al8400 Diodes, Inc., al8400 Datasheet - Page 6

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

Application Information
Bipolar transistor as the pass element
There are two important equations for the circuit:
LED circuit path:
Control drive circuit path
The maximum total LED voltage plus the reference voltage determines the minimum supply voltage. Substituting into
equation 1 yields:
For a bipolar transistor the voltage (V
the AL8400. So rearranging equation 2 yields the boundaries for allowable R
where I
current gain of the transistor.
Finally, the bipolar selection is also influenced by the maximum power dissipation
Since this determines the package choice (θ
Bipolar Example
The driver is required to control 2 series connected LEDs at 150mA ±10%, each having a forward voltage of 3V minimum and
3.6V maximum. Hence the minimum operating supply voltage is 3.6*2 + 0.2 = 7.4V. The actual supply voltage given is
8V ±5%, i.e. 7.6V minimum. We will use the DNLS320E bipolar transistor (Q2).
The DNLS320E datasheet shows:
The datasheet graph shows a very slow variation at his current, so a value of 500 is appropriate.
Then
The minimum recommended I
is approximately 0.8V at -55°C.
From these the maximum allowed bias resistor value is:
To ensure that the output capability of the AL8400 is not exceeded at maximum V
values should be substituted back into the R
h
AL8400
Document number: DS35115 Rev. 2 - 2
I
R
B
FEmax
max
B
max
1 ..... V
2 ..... V
=
I
B
is about 1200 @ I
=
Bmax
max
h
P
T
h
I
R
I
V
V
FE
B
LED
FEmin
CC
CC
TOT
J
B
CC
CC
min
min
= T
is the maximum transistor base current
=
max
min
min
I
= I
150
500
OUT
=
A
is 500 @ I
where h
= (V
= (V
−
=
LED
+ P
1200
=
150
min
V
. 0
V
= 0.3mA
7
BE
LED
RB
* (V
TOT
0003
LED
6 .
+
max
= 0.125mA
+ V
I
−
B
FEmin
+ V
CC
• θ
C
+
max
0
C
−
+
= 100mA, and a temperature of 85˚C which results in:
BE
8 .
JA
– V
V
= 100mA, 400 @ I
CE
. 0
V
CEsat
is the minimum DC
−
+ V
REF
0003
+ V
LED
OUT
0
2 .
REF
REF
+
– V
for AL8400 is 0.3mA and the maximum V
(Continued)
)
= 11kΩ
V
)
REF
REF
RB
) across bias resistor R
) = I
LED
JA
B
C
equation to determine the minimum allowable value for R
) in order to keep the junction temperature below the maximum value allowed.
(Continued)
= 2A,
* V
CE
www.diodes.com
where I
the transistor.
I
B
min
6 of 12
=
Bmin
h
B
I
FE
R
consists of the base current of Q2 and the output current of
LED
B
max
LINEAR LED DRIVER-CONTROLLER with
is the minimum transistor base current
min
where h
=
V
200mV CURRENT SENSE VOLTAGE
CC
BE
B
max
, according to the DNLS320E datasheet graph,
values:
I
OUT
FEmax
−
max
IN
V
is the maximum DC current gain of
, maximum h
BE
+
min
I
B
min
−
V
REF
FE
and minimum V
B
.
AL8400
© Diodes Incorporated
March 2011
BE
, these