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AL8400 /AL8400Q
Application Information (cont.)
MOSFET Example Choosing R B and C L (cont.)
R Bmax
The minimum recommended I OUT for AL8400 is 0.3mA.
The maximum V GS is not stated explicitly, but from the datasheet graphs (Figures 8 and 9) it is expected to be approximately 3.8V at -50°C.
R B max =
V CC min ? V GS max ? V FB
I OUT min
=
=
11 . 4 V ? 3 . 8 V ? 0 . 2 V
0 . 3 mA
= 24.7k ?
To ensure that the output capability of the AL8400 is not exceeded at maximum V IN and minimum V GS these values should be substituted back
into the R B equation to determine the minimum allowable value for R B .
R Bmin
The maximum recommended I OUT for the AL8400 is 15mA. The minimum V GS is about 1V and assuming V CCmax = 8.4V:
R B min =
=
V CC max ? V GS min ? V FB
I OUT max
12 . 6 V ? 1V ? 0 . 2 V
15 mA
=
= 480 ?
this is less than 12k ? and so the AL8400 output current is within its ratings.
Figure 8 Typical Transfer Characteristics
Figure 9 Normalised Curves and Temperature
Assuming V GS ~ 3V and choosing an R B = 8.2k ? satisfies the requirements for the AL8400 conformance and sets approximately 1mA in the OUT
pin. The required compensation capacitor can therefore be calculated from:
C L ≈
2 ms
8 . 2 k Ω
≈ 0 . 243 μ F
220nF
The value of R SET is V REF /I LED
R SET
= 0.2/0.2 = 1 ?
Finally, the maximum power dissipation of the external MOSFET is:
P TOT
= I LED x V DSMAX
= I LED x (V CCmax – V LEDMIN – V FB )
= 0.2 x( 12.6 – 9 -0.2)
= 0.68W
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 x θ JA
= 60 + 0.68 x 62.5
= +102.5 ? C
AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
10 of 13
www.diodes.com
August 2012
? Diodes Incorporated
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