AAT2833/34
240mA Total Display Solution
for Portable Devices
Applications Information
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
PIN
VIN · IIN
Adjusting the Maximum Backlight LED
Current Level
4 · VLEDX · ILEDX
VIN · IIN
η =
η =
; x = 1, 2, 3 or 4 and IIN = 4 · ILEDX
The value of RBSET determines the maximum LED
current level for the backlight section. In the typical
application, selecting RBSET = 280kΩ results in a
maximum 30mA/channel LED current level.
VLEDX
VIN
Due to the very low 1X mode quiescent current, the
input current nearly equals the total output current
delivered to the LEDs. Further, the low resistance
bypass switch introduces a negligible voltage drop
from input to output.
The relationship between RBSET and the maximum
backlight current is illustrated graphically in Figure 4.
The AAT2833/34's LED current control circuits have
been optimized for full-scale current settings higher
than 15mA, RBSET > 550kΩ.
The AAT2833/34 further maintains optimized per-
formance and efficiency by detecting when the
input voltage is not sufficient to sustain the LED
bias current. The device automatically switches to
1.5X mode when the input voltage drops too low in
relation to the LED forward voltages.
45
40
35
30
25
20
15
10
5
In 1.5X mode, the output voltage can be boosted to
1.5X the input voltage. The 1.5X conversion ratio
introduces a corresponding 0.5X increase in input
current. For ideal conversion, the 1.5X mode effi-
ciency is given by:
0
100
200
300
400
500
600
700
800
900
1000
RBSET (kΩ)
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
Figure 4: Maximum Backlight LED
PIN
VIN · IIN
Current vs RBSET
.
4 · VLEDX · ILEDX
VIN · IIN
η =
η =
; x = 1, 2, 3 or 4 and IIN = 1.5(4 · ILEDX)
Device Power Efficiency
VLEDX
1.5VIN
The AAT2833/34's power conversion efficiency
depends on the charge pump mode of operation.
By definition, device efficiency is expressed as the
output power delivered to the LEDs divided by the
total input power consumed.
Similarly, when the input falls further, such that
1.5X mode can no longer sustain the LED bias cur-
rents, the AAT2833/34 will automatically switch to
2X mode. In 2X mode, the output voltage can be
boosted to 2X the input voltage. The 2X conversion
ratio introduces a corresponding 1X increase in
input current. For ideal conversion, the 2X mode
efficiency is given by:
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
PIN
VIN · IIN
When the input voltage is sufficiently greater than
the LED forward voltages, the device optimizes
efficiency by operating in 1X mode. In 1X mode,
the device is working as a bypass switch and pass-
ing the input supply directly to the output. By sim-
plifying the conditions such that the LEDs have uni-
form VF, the power conversion efficiency can be
approximated by:
PLEDs VLED1 · ILED1 + ... + VLED4 · ILED4
=
η =
PIN
VIN · IIN
4 · VLEDX · ILEDX
VIN · IIN
η =
η =
; x = 1, 2, 3 or 4 and IIN = 2(4 · ILEDX)
VLEDX
2VIN
2833.2007.09.1.0
15
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
