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UC1108 Scheda tecnica(PDF) 5 Page - Unisonic Technologies |
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UC1108 Scheda tecnica(HTML) 5 Page - Unisonic Technologies |
5 / 8 page UC1108 Preliminary CMOS IC UNISONICTECHNOLOGIESCO.,LTD 5 of 8 www.unisonic.com.tw QW-R103-098.a OPERATION DESCRIPTION (Cont.) Constant Current Operation When the sampled voltage is below 4V reference voltage and the error amplifier output reaches its maximum, thus UTC UC1108 operates in constant-current (CC) mode. The CC point and maximum output power can be externally adjusted by external current sense resistor Rcs. The larger Rcs, the smaller CC point is, and the smaller output power becomes. Figure3. Adjustable output power by changing Rcs Figure4. Secondary current waveform In CC operation, the CC loop control function of UTC UC1108 will keep a fixed proportion between secondary inductance de-magnetization time (Tdemag) and switching cycle time (Tsw). The fixed proportion is 7 4 = Tsw Tdemag (2) Thus the output current is given by: Ipk × N N × 7 2 = Tsw Tdemag × Ipk × N N × 2 1 = Iout S P S P (3) Current Sensing and Leading Edge Blanking Cycle-by-cycle current limiting is offered in UTC UC1108. The switch current is detected by a sense resistor into the CS pin. When the power switch is turned on, a turn-on spike will occur on this resistor. A 500ns leading-edge blanking is built in to avoid false-termination of the switching pulse so that the external RC filtering is no longer needed. Drive BJT The driver is a push pull stage with the supply voltage VCC. It provides the driving current for the external power bipolar transistor. The output signal is current-limited to IOUT(SOURCE). |
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