AAT2846 Datasheet, PDF (17/25 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight and Flash Applications

AAT2846
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 pro-
grammed by an external resistor divider network.
As shown below, the selection of R1 and R2 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 capaci-
tance to become a larger factor in circuit perform-
ance whereas lower resistor values increase bias
current and decrease efficiency.
OUT(A/B)
FB(A/B)
V_REF(A/B) = 1.2V
R2(A/B)
VOUT(A/B)
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, cal-
culate R2 according to the equation below. An exam-
ple calculation follows.
An R1 value of 120K is chosen, resulting in a small
feedback 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.
R₂
=
R₁(V_OUT - 1.2V)
1.2V
The result is R2 = 60K. Since 60K is not a standard
1%-value, 60.4K is selected. From this example
calculation, for V_OUT = 1.8V, use R1 = 120K and R2
= 60.4K. Example output voltages and correspon-
ding resistor values are provided in Table 11.
R2 Standard 1% Values (R1 = 120K)
V_OUT (V)
2.8
2.5
2.0
1.8
1.5
R2 (Ω)
160K
130K
79.6K
60.4K
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 typ-
ical application, selecting R_FSET = 280K results in
300mA/ch LED current. From this reference point,
the maximum current level can be modified by cal-
culating 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.
2846.2007.08.1.0
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