PRODUCT DATASHEET
AAT3159
ChargePumpTM
High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications
Applications Information
45
40
35
30
25
20
15
10
5
LED Selection
Although the AAT3159 is specifically intended for driving
white LEDs, the device can also be used to drive most
types of LEDs with forward voltage specifications rang-
ing from 2.0V to 4.7V. LED applications may include
main and sub-LCD display backlighting, camera photo-
flash applications, color (RGB) LEDs, infrared (IR) diodes
for remotes, and other loads benefiting from a con-
trolled output current generated from a varying input
voltage. Since the D1 to D4 input current sinks are
matched with negligible voltage dependence, the LED
brightness will be matched regardless of the specific LED
forward voltage (VF) levels. In some instances (e.g., in
high-luminous-output applications such as photo flash),
it may be necessary to drive high-VF type LEDs. The low
dropout current sinks in the AAT3159 make it capable of
driving LEDs with forward voltages as high as 4.7V at
full current from an input supply as low as 3.0V. Outputs
can be paralleled to drive high-current LEDs without
complication.
0
10 15 20 25 30 35 40 45 50 55 60 65 70 75
RSET (kΩ)
Figure 1: Maximum LED Current vs. RSET
.
AAT3159
HI/LO
or
VDAC
R2
SET
R1
Determining the Maximum LED Current
Level
The value of RSET determines the maximum LED current
level. In the typical application, selecting RSET = 26.1kΩ
results in 20mA/channel LED current. From this refer-
ence point, the RSET value required for other current
levels can be calculated as:
Figure 2: SET Pin Configuration Using
Two Resistors.
When the digital output is asserted high, the resulting
brightness level is LO and the individual LED currents
are:
20mA · 26.1kΩ
ILED(MAX)
RSET
=
⎛ 0.6V
VIO⎞
-
A visual representation of the maximum LED current per
channel versus RSET value is shown in Figure 1. Since the
AAT3159’s LED current control circuits were optimzed for
full-scale current settings higher than 15mA, RSET values
smaller than or equal to 33.2kΩ are recommended.
ILED(LO) = ISET
⎝R1 // R2 R2 ⎠
When the digital output is asserted low, the resulting
brightness level is HI and the individual LED currents
are:
Brightness Control Using the SET Pin
⎛ 0.6V ⎞
⎝R1 // R2⎠
ILED(HI) = ISET
Additional methods of brightness control can be achieved
with the SET pin. For example, using an additional resis-
tor to connect the SET pin with a digital output provides
a HI/LO control. Figure 2 illustrates a configuration of
the SET pin utilizing two resistors.
w w w . a n a l o g i c t e c h . c o m
10
3159.2007.10.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
