AAT2782_12 Datasheet, PDF (17/22 Pages) Skyworks Solutions Inc. – Triple-Output PMIC Dual Buck with Low-VIN LDO

DATA SHEET
AAT2782
Triple-Output PMIC: Dual Buck with Low-V_IN LDO
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: AAT2782 Resistor Values for Various
Output Voltages.
Channel 3: 400mA Low
Dropout (LDO) Converter
The Channel 3 LDO converter can operate independently
or can be used to post-regulate one of the step-down
buck converters and provide a quiet output voltage. The
input voltage can be as low as 1.7V for an output voltage
of 1.2V; 2.5V, 2.8V, and 3.3V versions are also available.
The output current is 400mA. If the channel 2 buck con-
verter efficiency at full load is 85% and the LDO effi-
ciency is 70%, the combined post-regulated efficiency
would be 60%. If the LDO is used to post-regulate from
a buck, then EN3 should be brought high after the buck
comes into regulation.
Component Selection
Input Capacitor
Typically, a 10μF or larger capacitor is required for C_IN in
most applications and capacitance value of greater than
or equal to the value of the output capacitor is recom-
mended. A C_IN capacitor is required for basic LDO regula-
tor operation. If the AAT2782 is physically located more
than three centimeters from an 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 10μF 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 capacitors are recommended
for C_IN due to their inherent capability over tantalum
capacitors to withstand input current surges from low
impedance sources such as batteries in portable devices.
Care should also be taken when choosing the voltage rat-
ing for the capacitors. A 6.3V rated capacitor should not
be used to bypass a 5V input. In such cases, a 10V rated
capacitor should be used.
Output Capacitor
For proper load voltage regulation and operational stabil-
ity, a capacitor is required between pins VOUT3 and GND.
The C_OUT3 capacitor connection to the LDO regulator
ground pin should be made as direct as practically possi-
ble for maximum device performance. The AAT2782 has
been specifically designed to function with very low ESR
ceramic capacitors. For best performance, ceramic capac-
itors are recommended. Typical output capacitor values
for maximum output current conditions range from 4.7μF
to 10μF. Applications utilizing the exceptionally low output
noise and optimum power supply ripple rejection charac-
teristics of the AAT2782 should use 10μF or greater for
C_OUT3. If desired, C_OUT3 may be increased without limit.
Thermal Calculations
There are three types of losses associated with the
AAT2782 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_OSC · I_OUT + I_Q) · V_IN]
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202038A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
17