AL8862Q
Application Information
AL8862Q Operation
In normal operation, when normal input voltage is applied at VIN pin, the AL8862Q internal switch will turn on. Current starts to flow through the
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 AL8862Q senses the voltage across RSET and applies a
proportional voltage to the input of the internal comparator. When this voltage reaches an internal upper-set 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 AL8862Q. A voltage proportional to the sense voltage across
RSET 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.
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
pin and SET pin, and is given by:
0.1
0.1
IOUT
NOM
RSET // RSET_INT
RSET // 500
If the RSET < 1Ω, the current setting equation is simplified as below:
0.1
RSET
IOUT
NOM
The table below gives values of the nominal average output current for setting resistor (RSET) in the Typical Applications Circuit shown on page 2.
Nominal Average Output Current (mA)
RSET (Ω)
0.1
1000
667
0.15
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 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 be clamped to 100% of IOUT_NOM, and the output switch will turn off if the CTRL
voltage falls below 0.3V.
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 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 AL8862Q. 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 a push-pull
output stage.
9 of 17
www.diodes.com
July 2023
AL8862Q
© 2023 Copyright Diodes Incorporated. All Rights Reserved.
Document number: DS42199 Rev. 5 - 2
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