AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
9 of 13
www.diodes.com
August 2012
© Diodes Incorporated
AL8400 /
L8400Q
Application Information (cont.)
N-Channel MOSFET as the Pass Element (cont.)
The equations (1 and 2) for the bipolar transistor are transformed into:
LED circuit path:
5. V
CC
= (V
LED
+ V
DS
+ V
FB
) where VFB is approximately the internal reference voltage of 200mV.
Control drive circuit path
6. V
CC
= (V
RB
+ V
GS
+ V
FB
)
The maximum total LED voltage plus the reference voltage determines the minimum supply voltage. Substituting into equation 5 yields:
FBDSMINLEDminCC
VVVV ++=
The MOSFET DC gate current is negligible, so the bias resistor R
B
has only to provide the minimum output current of the AL8400. So
rearranging equation 6 yields the boundaries for allowable R
B
values:
7.
maxOUT
FBminGSmaxCC
minB
I
VVV
R
−−
=
8.
minOUT
FBmaxGSminCC
maxB
I
VVV
R
−−
=
Where I
OUTmax
is the AL8400 maximum output current Where I
OUTmin
is the AL8400 minimum output current
Once the value of R
B
has been determined, somewhere between R
Bmax
and R
Bmin
– trying to get I
OUT
close to 1mA for all variations, the value for
compensation capacitor, C
L
, should be calculated.
The MOSFET selection is also influenced by the maximum power dissipation
P
TOT
= I
LED
* (V
CC
– V
LED
– V
FB
) = I
LED
* V
DS
Since this determines the package choice (θ
JA
) in order to keep the junction temperature below the maximum value allowed.
T
J
= T
A
+ P
TOT
• θ
JA
where
T
J(MAX)
is the maximum operating junction temperature,
T
A
is the ambient temperature,
θ
JA
is the junction to ambient thermal resistance.
Low Supply Voltages and MOSFET as Pass Element
When driving a single LED at low supply voltages, a low threshold MOSFET or high gain NPN bipolar transistor should be used as the LED driving
pass transistor.
This is because a standard threshold voltage MOSFET might not have enough Gate-Source voltage to ensure that it is sufficiently enhanced to
regulate the LED current.
MOSFET Example Choosing R
B
and C
L
The driver is required to control 3 series connected LEDs at 200mA ±10% from an 12V ±5% supply. Each LED has a forward voltage of 3V
minimum and of 3.6V maximum.
Therefore the minimum supply voltage is 11.4V and the maximum LED chain voltage is 10.8V.
Rearranging equation 5 (page 9); the minimum voltage drop across the MOSFET is required to be:
=−−=×
FBmaxLEDminCCDSLED
VVVRI
V2.0V8.10V4.11
= 0.4V Æ R
DS(ON)
2
We will use the DMN6068SE N-channel MOSFET (Q2) with a maximum R
DS(ON)
of 100m at V
GS
= 4.5V.
