Diodes AL8806Q - Manual (Page 10)

AL8806Q
Document number: DS36905 Rev. 1 - 2
10 of 13
www.diodes.com
April 2014
© Diodes Incorporated
AL8806Q
Application Information
(cont.)
Reducing Output Ripple
Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown already in the circuit
schematic.
A value of 1μF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor
values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of
LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version
without altering the mean current value.
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AL8806Q applications. X5R and X7R types are recommended because they retain their
capacitance over wider voltage and temperature ranges than other types such as Z5U.
A 2.2μF input capacitor is sufficient for most intended applications of AL8806Q; however a 4.7μF input capacitor is suggested for input voltages
approaching 30V.
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the
maximum operating voltage and temperature. The Schottky diode also provides better efficiency than silicon PN diodes, due to a combination of
lower forward voltage and reduced recovery time.
It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output
load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than the operating voltage to ensure safe operation
during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating the power
loss through the diode.
Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8806Q applications.
Thermal and Layout Considerations
For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation
depends on several factors: the thermal resistance of the IC package
θ
JA
, PCB layout, airflow surrounding the IC, and difference between junction
and ambient temperature.
The maximum power dissipation can be calculated using the following formula:
P
D(MAX)
= (T
J(MAX)
− T
A
) / θ
JA where
T
J(MAX)
is the maximum operating junction temperature,
T
A
is the ambient temperature, and
θ
JA
is the junction to ambient thermal resistance.
The recommended maximum operating junction temperature, T
J
, is 125°C and so maximum ambient temperature is determined by the AL8806Q’s
junction to ambient thermal resistance, θ
JA
. To support high LED drive at higher ambient temperatures the AL8806Q has been packaged in
thermally enhanced MSOP-8EP package.
θ
JA
, is layout dependent and the AL8806Q’s θ
JA
in MSOP-8EP on a
51 x 51mm double layer PCB with 2oz copper standing in still air is
approximately 69°C/W.
Therefore the maximum power dissipation at T
A
= 25°C is:
( )
W 45 . 1
W / C 69
C 25 C 125
P
) MAX ( D
=
°
° − °
=
Figure 5, shows the power derating of the AL8806Q on an FR4
51x51mm PCB with 2oz copper standing in still air.
As the ambient temperature increases and/or the PCB area reduces
the maximum allowable power dissipated by the AL8806Q will
decrease.
Figure 5 Derating Curve
MSOP-8EP
0
200
400
600
800
1000
1200
1400
1600
-40 -25 -10 5 20 35 50 65 80 95 110 125
Ambient temperature (°C)
Pow
e
r di
s
s
ip
a
ti
o
n
(
m
W
)