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AL8807BQ Datasheet(PDF) 9 Page - Diodes Incorporated |
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AL8807BQ Datasheet(HTML) 9 Page - Diodes Incorporated |
9 / 18 page AL8807BQ Document number: DS37011 Rev. 1 - 2 9 of 18 www.diodes.com October 2014 © Diodes Incorporated AL8807BQ Application Information The AL8807BQ is a hysteretic (also known as equal ripple) LED driver with integrated power switch. It is available in two packages that provide a PCB area-power dissipation capability compromise. It is recommended that at higher LED currents/smaller PCBs that the MSOP-8EP version is used to maximize the allowable LED current over a wider ambient temperature range. AL8807BQ Operation In normal operation, when voltage is applied at +VIN, the AL8807BQ 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 AL8807BQ 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 AL8807BQ. 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 31. Figure 31 Typical Application Circuit Figure 32 Typical Operating Waveform (C2 not fitted) Connected between VIN and SET the nominal average output current in the LED(s) is defined as: 1 R V I THD LED For example for a desired LED current of 660mA and a default voltage VCTRL=2.5V the resulting resistor is: m 150 66 . 0 1 . 0 I V 1 R LED THD VIN = 12V TA =25ºC 2 LEDs 20ns/div No C2 Ch2: 2V/div Ch4: 100mA/div Ch4: LED Current Ch2: SW Pin |
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