Al8807q, Application information – Diodes AL8807Q User Manual
Page 13
AL8807Q
Document number: DS36904 Rev. 1 - 2
13 of 16
March 2014
© Diodes Incorporated
AL8807Q
Application Information
(cont.)
Inductor Selection
(cont.)
The following equations can be used as a guide, with reference to Figure 1 - Operating waveforms.
Switch ‘On’ time Switch ‘Off’ time
R
r
R
x
I
V
V
I
L
t
SW
L
S
AVG
LED
IN
ON
r
R
x
I
V
V
I
L
t
L
S
AVG
D
LED
OFF
Where:
L is the coil inductance (H)
r
L
is the coil resistance (Ω)R
S
is the current sense resistance (Ω)
I
avg
is the required LED current (A)
∆I is the coil peak-peak ripple current (A) {Internally set to 0.3 x Iavg}
V
IN
is the supply voltage (V)
V
LED
is the total LED forward voltage (V)
R
SW
is the switch resistance (Ω) {=0.5Ω nominal}
V
D
is the diode forward voltage at the required load current (V)
Thermal 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
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
) /
JA
T
J(MAX)
is the maximum operating junction temperature,
T
A
is the ambient temperature, and
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 AL8807Q’s
junction to ambient thermal resistance,
JA
and device power dissipation. To support high LED drive at higher ambient temperatures the AL8807Q
has been packaged in thermally enhanced MSOP-8EP package.
JA
, is layout dependent and the AL8806Q’s
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 35, shows the power derating of the AL8807QMP 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 AL8807Q will
decrease.
Figure 35 Derating Curve for Different PCB
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)
P
o
w
e
r di
ssi
pat
ion
(
m
W
)