AL8843

Document number: DS40367 Rev. 1 - 2

13 of 18

www.diodes.com

December 2017

© Diodes Incorporated

AL8843

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

SW

Switch ‗Off‘ Time

is the required LED current; ΔI is the coil peak-

Ω

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

and the SW pin will then fall to GND. No excessive voltages will be seen by 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.