AAT2842
2
High-Current Charge Pump with S Cwire™
Control and Dual LDO for Backlight and Flash
Applications Information
FL% of FSET
Data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
100
81
72
66
LDO Output Voltage Programming
The output voltages for LDOA and LDOB are pro-
grammed by an external resistor divider network.
As shown in Figure 2, the selection of R1 and R2 is
a straightforward matter.
60
52.5
46.2
41.4
36.6
31.7
28.5
25.0
21.7
18.4
16.7
15.0
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)
VOUT(A/B)
R2(A/B)
FB(A/B)
Table 2: Flash Current Register: FL1-FL4
VREF = 1.2V
(RFSET = 280kΩ).
R1(A/B)
Shutdown
Since the sink switches are the only power returns
for all loads, there is no leakage current when all of
the sink switches are disabled. To activate the shut-
down mode, hold both the BENS and FENS inputs
low for longer than TBENS(OFF) or TFENS(OFF) (500µs).
In this state, the AAT2842 typically draws less than
1µA from the input. Data and address registers are
reset to 0 in shutdown.
Figure 2: Selection of External Resistors.
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.
Low Dropout Regulators
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 AAT2842 includes two LDO linear regulators.
The regulators run from the same 2.7V to 5.5V input
voltage as the charge pump. The regulators use a
single on/off control input, ENL. The LDO output
voltages are set through a resistive voltage divider
from the output (OUTA or OUTB) to the feedback
input (FBA or FBB). The ratio of resistor values
determines the LDO output voltage. The low 200mV
dropout voltage at 200mA load current allows the
regulator to maintain output voltage regulation.
R1(VOUT - 1.2V)
R2 =
1.2V
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. Example output voltages and correspon-
ding resistor values are provided in Table 3.
Each LDO regulator can supply a continuous load
current up to 200mA. Both LDOs include current
limiting and thermal overload protection to prevent
damage to the load or to the LDOs.
12
2842.2007.09.1.2
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