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AL8400 /AL8400Q
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:
V CC min = V LED + V DSMIN + V FB
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 .
R B min =
V CC max ? V GS min ? V FB
I OUT max
8 .
R B max =
V CC min ? V GS max ? V FB
I OUT min
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:
I LED × R DS = V CC min ? V LED max ? V FB =
= 11 . 4 V ? 10 . 8 V ? 0 . 2 V = 0.4V
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.
AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
9 of 13
www.diodes.com
August 2012
? Diodes Incorporated
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