al8400 Diodes, Inc., al8400 Datasheet - Page 7

• Manufacturer Part Number: al8400
• Description: Linear Led Driver-controller With 200mv Current Sense Voltage
• Manufacturer: Diodes, Inc.

Application Information
Bipolar Example
The maximum recommended I
The minimum V
then the bias resistor value is:
The value of R
Finally, the maximum power dissipation of the external bipolar transistor is:
This determines the package choice (θ
allowed.
N-channel MOSFET as the pass element
Alternatively, an N-channel MOSFET may be used in the same configuration. The current in R
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 3.
The equations (1 and 2) for the bipolar transistor are transformed into:
LED circuit path:
Control drive circuit path
AL8400
Document number: DS35115 Rev. 2 - 2
1 ..... V
2 ..... V
R
P
T
R
TOT
J
CC
CC
SET
B
min
= (V
SET
BE
=
= 0.2/0.15 = 1.333Ω
= I
= I
= T
= T
= (V
LED
, according to the DNLS320E datasheet graph, is approximately 0.4V at 85°C and assuming V
. 0
is V
LED
LED
A
A
8
015
RB
(Continued)
+ P
+ 0.27*125 = T
4 .
+ V
* (V
REF
* V
+ V
−
+
TOT
DS
/I
CEMAX
0
CC
. 0
LED
GS
4 .
+ V
000125
• θ
– V
−
+ V
so:
OUT
0
JA
REF
LED_MIN
2 .
REF
)
for AL8400 is 15mA.
(Continued)
)
A
= 516Ω this is less than 11kΩ and so the AL8400 output current is within its ratings.
Figure 3. Application Circuit Using MOSFET
+ 33.75˚C
– V
JA
1.3Ω is practical.
) in order to keep the junction temperature of the bipolar below the maximum value
REF
) = 0.27W
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LINEAR LED DRIVER-CONTROLLER with
200mV CURRENT SENSE VOLTAGE
B
is then reduced compared to
AL8400
© Diodes Incorporated
CCmax
March 2011
= 8.4V,