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AL8806 Datasheet(PDF) 10 Page - Diodes Incorporated |
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AL8806 Datasheet(HTML) 10 Page - Diodes Incorporated |
10 / 15 page AL8806 HIGH EFFICIENCY 30V 1.5A BUCK LED DRIVER AL8806 Document number: DS35144 Rev. 2 - 2 10 of 15 www.diodes.com June 2011 © Diodes Incorporated Applications Information (continued) Reducing output ripple Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown already in the circuit schematic. A value of 1 μF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version without altering the mean current value. Capacitor Selection The small size of ceramic capacitors makes them ideal for AL8806 applications. X5R and X7R types are recommended because they retain their capacitance over wider voltage and temperature ranges than other types such as Z5U. A 2.2 μF input capacitor is sufficient for most intended applications of AL8806; however a 4.7μF input capacitor is suggested for input voltages approaching 30V. 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. The Schottky diode also provides better efficiency than silicon PN 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. In particular, it is recommended to have a diode voltage rating at least 15% higher than the operating voltage to ensure safe operation during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating the power loss through the diode. Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8806 applications. Thermal and layout considerations For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation depends on several factors: the thermal resistance of the IC package θ JA, PCB layout, airflow surrounding the IC, and difference between junction and ambient temperature. The maximum power dissipation can be calculated using the following formula: () W 45 . 1 W / C 69 C 25 C 125 P ) MAX ( D = ° ° − ° = where TJ(MAX) is the maximum operating junction temperature, TA is the ambient temperature, and θ JA is the junction to ambient thermal resistance. The recommended maximum operating junction temperature, TJ, is 125°C and so maximum ambient temperature is determined by the AL8806’s junction to ambient thermal resistance, θ JA. To support high LED drive at higher ambient temperatures the AL8806 has been packaged in thermally enhanced MSOP-8EP package. θ JA, is layout dependent and the AL8806 in MSOP-8EP’s θJA on a 51 x 51mm single layer PCB with 2oz copper standing in still air is approximately 69°C/W. Therefore the maximum power dissipation at TA = 25°C is: () W 45 . 1 W / C 69 C 25 C 125 P ) MAX ( D = ° ° − ° = Figure 5, shows the power derating of the AL8806 on an FR4 51x51mm PCB with 2oz copper standing in still air. As the ambient temperature increases and/or the PCB area reduces the maximum power dissipated by the AL8806 will decrease. MSOP-8EP 0 200 400 600 800 1000 1200 1400 1600 -40 -25 -10 5 20 35 50 65 80 95 110 125 Ambient temperature (°C) Figure 5. Derating Curve |
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