Technologies |
5 / 9 page UC3875C Preliminary LINEAR INTEGRATED CIRCUIT UNISONICTECHNOLOGIESCO.,LTD 5 of 9 www.unisonic.com.tw QW-R103-075.b OPERATION DESCRIPTION The UTC UC3875C 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 UC3875C 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 2.5~3MΩ, 1/8W startup resistor could be used together with a VDD capacitor to provide a fast startup and low power 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 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 UC3875C enter burst mode at standby condition to minimize the switching loss and reduces the standby power consumption. Power supplies using the UTC UC3875C 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 modulated by output power POUT during IC 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 load condition, The IC will operate at from Burst mode to Reducing Frequency Mode. The relation curve between fSW and POUT/POUT (MAX) as followed Fig.1. The maximum switch frequency is set through the RI-pin resistor RI: FSW=6500/RI (KΩ) KHz. Fig.1 The relation curve between fSW and relative output power POUT/ POUT (MAX) |
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