Diodes AL8807B - Manual (Page 16)

AL88070B
Document number: DS36191 Rev. 1 - 2
16 of 18
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March 2014
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AL8807B
Applications Information
(cont.)
Fault Condition Operation
Open Circuit LEDs
The AL8807B has by default open LED protection. If the LEDs should become open circuit the AL8807B 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 by the AL8807B.
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 AL8807B will
continue to switch and the current through the AL8807B’s internal switch will still be at the expected current - so no excessive heat will be
generated within the AL8807B. However, the duty cycle at which it operates will change dramatically and the switching frequency will most likely
decrease. See Figure 43 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
43) causing a much slower decay in inductor current.
Figure 43 Switching Characteristics (normal operation to LED chain shorted out)
High Temperature Operation and Protection
The AL8807B is a high efficiency switching LED driver capable of operating junction temperatures up to +125°C. This allows it operate with
ambient temperature in excess of +100°C given the correct thermal impedance to free air. If a fault should occur that leads to increased ambient
temperatures and hence junction temperature then the Over-Temperature Protection (OTP) of the AL8807B will cut-in, turning the output of the
AL8807B off. This will allow the junction temperature of the AL8807B to cool down and potentially giving an opportunity for the fault to clear itself.
The OTP shutdown junction temperature of the AL8807B is approximately +155°C with a hysteresis of +55°C. This means that the AL8807B will
never switch-off with a junction temperature below +125°C allowing the designer to design the system thermally to fully utilize the wide operating
junction temperature of the AL8807B.