Datasheet AL8861Q (Diodes) - 9

AL8861Q Application Information
The AL8861Q is a hysteretic mode LED driver with integrated power switch, and is available in thermal enhanced MSOP-8EP package.
AL8861Q Operation
In normal operation, when normal input voltage is applied at VIN pin, the AL8861Q internal switch will turn on. Current starts to flow through the sense resistor RS, inductor L1, and the LEDs. The current ramps up linearly, and the slew rate is determined by the input voltage VIN, VOUT and the inductor L1. This rising current produces a voltage ramp across RS. The internal circuit of the AL8861Q senses the voltage across RS and applies a
T
proportional voltage to the input of the internal comparator. When this voltage reaches an internal set upper threshold, the internal switch is turned
C
off. The inductor current continues to flow through RS, L1, LEDs and diode D1, and back to the supply rail, but it decays with the rate determined
U D
by the forward voltage drop of LEDs and the diode D1.
O R
This decaying current produces a falling voltage on RS, which is sensed by the AL8861Q. A voltage proportional to the sense voltage across RS
P
will be applied at the input of internal comparator. When this voltage falls to the internal lower-set threshold, the internal switch is turned on again.
W E
This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor RS.
N LED Current Configuration
The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RS) connected between VIN pin and ISENSE pin, and is given by: The table below provides values of the nominal average output current for setting resistor (RS) in the Typical Application Circuit shown on Page 2.
RS (Ω) Nominal Average Output Current (mA)
0.066 1,500 0.1 1,000 0.13 760 0.15 667 0.3 333 The above values assume that the VSET pin is floating and at a nominal reference voltage for internal comparator. It is possible to use different values of RS if the VSET pin is driven by an external dimming signal.
Analog Dimming
Applying a DC voltage from 0.3V to 2.5V on the VSET pin can adjust output current from 0 to 100% of IOUTNOM, as shown in Figure 1. The recommended dimming range is from 5% to 100%. If the VSET pin is brought higher than 2.5V, the LED current will be clamped to 100% of IOUTNOM. And the output switch will turn off if the VSET pin voltage falls below the threshold of 0.3V.
PWM Dimming
The LED current can be adjusted digitally by applying a low frequency Pulse-Width-Modulated (PWM) logic signal to the VSET pin to turn the device on and off. This will 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 high 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 AL8861Q. For best accuracy/resolution, the switching frequency should be increased while the PWM frequency should be reduced. The VSET pin is designed to be driven by both 3.3V and 5V logic levels directly from a logic output with either an open drain output or a push-pull output stage. AL8861Q 9 of 15 May 2019 Document number: DS41798 Rev. 2 - 2
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