AAT2842 Datasheet, PDF (14/21 Pages) Advanced Analogic Technologies – High-Current Charge Pump with S2Cwire™ Control and Dual LDO for Backlight and Flash

AAT2842
High-Current Charge Pump with S²Cwire™
Control and Dual LDO for Backlight and Flash
Using an additional resistor to connect the BSET
pin with a digital output provides a LO/HI control.
When the digital output is asserted high, the result-
ing brightness level for the backlighting section is
LO and the individual LED current levels are:
I_LED(LO) = 12 · 10³
⎛ 0.7
⎝ ^R1 ^{// R}2
-
V_IO⎞
R_2⎠
The same can be applied to the FSET pin. When
the digital output is asserted high, the resulting
brightness level for the flash section is LO and the
individual LED current levels are:
I_LED(LO) = 60 ·
10³
⎛ 0.7
⎝ ^R1 ^{// R}2
-
V_IO⎞
R_2⎠
When the digital output is asserted low, the result-
ing brightness level for the backlighting section is
HI and the individual LED current levels are:
I_LED(HI)
=
12
·
10³
⎛ 0.6 ⎞
⎝ ^R1 ^{// R}2⎠
The same can be applied to the FSET pin. When
the digital output is asserted low, the resulting
brightness level for the flash section is HI and the
individual LED current levels are:
I_LED(HI)
=
60
·
10³
⎛ 0.6 ⎞
⎝ ^R1 ^{// R}2⎠
Additionally, the output from a digital-to-analog
converter can be used with either SET pin to con-
trol the brightness level. The result is like the equa-
tions above, where V_IO is replaced with V_DAC. Using
the flash section as an example:
I_LED = 60 · 10³
⎛ 0.7
⎝ ^R1 ^{// R}2
-
V_DAC⎞
R_{2 âŽ} 
14
Device Power Efficiency
The AAT2842 power conversion efficiency
depends on the charge pump mode. By definition,
device efficiency is expressed as the output power
delivered to the LEDs divided by the total input
power consumed.
η
=
P_OUT
P_IN
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:
η=
V_F · I_LED
V_IN · I_IN
≈
V_F
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 negligible voltage drop
from input to output.
The AAT2842 further maintains optimized perform-
ance and efficiency by detecting when the input
voltage is not sufficient to sustain LED drive cur-
rent. The device automatically switches to 1.5X
mode when the input voltage drops too low in rela-
tion to the LED forward voltages.
In 1.5X mode, the output voltage can be boosted to
3/2 the input voltage. The 3/2 conversion ratio
introduces a corresponding 1/2 increase in input
current. For ideal conversion, the 1.5X mode effi-
ciency is given by:
η=
V_F · I_LED
V_IN · 1.5I_IN
≈
V_F
1.5 · V_IN
Similarly, when the input falls further, such that
1.5X mode can no longer sustain LED drive cur-
rent, the device will automatically switch to 2X
mode. In 2X mode, the output voltage can be
2842.2006.10.1.0