Diodes AL8400 /AL8400Q - Manual (Page 11)

AL8400/ AL8400Q
Document number: DS35115 Rev. 4 - 2
11 of 13
www.diodes.com
August 2012
© Diodes Incorporated
AL8400 /AL8400Q
Application Information
(cont.)
High Voltage Operation
The AL8400 also provides the capability to drive longer LED chains as the voltage across the LED chain is determined by the external switch.
The lower supply voltage for the AL8400 can be derived from the supply to the LED chain either by putting a series resistor to the AL8400’s V
CC
pin and putting a suitable zener diode from its V
CC
to GND Figure 10 or by tapping its V
CC
from the LED chain Figure 11.
Figure 10 High Voltage Operation with Zener Diode from VIN
Equations 1 and 2 (from page 7) now transform into:
LED Circuit Path:
1. V
IN
= (V
LED
+ V
CE
+ V
FB
)
Control Drive Circuit Path
2. V
CC
= (V
RB
+ V
BE
+ V
FB
)
When the supply voltage for the AL8400 is derived using a zener diode, care has to be taken in dimensioning the resistor R1. The current taken
through R1 from V
IN
has to be large enough to polarize the zener, bias the AL8400 supply current, AL8400 output current and the pass transistor
across all input voltage variations.
An alternative way of operating the AL8400 from rails greater than 18V is to take its power supply from the LED chain itself.
Figure 11 High Voltage Operation Tapping V
CC
from the LED String
When the supply voltage for the AL8400 is derived from the LED string, care has to be taken in dimensioning the resistor R
B
. The current spilled
from the LED chain can reduce the accuracy of the system and brightness matching between the LED.