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AL8805W5-7 Datasheet(PDF) 9 Page - Diodes Incorporated |
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AL8805W5-7 Datasheet(HTML) 9 Page - Diodes Incorporated |
9 / 16 page AL8805 HIGH EFFICIENCY 30V 1A BUCK LED DRIVER AL8805 Document number: DS35030 Rev. 1 - 2 9 of 16 www.diodes.com September 2010 © Diodes Incorporated Applications Information AL8805 Operation In normal operation, when voltage is applied at +VIN, the AL8805 internal switch is turned on. Current starts to flow through sense resistor R1, 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. This rising current produces a voltage ramp across R1. The internal circuit of the AL8805 senses the voltage across R1 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 R1, 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 AL8805. A voltage proportional to the sense voltage across R1 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 R1. LED Current Control The LED current is controlled by the resistor R1 in Figure 30. 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 660mA and a default voltage VCTRL=2.5V the resulting resistor is: Ω ≈ = = m 150 5 . 2 5 . 2 66 . 0 1 . 0 V V I V R REF CTRL LED THD SET R1 AL8805 L1 VIN SET SW CTRL GND C1 C2 D1 Figure 30: Typical Application Curcuit 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 in figure 2 for 4 different current levels. When the CTRL voltage falls below the threshold, 0.4V, the output switch is turned off which allows PWM dimming. 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. PWM Dimming LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device on and off. This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a max resolution of 10bit can be applied to the CTRL pin to change the output current to a value below the nominal average value set by resistor RSET. To achieve this resolution the PWM frequency has to be lower than 500Hz, however higher dimming frequencies can be used, at the expense of dimming dynamic range and accuracy. |
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