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AL8806MP8-13 Datasheet(PDF) 7 Page - Diodes Incorporated |
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AL8806MP8-13 Datasheet(HTML) 7 Page - Diodes Incorporated |
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7 / 15 page  AL8806 HIGH EFFICIENCY 30V 1.5A BUCK LED DRIVER AL8806 Document number: DS35144 Rev. 2 - 2 7 of 15 www.diodes.com June 2011 © Diodes Incorporated Application Information AL8806 Operation In normal operation, when voltage is applied at VIN, the AL8806 internal switch is turned on. Current starts to flow through sense resistor RSET, inductor L1, and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage VIN and the inductor L1. 33µH L1 AL8806 C3 1µF R SET 0R082 C2 100nF D1 Multi- die LED DFLS230LH C1 4.7µF 12V GND AL8806 C3 1µF R SET 0R082 C2 100nF D1 Multi- die LED DFLS230LH C1 4.7µF 12V GND Figure 1: Typical Application Circuit This rising current produces a voltage ramp across RSET. The internal circuit of the AL8806 senses the voltage across RSET and applies a proportional voltage to the input of the internal comparator. When this voltage reaches an internally set upper threshold, the internal switch is turned off. The inductor current continues to flow through RSET, L1, the LEDs and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward voltage drop of the LEDs and the schottky diode. This decaying current produces a falling voltage at R1, which is sensed by the AL8806. A voltage proportional to the sense voltage across RSET is applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the internal switch is turned on again. This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor RSET. LED Current Control The LED current is controlled by the resistor RSET in Figure 1. Connected between VIN and SET the nominal average output current in the LED(s) is defined as: SET THD LED R V I = If the CTRL pin is driven by an external voltage (higher than 0.4V and lower than 2.5V), the average LED current is: SET THD REF CTRL LED R V V V I = For example for a desired LED current of 1.33A and a default voltage VCTRL=2.5V the resulting resistor is: Ω ≈ = = m 75 5 . 2 5 . 2 33 . 1 1 . 0 V V I V R REF CTRL LED THD SET DC Dimming The CTRL pin can be driven by an external DC voltage (VCTRL), to adjust the output current to a value below the nominal average value defined by RSET. The LED current decreases linearly with the CTRL voltage when 0.5V ≤ VCTRL ≤ 2.5V, as shown on page 4 for 4 different current levels. Note that 100% brightness setting corresponds to VCTRL = VREF, nominally 2.5V. For any voltage applied on the CTRL pin that is higher than VREF, the device will not overdrive the LED current and will still set the current according to the equation VCTRL = VREF. When the CTRL voltage falls below the threshold, 0.4V, the output switch is turned off which allows PWM dimming. |
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