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7447714680 Scheda tecnica(PDF) 11 Page - Union Semiconductor, Inc. |
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7447714680 Scheda tecnica(HTML) 11 Page - Union Semiconductor, Inc. |
11 / 13 page ________________________________________________________________________ http://www.union-ic.com Rev.05 Apr.2016 11/13 UM1360 Load Current Inductor Saturation Current IOUT>1A 27-47μH 1.3-1.5 Times of Load Current 0.8A<IOUT≤1A 33-82μH 0.4A<IOUT≤0.8A 47-100μH IOUT≤0.4A 68-220μH Order Code L(μH) Size DCR (mΩ) Saturation Current (mA) Manufacturer 744066330 33 1038 92 1800 Würth Elektronik www.we-online.com 7447714470 47 1050 82.5 2.5 7447714680 68 1050 110 2200 7447714101 100 1050 165 1800 The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times within the specified limits over the supply voltage and load current range. The following equations can be used as a guide. LX Switch 'On' time TON=(L1*ΔI)/(VIN-VLED-ILED*(RS+RL+RLX)) LX Switch 'Off' time TOFF=(L1*ΔI)/(VLED+VD+ILED*(RS+RL)) Where: L1 is the coil inductance (H). RL is the coil resistance (Ω). RS is the current sense resistance (Ω). ILED is the required LED current (A). ΔI is the coil peak-peak ripple current (A) {internally set to 0.3×I LED}. VIN is the supply voltage (V). VLED is the total LED forward voltage (V). RLX is the switch resistance (Ω) {=0.5Ω nominal}. VD is the diode forward voltage at the required load current (V). Diode Selection For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operating voltage and temperature. They also provide better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time. It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device and if close to the load may create a thermal runaway condition. The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the LX output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the LX pin including supply ripple, does not exceed the specified maximum value. PCB Layout Guidelines Careful PCB layout is critical to achieve low switching losses and stable operation. Minimize ground noise by connecting high current ground returns, the input bypass capacitor ground lead, and the output filter ground lead to a single point. Place Rs as close as possible to the ISENSE and VIN. For better noise immunity, a Kelvin connection is strongly recommended between ISENSE and Rs. |
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