AAT2833 Datasheet, PDF (15/20 Pages) Advanced Analogic Technologies – 240mA Total Display Solution for Portable Devices

Applications Information
Adjusting the Maximum Backlight LED
Current Level
The value of R_BSET determines the maximum LED
current level for the backlight section. In the typical
application, selecting R_BSET = 280kΩ results in a
maximum 30mA/channel LED current level.
The relationship between R_BSET 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, R_BSET > 550kΩ.
45
40
35
30
25
20
15
10
5
0
100 200 300 400 500 600 700 800 900 1000
R_BSET (kΩ)
Figure 4: Maximum Backlight LED
Current vs R_BSET.
Device Power Efficiency
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.
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ...
V_IN ·
+ V_LED4
I_IN
·
I_LED4
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 V_F, the power conversion efficiency can be
approximated by:
2833.2007.09.1.0
AAT2833/34
240mA Total Display Solution
for Portable Devices
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ...
V_IN ·
+ V_LED4
I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
4
·
I_LEDX
η
=
V_LEDX
V_IN
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 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.
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:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ...
V_IN ·
+ V_LED4
I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
1.5(4
·
I_LEDX)
η
=
V_LEDX
1.5V_IN
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:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ...
V_IN ·
+ V_LED4
I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
2(4
·
I_LEDX)
η=
V_LEDX
2V_IN
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