AP8803

Document number: DS31970 Rev. 5 - 2

8 of 10

www.diodes.com

August 2012

© Diodes Incorporated

AP8803

Fault Condition Operation

The AP8803 has by default open LED protection. If the LEDs should become open circuit the AP8803 will stop oscillating; the SET pin will rise

If the LEDs should become shorted together the AP8803 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 7) 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 AP8803.

Figure 7 Switching Characteristics (normal open to short LED chain)

Thermal Considerations

The graph below in Figure 8, gives details for power dissipation derating. This assumes the device to be mounted on a 25 x 25mm PCB with

1oz copper standing in still air.

0

0.2

0.4

0.6

0.8

1

1.2

-50

-25

0

25

50

75

100

125

150

Ambient Temperature (°C)

Figure 8 Power Dissipation Derating Curve

The maximum power dissipation is affected by PCB area and the area of copper associated with the LX pin as well as other components on the

PCB generating heat – such as the inductor, capacitor or rectifiers.