Datasheet AL8843Q (Diodes) - 11

AL8843Q Application Information AL8843Q Operation
In normal operation, when normal input voltage is applied at VIN, the AL8843Q internal switch will turn on. Current starts to flow through sense resistor RSET, inductor L1, and the LEDs. The current ramps up linearly, and the ramp-up rate is determined by VIN, VOUT and the inductor L1. This rising current produces a voltage ramp across RSET. The internal circuit of the AL8843Q 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 will be turned off. The inductor current continues to flow through RSET, L1, LEDs and diode D1, and back to the supply rail, but then it decays with the rate determined by the forward voltage drop of LEDs and the diode D1. This decaying current produces a falling voltage on RSET, which is sensed by the AL8843Q. A voltage proportional to the sense voltage across RSET will be applied at the input of internal comparator. When this voltage falls to the internally set lower threshold, the internal switch will be turned on again. This switch-on-and-off cycle continues to provide the average LED current, set by the sense resistor RSET.
LED Current Configuration
The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RSET), which is connected between VIN and SET pins, and is given by: 0.1 I = OUT(NOM) R SET The table below gives values of nominal average output current for several preferred values of current setting resistor (RSET) in the Typical Application Circuit shown on Page 2.
RSET (Ω) Nominal Average Output Current (mA)
0.033 3,000 0.05 2,000 0.067 1,500 0.1 1,000 0.15 667 0.3 333 The above values assume that the CTRL pin is floating and at a nominal reference voltage for internal comparator. It is possible to use different values of RSET if the CTRL pin is driven by an external dimming signal.
Analog Dimming
Applying a DC voltage from 0.4V to 2.5V on the CTRL pin can adjust output current from 10% to 100% of IOUT_NOM, as shown in Figure 1. If the CTRL pin is brought higher than 2.5V, the LED current wil be clamped to 100% of IOUT_NOM while if the CTRL voltage falls below the threshold of 0.3V, the output switch wil turn off.
PWM Dimming
The 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 wil produce an average output current proportional to the duty cycle of the control signal. To achieve a high resolution, the PWM frequency is recommended to be lower than 500Hz, however higher dimming frequencies can be used at the expense of dimming dynamic range and accuracy. Typically, for a PWM frequency of 500Hz, the accuracy is better than 1% for PWM ranging from 1% to 100%. The accuracy of the low duty cycle dimming is affected by both the PWM frequency and the switching frequency of the AL8843Q. For best accuracy/resolution, the switching frequency should be increased while the PWM frequency should be reduced. AL8843Q 11 of 18 October 2019 Document number: DS41943 Rev. 2 - 2
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© Diodes Incorporated Document Outline Pin Assignments Description Applications Features