Application information – Diodes AL5802 User Manual
Page 7
AL5802
Document number: DS35516 Rev. 9 - 2
7 of 11
March 2014
© Diodes Incorporated
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AL5802
Application Information
Fig. 14 Typical Application Circuit for
Linear Mode Current Sink LED Driver
Fig. 15 Application Circuit for Increasing LED Current
The AL5802 has been designed for driving low current LEDs with typical
LED current of 20mA to 100mA. It provides a cost effective way for
driving low current LEDs compared with more complex switching
regulator solutions. Furthermore, it reduces the PCB board area of the
solution as there is no need for external components like inductors,
capacitors and switching diodes.
Figure 14 shows a typical application circuit diagram for driving an LED
or string of LEDs. The NPN transistor Q1 measures the LED current by
sensing the voltage across an external resistor R
EXT
. Q1 uses its V
BE
as
reference to set the voltage across R
EXT
and controls the base current
into Q2. Q2 operates in linear mode to regulate the LED current. The
LED current is
I
LED
= V
BE(Q1)
/ R
EXT
From this, for any required LED current the necessary external resistor
R
EXT
can be calculated from
R
EXT
= V
BE(Q1)
/ I
LED
Two or more AL5802 can be connected in parallel to construct higher
current LED strings as shown in Figure 15.
Consideration of the expected linear mode power dissipation must be
factored into the design, with respect to the AL5802's thermal
resistance. The maximum voltage across the device can be calculated
by taking the maximum supply voltage less the voltage across the LED
string.
V
CE(Q2)
= V
CC
– V
LED
– V
BE(Q1)
P
D
= V
CE(Q2)
* I
LED
+ ( V
CC
– V
BE(Q2)
– V
BE(Q1)
)
2
/ R
1
As the output current of AL5802 increases, it is necessary to provide
appropriate thermal relief to the device. The power dissipation
supported by the device is dependent upon the PCB board material, the
copper area and the ambient temperature. The maximum dissipation
the device can handle is given by:
P
D
= ( T
J(MAX)
- T
A
) /
R
θJA
Refer to the thermal characteristic graphs on page 4 for selecting the
appropriate PCB copper area.