Boost Converter with USB Power Switch
Functional Block Diagram
LIN
SW
AAT1275
IN
OUT1
VCC
Boost Regulator Control
EN
FB
OUT2
SET
Load Switch Control
FLT
GND
Functional Description
The AAT1275 is a 500mA synchronous boost con-
verter with a current-limited load switch targeted for
single-cell Lithium-ion/polymer devices acting as a
portable host for USB power.
The AAT1275 has integrated control and synchro-
nous MOSFETs, minimizing the cost and the num-
ber of external components. Additional features
include a soft-start function which allows the load
voltage to ramp up in a controlled manner, elimi-
nating output voltage overshoot and minimizing
inrush current. Typical soft-start time for the boost
converter is approximately 300µs.
The AAT1275 also has a load switch with user-pro-
grammable current limiting. The load switch reports
over-current and over-temperature conditions
through an open-drain fault reporting signal (FLT).
The fault reporting signal has a 4ms turn-on delay.
Control Scheme
The control circuit uses hysteretic current mode
control with internal inductor current sensing for
very high efficiency over a wide output current
range. For heavy load, the boost converter oper-
ates in continuous conduction mode (CCM). This
minimizes the RMS current and optimizes the effi-
ciency at load conditions where the losses are
dominated by the power MOSFET R
DS(ON)
. This
also keeps the ripple current to a minimum and
minimizes the output voltage ripple and the output
capacitor size. A zero current comparator senses
the inductor current and prevents reverse current
flow for optimum light load efficiency.
1275.2007.01.1.3
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ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
