AAT2783_12 Datasheet, PDF (17/22 Pages) Skyworks Solutions Inc. – Triple-Output PMIC Dual Buck with Low-VIN LDO

DATA SHEET
AAT2783
Triple-Output PMIC: Dual Buck with Low-V_IN LDO
R2, R4 is 59k. Although a larger value will further
reduce quiescent current, it will also increase the imped-
ance of the feedback node, making it more sensitive to
external noise and interference. Table 1 summarizes the
resistor values for various output voltages with R2, R4
set to either 59kfor good noise immunity or 221kfor
reduced no load input current.
V_OUT = 0.6V ·
1+
R3, R5
R2, R4
V_OUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
R2, R4 = 59kΩ
R3, R5 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R2, R4 = 221kΩ
R3, R5 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 1: AAT2783 Resistor Values for Various
Output Voltages.
input power source, a C_IN capacitor will be needed for
stable operation. C_IN should be located as closely to the
device VIN3 pin as practically possible. C_IN values greater
than 2.2μF will offer superior input line transient response
and will assist in maximizing the highest possible power
supply ripple rejection. Ceramic, tantalum, or aluminum
electrolytic capacitors may be selected for C_IN. There is no
specific capacitor ESR requirement for C_IN. However, for
400mA LDO regulator output operation, ceramic capaci-
tors are recommended for C_IN due to their inherent capa-
bility over tantalum capacitors to withstand input current
surges from low impedance sources such as batteries in
portable devices.
Output Capacitor
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between pins VOUT3 and
GND. The C_OUT capacitor connection to the LDO regulator
ground pin should be made as direct as practically pos-
sible for maximum device performance. The AAT2783
has been specifically designed to function with very low
ESR ceramic capacitors. For best performance, ceramic
capacitors are recommended. Typical output capacitor
values for maximum output current conditions range
from 4.7μF to 10μF. Applications utilizing the exception-
ally low output noise and optimum power supply ripple
rejection characteristics of the AAT2783 should use 10μF
or greater for C_OUT. If desired, C_OUT may be increased
without limit.
Channel 3: 400mA Low
Dropout (LDO) Converter
The Channel 3 LDO converter is used to post-regulate the
channel 2 step-down buck converter and provide a quiet
output voltage. The input voltage is 1.7V and the output
voltage is set to 1.2V. The output current is 400 mA. The
channel 2 buck converter efficiency at full load is 85%
and the LDO efficiency is 70%. Combined post-regulated
efficiency at full load is 60%. The LDO will turn on when
EN3 is high and V_OUT2 has come into regulation.
Component Selection
Input Capacitor
Typically, a 2.2μF or larger capacitor is recommended for
C_IN in most applications. A C_IN capacitor is not required for
basic LDO regulator operation. However, if the AAT2783 is
physically located more than three centimeters from an
Thermal Calculations
There are three types of losses associated with the
AAT2783 step-down converters: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the R_DS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the losses for
each step-down is given by:
P_{TOTAL(Step-down)}
=
I_O2
·
(R_DS(ON)H
·
V_O
+
R_DS(ON)L
V_IN
·
[V_IN
-
V_O])
+ [(t_sw · F_S · I_OUT + I_Q) · V_IN]
I_Q is the step-down converter quiescent current. The
term t_sw is used to estimate the full load step-down con-
verter switching losses. For the condition where the
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202039A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
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