AL8400 /AL8400Q
Application Information (cont.)
Bipolar Example – Choosing R B and C L
The driver is required to control 3 series connected LEDs at 150mA ±10% from a 12V ±5% supply. Each LED has a forward voltage of 3V
minimum and of 3.6V maximum.
From this information the minimum supply voltage is 11.4V and the maximum LED chain voltage is 10.8V. Rearranging equation 1 (page 7); the
minimum voltage drop across the bipolar transistor is determined to be:
V CE = V CC min ? V LED max ? V FB = 11 . 4 V ? 10 . 8 V ? 0 . 2 V = 0 . 4 V
We will use the DNLS320E bipolar transistor (Q2.)
R Bmax
The DNLS320E datasheet table states:
V CE(SAT)max = 0.1V at I C = 100mA, I B = 0.5mA
h FEmin = 500 @ I C = 100mA, V CE = 2V;
The datasheet graph (see left) shows a very slow variation at 100mA, so a value of 500 is considered appropriate.
Then I B max =
150mA
500
= 0.3mA
The minimum recommended I OUT for AL8400 is 0.3mA and the maximum V BE , according to the DNLS320E datasheet graph (Figure 6), is
approximately 0.8V at -55°C.
From these and equation 3, the maximum allowed bias resistor value is:
R B max =
V CC min ? V BE max ? V FB
I OUT min + I B max
=
=
11 . 4 ? 0 . 8 ? 0 . 2
0 . 0003 + 0 . 0003
= 17.3k ?
Figure 5 DNLS320E H FE vs. I C
Figure 6 DNLS320E V BE vs. I C
AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
7 of 13
www.diodes.com
August 2012
? Diodes Incorporated
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