AAT2845
Four-Channel Backlight Driver with Dual LDOs
To select appropriate resistor values, first choose
R1 such that the feedback network bias current is
reasonable. Then, according to the desired VOUT
calculate R2 according to the equation below. An
example calculation follows.
This makes quantifying output power a difficult task
when taken in the context of comparing to other
white LED driver circuit topologies. A better way to
quantify total device efficiency is to observe the total
input power to the device for a given LED current
drive level. The best white LED driver for a given
application should be based on trade-offs of size,
external component count, reliability, operating
range, and total energy usage...not just % efficiency.
,
R1 is chosen to be 120K, resulting in a small feed-
back network bias current of 1.2V/120K = 10µA.
The desired output voltage is 1.8V. From this infor-
mation, R2 is calculated from the equation below.
The AAT2845 efficiency may be quantified under
very specific conditions and is dependent upon the
input voltage versus the output voltage seen
across the loads applied to outputs D1 through D4
for a given constant current setting. Depending on
the combination of VIN and voltages sensed at the
current sinks, the device will operate in load switch
mode. When any one of the voltages sensed at the
current sinks nears dropout, the device will operate
in 1.5X or 2X charge pump mode. Each of these
modes will yield different efficiency values. Refer
to the following two sections for explanations for
each operational mode.
R1(VOUT - 1.2)
R2 =
1.2
The result is R2 = 60K. Since 60K is not a standard
1% value, 60.4K is selected. From this example
calculation, for VOUT = 1.8V, use R1 = 120K and R2
= 60.4K. A table of example output voltages and
corresponding resistor values is provided below.
R2 Standard 1% Values (R1 = 120K)
VOUT (V)
R2 (Ω)
2.8
2.5
2.0
1.8
1.5
160K
130K
79.6K
60.4K
30.1K
1X Mode Efficiency
The AAT2845 1X mode is operational at all times
and functions alone to enhance device power con-
version efficiency when VIN is higher than the volt-
age across the load. When in 1X mode, voltage
conversion efficiency is defined as output power
divided by input power:
Table 2: Example Output Voltages and
Corresponding Resistor Values
Power Efficiency and Device Evaluation
POUT
PIN
η =
The charge pump efficiency discussion in the fol-
lowing sections accounts only for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT2845, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
The expression to define the ideal efficiency (η)
can be rewritten as:
POUT VOUT · IOUT VOUT
Since the AAT2845 outputs are pure constant cur-
rent sinks and typically drive individual loads, it is
difficult to measure the output voltage for a given
output (D1 to D4) to derive an overall output power
measurement. For any given application, white
LED forward voltage levels can differ, yet the out-
put drive current will be maintained as a constant.
η =
=
=
PIN
VIN · IOUT
VIN
-or-
V
⎛
⎞
⎠
OUT
η(%) = 100
VIN
⎝
2845.2006.12.1.0
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ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
