AAT2612 Datasheet, PDF (10/18 Pages) Skyworks Solutions Inc. – Step-Down DC/DC Converter With Three High PSRR LDOs

DATA SHEET
AAT2612
Step-Down DC/DC Converter With Three High PSRR LDOs
Functional Description
The AAT2612 is a compact power management solution.
It integrates a step-down converter with three high PSRR
low-dropout regulators to provide power from a wall
adapter, USB port, or a single-cell Lithium Ion/Polymer
battery or dual cell alkaline battery.
The AAT2612 uses fixed-frequency peak current control
architecture. Light load mode is used to enhance light
load efficiency. Compensation is integrated to reduce the
number of external components and achieve excellent
transient response and load/line regulation.
The ideal 1.5MHz switching frequency allows the use of
smaller output filter components for improved power
density, reduced external component size, and optimized
output voltage ripple.
The AAT2612 has four separate enable pins to control
buck converter and three LDO regulator outputs’ startup.
Also see the “Enable Function” section in the Applications
Information section of this datasheet.
Synchronous Step-Down Converter
The AAT2612 contains one high performance 600mA,
1.5MHz synchronous step-down converter. The step-
down converter operates to ensure high efficiency per-
formance over all load conditions.
The input voltage range is from 2.5V to 5.5V, and the
output voltage is programmable from 85% of V_IN to as
low as 0.9V with external resistor divider. Power devices
are sized for 600mA current capability while maintaining
over 85% efficiency at full load. High efficiency is main-
tained at lower currents
A high DC gain error amplifier with internal compensa-
tion controls the output. It provides excellent transient
response and load/line regulation. The converter has soft
start control to limit inrush current.
Apart from the resistor divider and input capacitor, only
a small L-C filter is required at the output side for the
step-down converter to operate properly. Typically, a
2.2μH inductor and a 10μF ceramic capacitor are recom-
mended for low output voltage ripple and small compo-
nent size.
Control Loop
The converter is a peak current mode step-down con-
verter. The inner, wide bandwidth loop controls the
inductor peak current. The inductor current is sensed
through the P-channel MOSFET (high side) and is also
used for short circuit and overload protection. A fixed
slope compensation signal is added to the sensed cur-
rent to maintain stability for duty cycles greater than
50%. The peak current mode loop appears as a voltage
programmed current source in parallel with the output
capacitor. The output of the voltage error amplifier pro-
grams the current mode loop for the necessary peak
inductor current to force a constant output voltage for all
load and line conditions. The voltage feedback resistive
divider is external and the error amplifier reference volt-
age is 0.6V. The voltage loop has a high DC gain making
for excellent DC load and line regulation. The internal
voltage loop compensation is located at the output of the
transconductance voltage error amplifier.
Soft-Start
Soft start increases the inductor current limit point lin-
early when the input voltage or enable input is applied.
It limits the current surge seen at the input and elimi-
nates output voltage overshoot.
Current Limit and
Over-Temperature Protection
For overload conditions the peak input current is limited.
As load impedance decreases and the output voltage
falls closer to zero, more power is dissipated internally,
raising the device temperature. Thermal protection com-
pletely disables switching when internal dissipation
becomes excessive, protecting the device from damage.
The junction over-temperature threshold is 140°C with
15°C of hysteresis.
LDO Regulator
The advanced circuit design of the linear regulator is
specifically optimized for very fast start-up and shut-
down timing. This proprietary LDO is also tailored for
superior transient response characteristics. These traits
are particularly important for applications which require
fast power supply timing.
The high-speed turn-on capability is enabled through the
implementation of a fast start control circuit, which
accelerates the power up behavior of fundamental con-
trol and feedback circuits within the LDO regulator. Fast
turn-off time response is achieved by an active output
pull down circuit, which is enabled when the LDO regula-
tor is placed in the shutdown mode. This active fast
shutdown circuit has no adverse effect on normal device
operation. The LDO regulator output has been specifi-
cally optimized to function with low cost, low ESR ceram-
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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202407B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 20, 2013