Ap8801, 500ma led step-down converter, Application information (cont.) – Diodes AP8801 User Manual
Page 10
AP8801
500mA LED STEP-DOWN CONVERTER
AP8801
Document number: DS31765 Rev. 7 - 2
10 of 14
June 2012
© Diodes Incorporated
Application Information (cont.)
Soft-Start
An external capacitor from the CTRL pin to ground will provide a soft-start delay, by increasing the time taken for the voltage on this pin to
rise to the turn-on threshold and by slowing down the rate of rise of the control voltage at the input of the comparator. Adding capacitance
increases this delay by approximately 200µs/nF. The graph below shows the variation of soft-start time for different values of capacitor.
0
20
40
60
80
100
120
CAPACITANCE (nf)
Figure. 19 Soft-Start Time vs. Capacitance
from ADJ Pin to Ground
16
14
0
-2
12
10
4
2
8
6
S
O
F
T
-S
T
A
R
T
T
IM
E
(
m
s
)
Thermal Considerations
The graph below in figure 20, gives details for power derating. This assumes the device to be on a FR-4 PCB, 2”x2”, 2oz copper, minimum
recommended pad layout on top layer and thermal vias to bottom layer ground plane standing in still air.
-50
-25
0
50
75
100
125
AMBIENT TEMPERATURE (°C)
Figure. 20 Maximum Power Dissipation
25
1.2
1.0
0.2
0
0.8
0.4
0.6
P
O
WE
R
DISS
IP
A
T
IO
N (
W
)
SO-8
MSOP-8