AL8862
Application Information
AL8862 Operation
In normal operation, when normal input voltage is applied at VIN, the AL8862 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 the input voltage VIN, VOUT and the
inductor L1.
This rising current produces a voltage ramp across RSET. The internal circuit of the AL8862 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, LEDs, and diode D1, and back to the supply rail; but 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 AL8862. 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 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 Configuration
The nominal average output current in the LED(s) is determined by the value of the external current sense resistor (RSET) connected between VIN
and SET and is given by:
0.1
IOUT
NOM
RSET
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 1.
RSET (Ω)
0.1
Nominal Average Output Current (mA)
1000
667
333
0.15
0.3
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
Application of a DC voltage from 0.4V to 2.5V on the CTRL pin can adjust output current from 10% to 100% of IOUT_NOM linearly, as shown in
Figure 1. If the CTRL pin is brought higher than 2.5V, the LED current will clamp to 100% of IOUT_NOM. If the CTRL voltage falls below 0.3V, the
output switch will turn off.
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. 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 also the switching frequency of the AL8862. For best
accuracy/resolution, the switching frequency should be increased while the PWM frequency should be reduced.
The CTRL 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 push pull
output stage.
10 of 17
www.diodes.com
September 2022
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AL8862
Document number: DS40375 Rev. 5 - 2
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