AAT3601_08 Datasheet, PDF (29/37 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

PRODUCT DATASHEET
AAT3601178
Total Power Solution for Portable Applications
Synchronous Step-Down
(Buck) Converter
The AAT3601 contains a high performance 300mA,
1.5MHz synchronous step-down converter. The step-
down converter operates to ensure high efficiency per-
formance over all load conditions. It requires only 3
external power components (C_IN, C_OUT, and L). A high DC
gain error amplifier with internal compensation controls
the output. It provides excellent transient response and
load/line regulation. Transient response time is typically
less than 20μs. The converter has soft start control to
limit inrush current and transitions to 100% duty cycle
at drop out.
The step-down converter input pin PVIN should be con-
nected to the SYSOUT LDO output pin. The output volt-
age is internally fixed at 1.24V. Power devices are sized
for 300mA current capability while maintaining over
90% efficiency at full load.
Input/Output Capacitor and Inductor
Apart from the input capacitor that is shared with the LDO
inputs, 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 such as the Sumida CDRH2D11NP-
2R2NC and a 4.7μF ceramic output capacitor are recom-
mended for low output voltage ripple and small compo-
nent size. Ceramic capacitors with X5R or X7R dielectrics
are highly recommended because of their low ESR and
small temperature coefficients. A 10μF ceramic input
capacitor is sufficient for most applications.
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) which 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 programs the
current mode loop for the necessary peak inductor cur-
rent to force a constant output voltage for all load and
line conditions. The voltage feedback resistive divider is
internal and the error amplifier reference voltage is
0.45V. The voltage loop has a high DC gain making for
excellent DC load and line regulation. The internal volt-
age loop compensation is located at the output of the
transconductance voltage error amplifier.
Soft-Start
Soft start slowly increases the internal reference voltage
when the input voltage or enable input is initially 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.
Linear LDO Regulators (OUT1-5)
The advanced circuit design of the linear regulators has
been specifically optimized for very fast start-up and
shutdown timing. These proprietary LDOs are tailored for
superior transient response characteristics. These traits
are particularly important for applications which require
fast power supply timing.
There are two LDO input pins AVIN1/2 which should be
connected to the SYSOUT LDO output pin. All LDO out-
puts are initially fixed at default levels. The user can
program the output voltages for all the LDOs using I²C
(see Table 8).
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 an LDO regula-
tor is placed in the shutdown mode. This active fast
shutdown circuit has no adverse effect on normal device
operation.
3601.2008.07.1.1
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