Ap8800, Application information – Diodes AP8800 User Manual
Page 8
AP8800
Document number: DS31764 Rev. 7 - 2
8 of 13
August 2012
© Diodes Incorporated
AP8800
Application Information
(cont.)
Fault conditions
The AP8800 is inherently protected against open-LED conditions. If one LED becomes open circuit the device automatically stops switching and
will only retart if the open-LED fault is removed.
If one or more LEDs should become shorted together then the switching frequency and duty cycle will change. If one or more LEDs get shorted
together, the ramp-up time of LED current will become shorter due to there being a larger voltage across the inductor. However, the ramp-down
time of the LED current will increase due to the voltage across the inductor becoming smaller.
Figure 7 below shows the AP8800 driving 3 LEDs when all 3 LEDs become shorted together. Due to the large voltage change across the
inductor during both LED current ramp-up and ramp-down we see a large difference in switching frequency.
Figure 7 LED Short Fault Condition
Thermal Considerations
The graph below in Figure 8, gives details for power derating. This assumes the device to be mounted on a 25 x 25mm PCB with 1oz copper
standing in still air.
AMBIENT TEMPERATURE (°C)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0
25
50
75
100
125
150
DFN3030-10
SO-8
MSOP-8
PO
W
E
R
(W
)
Figure 8 Power Dissipation Derating Curve