AAT2846_08 Datasheet, PDF (15/22 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight and Flash Applications

PRODUCT DATASHEET
AAT2846
ChargePump^TM High Current Charge Pump with Dual LDO for Backlight and Flash Applications
Applications Information
LDO Output Voltage Programming
The output voltages for LDOA and LDOB are programmed
by an external resistor divider network. As shown below,
the selection of R₁ and R₁ is a straight forward matter.
R₁ is chosen by considering the tradeoff between the
feedback network bias current and resistor value. Higher
resistor values allow stray capacitance to become a
larger factor in circuit performance whereas lower resis-
tor values increase bias current and decrease efficiency.
OUT(A/B)
R2(A/B)
VOUT(A/B)
FB(A/B)
V_REF(A/B) = 1.2V
R1(A/B)
To select appropriate resistor values, first choose R₁ such
that the feedback network bias current is less than 10µA.
Then, according to the desired V_OUT, calculate R₁ according
to the equation below. An example calculation follows.
An R₁ value of 120K is chosen, resulting in a small feed-
back network bias current of 1.2V/120K = 10µA. The
desired output voltage is 1.8V. From this information, R₁
is calculated from the equation below.
R₂
=
R₁(V_OUT - 1.2V)
1.2V
The result is R₁ = 60K. Since 60K is not a standard
1%-value, 60.4K is selected. From this example calcula-
tion, for V_OUT = 1.8V, use R₁ = 120K and R₁ = 60.4K.
Example output voltages and corresponding resistor
values are provided in Table 11.
R₂ Standard 1% Values (R₁ = 120K)
V_OUT (V)
R₂ (Ω)
2.8
160K
2.5
130K
2.0
79.6K
1.8
60.4K
1.5
30.1K
Table 10: Example Output Voltages and
Corresponding Resistor Values
Altering the Maximum
Flash LED Current Level
R_FSET determines the maximum LED current level at each
of the F1 and F2 flash LED outputs. In the typical appli-
cation, selecting R_FSET = 280K results in 300mA/ch LED
current. From this reference point, the maximum current
level can be modified by calculating an alternative R_FSET
value:
300mA · 280kΩ
R_FSET =
I_LED(MAX)
This is illustrated in Figure 4.
450
400
350
300
250
200
150
100
50
0
100 200 300 400 500 600 700 800 900 1000 1100
R_FSET (kΩ)
Figure 4: Maximum Flash LED Current
vs. R_FSET.
Selection of set resistor values outside of the typical
application must be carefully evaluated to ensure that the
application’s performance requirements can still be met.
Device Power Efficiency
The AAT2846 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 passing the input supply directly
to the output. By simplifying the conditions such that the
LEDs have uniform V_F, the power conversion efficiency
can be approximated by:
2846.2008.03.1.1
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