al8400 Diodes, Inc., al8400 Datasheet - Page 8

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

Application Information
N-channel MOSFET as the pass element
The maximum total LED voltage plus the reference voltage determines the minimum supply voltage. Substituting into
equation 3 yields:
The MOSFET DC gate current is negligible, so the bias resistor R
AL8400. So rearranging equation 4 yields the boundaries for allowable R
Where I
Note that in the case of a single LED, the MOSFET gate-source voltage may be too high for operation over the desired supply
voltage range. If the gate source voltage at the operating current is V
V
15mA.
The supply voltage is usually the LED voltage plus a margin for transistor saturation voltage, plus V
the voltage across R
V
Finally, the MOSFET selection is also influenced by the maximum power dissipation
Since this determines the package choice (θ
MOSFET Example
The driver is required to control 2 series connected LEDs at 150mA ±10%, each having a forward voltage of 3V minimum and
of 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 DMN6068SE N-channel MOSFET (Q2),
The minimum recommended I
The maximum V
(Here we have used the graphs of Typical Transfer and Normalized V
So
To ensure that the output capability of the AL8400 is not exceeded at maximum V
substituted back into the R
The maximum recommended I
The value of R
AL8400
Document number: DS35115 Rev. 2 - 2
RBmin
GS
is too high. Then either a MOSFET with lower V
is determined by the operating voltage range. At the top of the range, the current is required to be not greater than
OUTmax
R
V
P
T
R
V
R
R
RBmin
TOT
J
SET
Bmin
CC
B
Bmax
= T
min
min
= I
is the AL8400 maximum output current
SET
=
A
+ V
= (7.6 – 3.8 - 0.2) / 0.0003 = 12kΩ
=
= 0.2/0.15 = 1.333Ω
LED
+ P
GS
=
8
V
is V
4 .
V
GSmax
CC
is not stated explicitly, but from the datasheet graphs it is expected to be approximately 3.8V at -50°C.
* (V
TOT
. 0
LED
B
−
max
REF
015
plus V
1
CC
• θ
−
+
+ V
/I
0
−
I
LED
V
JA
– V
B
OUT
2 .
V
DSMIN
REF
equation to determine the minimum allowable value for R
REF
GS
LED
OUT
max
= 480Ω
OUT
min
< V
(0.2V). Therefore the use of the MOSFET may not be practical for driving a single LED if the
– V
for AL8400 is 0.3mA.
+
for the AL8400 is 15mA. The minimum V
(Continued)
−
CC
V
V
REF
REF
where V
REF
1.3Ω is practical.
) = I
LED
JA
this is less than 12kΩ and so the AL8400 output current is within its ratings.
) in order to keep the junction temperature below the maximum value allowed.
RBmin
* V
(Continued)
DS
is the minimum voltage drop across R
GS
www.diodes.com
must be selected, or a bipolar NPN device must be used.
8 of 12
Where I
R
B
max
LINEAR LED DRIVER-CONTROLLER with
B
GSMAX
=
GS(th)
OUTmin
has only to provide the minimum output current of the
V
B
CC
, we must have:
versus temperature.)
200mV CURRENT SENSE VOLTAGE
values:
min
GS
is the AL8400 minimum output current
is about 1V and assuming V
−
I
OUT
V
IN
GS
min
and minimum V
max
B
.
−
B
V
.
REF
GS
REF
these values should be
. The bias amounts to
CCmax
AL8400
© Diodes Incorporated
= 8.4V:
March 2011