AAT2845A Datasheet, PDF (13/19 Pages) Advanced Analogic Technologies – Four-Channel Backlight Driver with Dual LDOs

AAT2845A
Four-Channel Backlight Driver with Dual LDOs
To select appropriate resistor values, first choose
R9 such that the feedback network bias current is
reasonable. Then, according to the desired V_OUT,
calculate R8 according to the equation below. An
example calculation follows.
R9 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, R8 is calculated from the equation below.
R8 =
R9(V_OUT - 1.2V)
1.2V
The result is R8 = 60K. Since 60K is not a standard
1% value, 60.4K is selected. From this example
calculation, for V_OUT = 1.8V, use R9 = 120K and R8
= 60.4K. A table of example output voltages and
corresponding resistor values is provided below.
Standard 1% Values (R9, R11 = 120K)
V_OUT (V)
R8, R10 (Ω)
2.8
160K
2.5
130K
2.0
79.6K
1.8
60.4K
1.5
30.1K
Table 2: Example Output Voltages and
Corresponding Resistor Values
Power Efficiency and Device Evaluation
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 AAT2845A, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT2845A 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.
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.
The AAT2845A 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 V_IN 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.
1X Mode Efficiency
The AAT2845A 1X mode is operational at all times
and functions alone to enhance device power con-
version efficiency when V_IN 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:
η
=
P_OUT
P_IN
The expression to define the ideal efficiency (η)
can be rewritten as:
η = ^POUT = ^{VOUT · IOUT} = ^VOUT
P_IN
V_IN · I_OUT
V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT⎞
V_IN ⎠
2845A.2007.05.1.0
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