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UC1108 Scheda tecnica(PDF) 4 Page - Unisonic Technologies |
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UC1108 Scheda tecnica(HTML) 4 Page - Unisonic Technologies |
4 / 8 page UC1108 Preliminary CMOS IC UNISONICTECHNOLOGIESCO.,LTD 4 of 8 www.unisonic.com.tw QW-R103-098.a OPERATION DESCRIPTION The UTC UC1108 is a primary control unit for switch mode charger and adapter applications. It operates in primary-side sensing and regulation. Opto-coupler and TL431 could be eliminated. Proprietary built-in CV and CC control can achieve high precision CV/CC performance. It uses Pulse Frequency Modulation (PFM) method to build discontinuous conduction mode (DCM) flyback power supplies. Startup control The voltage regulator provides the internal power supply VDD. For VCC and VDD external capacitors are required. The current biasing of UTC UC1108 is adjustable by an external resistor. Internal time constants, thresholds and the output current depend from the value of this resistor. The startup circuit activates the IC after startup and deactivates the IC in case of UVLO. Operating Current The Operating current of UTC UC1108 is as low as 400uA. Good efficiency and very low standby power can be achieved. Constant Voltage Operation The output voltage is defined by the transmission ratio between the secondary and auxiliary winding. The UTC UC1108 captures the auxiliary winding feedback voltage at FB pin and operates in constant-voltage (CV) mode to regulate the output voltage. The auxiliary voltage reflects the output voltage is given by: ) V ∆ + V ( × N N = V O S A AUX (1) △ Where V indicates the drop voltage of the output diode. Figure2. Auxiliary voltage waveform Via a resistor divider connected between the auxiliary winding and FB, the VAUX is sampled at the Tsampling end and it is hold until the next sampling. The sampled voltage is compared with 4V reference voltage and the error is amplified. The error amplifier output reflects the load condition and controls the Toff time to regulate the output voltage, thus constant output voltage can be achieved. |
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