Application information – Diodes AL8400 /AL8400Q User Manual
Page 8
AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
8 of 13
August 2012
© Diodes Incorporated
AL8400 /AL8400Q
Application Information
(cont.)
Bipolar Example – Choosing R
B
and C
L
(cont.)
R
Bmin
To ensure that the output capability of the AL8400 is not exceeded at maximum V
IN
, maximum h
FE
and minimum V
BE
, these values should be
substituted back into the R
B
equation to determine the minimum allowable value for R
B
.
h
FEmax
is about 1200 @ I
C
= 100mA, and a temperature of +85°C (Figure 5) which results in:
1200
150
I
min
B
=
= 0.125mA
The maximum recommended I
OUT
for AL8400 is 15mA.The minimum V
BE
, according to the DNLS320E datasheet graph (Figure 6), is
approximately 0.4V at 85°C and assuming V
CCmax
= 12.6V, then from equation 4 the bias resistor value is:
min
B
max
OUT
FB
min
BE
max
CC
min
B
I
I
V
V
V
R
+
−
−
=
=
000125
.
0
015
.
0
2
.
0
4
.
0
4
.
8
+
−
−
=
= 516
Ω this is less than 17kΩ and so the AL8400 output current is within its ratings.
C
L
Choosing R
B
= 11k
Ω satisfies the requirements for the AL8400 conformance and sets approximately 1mA in the OUT pin. The required
compensation capacitor can therefore be calculated from:
F
18
.
0
k
11
ms
2
C
L
μ
≈
Ω
≈
Æ 180nF
The value of R
SET
is V
REF
/I
LED
so:
R
SET
= 0.2/0.15 = 1.333
Ω Î Choosing two 2.7Ω yields 1.35Ω giving an approximate 1.3% difference from target.
Finally, the maximum power dissipation of the external bipolar transistor is:
P
TOT
= I
LED
x V
CEMAX
=
I
LED
x (V
CC_max
– V
LED_MIN
– V
FB
) = 0.51W
This determines the package choice (
θ
JA
) in order to keep the junction temperature of the bipolar transistor below the maximum value allowed. At
a maximum ambient temperature of +60°C the junction temperature becomes
T
J
= T
A
+ P
TOT
x
θ
JA
= 60 + 0.51 x 125 = +123.75°C
N-Channel MOSFET as the Pass Element
Alternatively, an N-channel MOSFET may be used in the same configuration. The current in R
B
is then reduced compared to the case in which
the bipolar transistor is used. For LED currents up to about 400mA a suitable MOSFET is DMN6068SE in the SOT223 package. The design
procedure is as follows, referring to Figure 7.
Figure 7 Application Circuit Using MOSFET