Datasheet AL8862Q (Diodes) - 10

AL8862Q Application Information
(continued)
Inductor Selection
Recommended inductor value for the AL8862Q is in the range of 33μH to 100μH. Higher inductance is 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 Pins Switch ‘On’ Time 𝑳∆𝑰 𝒕𝑶𝑵 = 𝑽𝑰𝑵 − 𝑽𝑳𝑬𝑫 − 𝑰𝑳𝑬𝑫(𝑹𝑺𝑬𝑻 + 𝑹𝑳 + 𝑹𝒔𝒘) SW Pins Switch ‘Off’ Time 𝑳∆𝑰 𝒕𝑶𝑭𝑭 = 𝑽𝑳𝑬𝑫 + 𝑽𝑫 + 𝑰𝑳𝑬𝑫(𝑹𝑺𝑬𝑻 + 𝑹𝑳) 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 x ILED); VIN is the supply voltage; VLED is the total LED forward voltage; RSW is the switch resistance (0.55Ω nominal); VD is the diode forward voltage at the required load current.
Thermal Protection
The AL8862Q 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 LED Protection
The AL8862Q has default open LED protection. If the LEDs become open circuit, the AL8862Q will stop oscillating; the voltage at the SET pin will rise to VIN and the voltage at the SW pin will then fall to GND. No excessive voltages will be seen by the AL8862Q.
LED Short-Circuit Protection
If the LED string becomes shorted together (the anode of the top LED becomes shorted to the cathode of the bottom LED), the AL8862Q will continue to switch and the current through the AL8862Q’s internal switch will still be at the expected current - so no excessive heat will be generated within the AL8862Q. However, the duty cycle 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. AL8862Q 10 of 16 December 2019 Document number: DS42199 Rev. 3 - 2
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