AAT2842 Analogic Corporation, AAT2842 Datasheet - Page 14

• Manufacturer Part Number: AAT2842
• Manufacturer: Analogic Corporation

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:
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:
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:
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:
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
the flash section as an example:
14
I
LED(LO)
I
I
LED
LED(LO)
I
LED(HI)
I
LED(HI)
= 60 · 10
= 12 · 10
= 60 · 10
= 60 · 10
= 12 · 10
IO
3
⎛
⎝
3
is replaced with V
R
3
⎛
⎝
R
1
⎛
⎝
3
0.7
3
R
// R
1
⎛
⎝
0.7
⎛
⎝
1
R
// R
0.7
R
// R
1
0.6
2
1
0.6
// R
2
// R
-
2
-
V
2
-
R
⎞
⎠
DAC
V
2
R
⎞
⎠
V
2
R
IO
2
⎞
⎠
IO
⎞
⎠
2
⎞
⎠
DAC
Control and Dual LDO for Backlight and Flash
. Using
High-Current Charge Pump with S
Device Power Efficiency
The AAT2842
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.
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
approximated by:
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:
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
F
, the power conversion efficiency can be
η =
η =
V
V
η =
V
V
IN
power
F
IN
F
· 1.5I
· I
· I
· I
P
LED
LED
P
IN
OUT
IN
IN
≈
conversion
≈
V
V
IN
1.5 · V
F
V
F
IN
2842.2006.10.1.0
2
Cwire™
efficiency