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US3651L-D08-T Datasheet(PDF) 6 Page - Unisonic Technologies |
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US3651L-D08-T Datasheet(HTML) 6 Page - Unisonic Technologies |
6 / 8 page US3651 Preliminary CMOS IC UNISONICTECHNOLOGIESCO.,LTD 6 of 8 www.unisonic.com.tw QW-R502-947.h OPERATION DESCRIPTION The UTC US3651 is a primary control 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. The device integrates PWM controller to enhance the performance of discontinuous conduction mode (DCM) flyback converters. Startup control The VDD pin of UTC US3651 is connected to the line input through a resistor. A large value startup resistor can be used to minimize the power loss in application because the start current of UTC US3651 is very low. When the VDD voltage reaches VTH(ON), the internal startup circuit is disabled and the IC turns on. Operating Current The Operating current of UTC US3651 is as low as 1.6mA. 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 US3651 captures the auxiliary winding feedback voltage at INV 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. Figure 2. Auxiliary voltage waveform Via a resistor divider connected between the auxiliary winding and INV, the VAUX is sampled at the Tsampling end and it is hold until the next sampling. The sampled voltage is compared with 2.0V reference voltage and the error is amplified. The error amplifier output reflects the load condition and controls the TOFF time and the Ipk to regulate the output voltage, thus constant output voltage can be achieved. Constant Current Operation When the sampled voltage is below 2.0V reference voltage and the error amplifier output reaches its maximum, thus UTC US3651 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. |
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