Diodes AP8802H - Manual (Page 11)

AP8802H
60V 1A LED STEP-DOWN CONVERTER
AP8802H
Document number: DS32227 Rev. 6 - 2
11 of 14
www.diodes.com
May 2012
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PRELIMINARY
NEW PROD
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T
Applications Information (cont.)
Fault Condition Operation
The AP8802H has by default open LED protection. If the LEDs should become open circuit the AP8802H will stop oscillating; the SET pin
will rise to V
IN
and the SW pin will then fall to GND. No excessive voltages will be seen by the AP8802H.
If the LEDs should become shorted together the AP8802H will continue to switch however the duty cycle at which it will operate will change
dramatically and the switching frequency will most likely decrease. The on-time of the internal power MOSFET switch will be significantly
reduced because almost all of the input voltage is now developed across the inductor. The off-time will be significantly increased because
the reverse voltage across the inductor is now just the Schottky diode voltage (See Figure 20) causing a much slower decay in inductor
current. During this condition the inductor current will remain within its controlled levels and so no excessive heat will be generated within
the AP8802H.
Fig. 20 Switching Characteristics (normal open to short LED chain)