AL8843
Application Information (continued)
Inductor Selection
Recommended inductor values for the AL8843 are in the range 33μH to 100μH. Higher inductance are recommended at higher supply voltages in
order to minimize output current tolerance due to switching delays, which will result in increased ripple and lower efficiency. Higher inductance
also results in a better line regulation. The inductor should be mounted as close to the device as possible with low resistance connections to SW
pins.
The chosen coil should have saturation current higher than the peak output current and a continuous current rating above the required mean
output current.
The inductor value should be chosen to maintain operating duty cycle and switch ‘on’/‘off’ times within the specified limits over the supply voltage
and load current range. The following equations can be used as a guide.
SW Switch ‘On’ Time
푳∆푰
풕푶푵
=
푽푰푵 − 푽푳푬푫 − 푰푳푬푫(푹푺푬푻 + 푹푳 + 푹퐒퐖
)
SW Switch ‘Off’ Time
푳∆푰
푽푳푬푫 + 푽푫 + 푰푳푬푫(푹푺푬푻 + 푹푳)
풕푶푭푭
=
Where: L is the coil inductance; RL is the coil resistance; RSET is the current sense resistance; ILED is the required LED current; ΔI is the coil peak-
peak ripple current (internally set to 0.26 × ILED); VIN is the supply voltage; VLED is the total LED forward voltage; RSW is the switch resistance
(0.2Ω nominal); VD is the diode forward voltage at the required load current.
Thermal Protection
The AL8843 includes Over-Temperature Protection (OTP) circuitry that will turn off the device if its junction temperature gets too high. This is to
protect the device from excessive heat damage. The OTP circuitry includes thermal hysteresis that will cause the device to restart normal
operation once its junction temperature has cooled down by approximately +30°C.
Open Circuit LEDs
The AL8843 has by default open LED protection. If the LEDs should become open circuit, the AL8843 will stop oscillating; the SET pin will rise to
VIN and the SW pin will then fall to GND. No excessive voltages will be seen with the AL8843.
LED Chain Shorted Together
If the LED chain should become shorted together (the anode of the top LED becomes shorted to the cathode of the bottom LED), the AL8843 will
continue to switch and the current through the AL8843’s internal switch will still be at the expected current - so no excessive heat will be generated
within the AL8843. However, the duty cycle at which it operates will change dramatically and the switching frequency will most likely decrease.
See Figure 2 for an example of this behavior at 24V input voltage driving 3 LEDs.
The on-time of the internal power MOSFET switch is significantly reduced because almost all of the input voltage is now developed across the
inductor. The off-time is significantly increased because the reverse voltage across the inductor is now just the Schottky diode voltage (See Figure
2) causing a much slower decay in inductor current.
12 of 16
www.diodes.com
August 2022
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AL8843
Document number: DS40367 Rev. 2 - 2
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