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USL3631 Fiches technique(PDF) 4 Page - Unisonic Technologies |
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USL3631 Fiches technique(HTML) 4 Page - Unisonic Technologies |
4 / 6 page USL3631 Preliminary LINEAR INTEGRATED CIRCUIT UNISONICTECHNOLOGIESCO.,LTD 4 of 6 www.unisonic.com.tw QW-R103-076.d APPLICATION INFORMATION The UTC USL3631 is a high performance non-isolated Buck converter specially designed for LED lighting. The device integrates a 600V power MOSFET. With very few external components, the converter achieves excellent constant current control. And it does not need auxiliary winding for powering the IC or voltage sensing, thus the system size and cost is greatly reduced. Start Up When system powered up, the VCC pin capacitor is charged up by the start up resistor. When the VCC pin voltage reaches the start up threshold, the internal circuits start operating. The UTC USL3631 integrates a 17V zener diode to clamp the VCC voltage. Constant Current Control Cycle by Cycle current sense is adjusted in UTC USL3631, the CS pin is connected to the current sense comparator, and the voltage on CS pin is compared with the internal 400mV reference voltage. The MOSFET will be switched off when the voltage on CS pin reaches the Peak Current threshold. The CS comparator includes a 350ns leading edge blanking time. After the MOSFET switched off, the OFF time controller would open the MOSFET again at time up. At that moment, CS pin voltage is 0mV usually. The Delta inductor current is given by: PK CS 400 I= (mA R ) Where, RCS is the current sense resistor value. The current in LED can be calculated by the equation: PK LED CS I 200 I= = (mA 2R ) Where, IPK is the Peak current of the inductor. Inductor Selection The UTC USL3631 works under inductor current critical conduction mode. When the power MOFET is switched on, the current in the inductor rises up from zero, the on time of the MOSFET can be calculated by the equation: LED IN PK on V V I L t - Where, L is the inductance value VIN is the DC bus voltage after the rectifier bridge VLED is the voltage on the LED After the power MOSFET is switched off, the current in the inductor decreases. When the inductor current reaches zero, the power MOSFET is turned on again by IC internal logic. The off time of the MOSFET is given by: LED PK off V I L t The inductance can be calculated by the equation: IN PK LED IN LED V I freq V V V L ) - ( The freq is the system switching frequency, which is proportional to the input voltage. So the minimum switching frequency is set at lowest input voltage, and the maximum switching frequency is set at highest input voltage. The minimum and maximum off time of UTC USL3631 is set at 4.5µs and 240µs, respectively. Referring to the equation of tOFF calculation, if the inductance is too small, the tOFF may be smaller than the minimum off time, system will operate in discontinuous conduction mode and the output current will be smaller than the designed value. If the inductance is too large, the tOFF may be larger than the maximum off time, the outputt current will not be constant follow design value. So it is important to choose a proper inductance, we recommend user to regulate the inductance value for 70KHz application. |
Numéro de pièce similaire - USL3631 |
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Description similaire - USL3631 |
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