AAT2846INJ-EE-T1 AnalogicTech, AAT2846INJ-EE-T1 Datasheet - Page 17

• Manufacturer Part Number: AAT2846INJ-EE-T1
• Description: High Current Charge Pump with Dual LDO for Backlight and Flash Applications
• Manufacturer: AnalogicTech

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.
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
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.
The result is R2 = 60K. Since 60K is not a standard
1%-value, 60.4K is selected. From this example
calculation, for V
= 60.4K. Example output voltages and correspon-
ding resistor values are provided in Table 11.
2846.2007.08.1.0
OUT(A/B)
FB(A/B)
V
REF(A/B)
R
OUT
2
= 1.2V
=
= 1.8V, use R1 = 120K and R2
R
1
(V
OUT
1.2V
- 1.2V)
R2(A/B)
R1(A/B)
V
OUT(A/B)
OUT
High Current Charge Pump with Dual LDO
, cal-
for Backlight and Flash Applications
Altering the Maximum Flash LED
Current Level
R
each of the F1 and F2 flash LED outputs. In the typ-
ical application, selecting R
300mA/ch LED current. From this reference point,
the maximum current level can be modified by cal-
culating an alternative R
This is illustrated in Figure 4.
FSET
Table 10: Example Output Voltages and
Figure 4: Maximum Flash LED Current
R2 Standard 1% Values (R1 = 120K)
determines the maximum LED current level at
450
400
350
300
250
200
150
100
50
0
100
Corresponding Resistor Values
V
OUT
200
2.8
2.5
2.0
1.8
1.5
R
(V)
FSET
300
=
400
vs. R
300mA · 280kΩ
500
R
FSET
FSET
I
FSET
LED(MAX)
600
FSET
(kΩ Ω )
value:
.
700
= 280K results in
AAT2846
800
R2 (Ω)
79.6K
60.4K
30.1K
160K
130K
900
1000 1100
17