AAT2856 AAT, AAT2856 Datasheet - Page 14

• Manufacturer Part Number: AAT2856
• Description: High Current Charge Pump
• Manufacturer: AAT

It is also important to note that Address 0 is disabled
when independent channel control mode is enabled.
Additionally, Address 3 is disabled and the very low
current settings cannot be used in this mode. Other
addresses are unmodified by this mode so that the
30mA backlight scale can still be used.
The LDO enables are always independent of
AS
Low Dropout Regulators
The AAT2856 includes two independent LDO linear
regulators. The regulators operate from a 2.7V to
5.5V input voltage at IN. The AAT2856 supplies
separate LDO enable inputs (ENA and ENB) to
control individually the operation of the LDOs. The
LDO output voltages are set through resistive volt-
age dividers from the output (OUTA or OUTB) to
the feedback input (FBA or FBB). The regulator
controls the output voltage such that the voltage
divider output is at the 1.2V feedback threshold.
The low 200mV dropout voltage at 200mA load cur-
rent allows the regulator to maintain output voltage
regulation.
Each LDO regulator can supply up to 200mA con-
tinuous current to the load. They include current
limiting and thermal overload protection to prevent
damage to the load or to the LDOs.
14
2
Cwire programming.
Dual LDO for BacklightApplications
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 13.
High Current Charge Pump with
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)
AAT2856
2856.2007.06.1.0
V
OUT(A/B)
OUT
, cal-