AAT1275
Boost Converter with USB Power Switch
Functional Block Diagram
LIN
SW
OUT1
IN
Boost Regulator Control
VCC
FB
EN
OUT2
Load Switch Control
SET
FLT
GND
through an open-drain fault reporting signal (FLT).
The fault reporting signal has a 4ms turn-on delay.
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.
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 RDS(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.
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
1275.2007.01.1.3
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ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
