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AL8400 /AL8400Q
Application Information (cont.)
Bipolar Example – Choosing R B and C L (cont.)
R Bmin
To ensure that the output capability of the AL8400 is not exceeded at maximum V IN , maximum h FE and minimum V BE , these values should be
substituted back into the R B equation to determine the minimum allowable value for R B .
h FEmax is about 1200 @ I C = 100mA, and a temperature of +85°C (Figure 5) which results in:
I B min =
150
1200
= 0.125mA
The maximum recommended I OUT for AL8400 is 15mA.The minimum V BE , according to the DNLS320E datasheet graph (Figure 6), is
approximately 0.4V at 85°C and assuming V CCmax = 12.6V, then from equation 4 the bias resistor value is:
R B min =
V CC max ? V BE min ? V FB
I OUT max + I B min
=
=
8 . 4 ? 0 . 4 ? 0 . 2
0 . 015 + 0 . 000125
= 516 ? this is less than 17k ? and so the AL8400 output current is within its ratings.
C L
Choosing R B = 11k ? 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 ≈
2ms
11 k Ω
≈ 0 . 18 μ F
180nF
The value of R SET is V REF /I LED so:
R SET
= 0.2/0.15 = 1.333 ?
Choosing two 2.7 ? yields 1.35 ? giving an approximate 1.3% difference from target.
Finally, the maximum power dissipation of the external bipolar transistor is:
P TOT
= I LED x V CEMAX
= I LED x (V CC_max – V LED_MIN – V FB ) = 0.51W
This determines the package choice ( θ JA ) in order to keep the junction temperature of the bipolar transistor below the maximum value allowed. At
a maximum ambient temperature of +60°C the junction temperature becomes
T J
= T A + P TOT x θ JA
= 60 + 0.51 x 125 = +123.75°C
N-Channel MOSFET as the Pass Element
Alternatively, an N-channel MOSFET may be used in the same configuration. The current in R B is then reduced compared to 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 7.
Figure 7 Application Circuit Using MOSFET
AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
8 of 13
www.diodes.com
August 2012
? Diodes Incorporated
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