Low Voltage 2MHz Step-Up DC/DC
Converter in SC70JW Package
nous MOSFET, forcing DCM operation at light load.
These controls, along with very low quiescent cur-
rent, help to maintain high efficiency over the com-
plete load range without the compromise of
increased output voltage ripple during light load
conditions.
AAT1265
Applications Information
Inductor Selection
The AAT1265 is designed to operate with a 2.2µH
inductor for all input/output voltage combinations.
The inductor saturation current rating should be
greater than the NMOS current limit specification
listed in the Electrical Characteristics table. If nec-
essary, the peak inductor current can exceed the
saturation level by a small amount with no signifi-
cant effect on performance. Table 1 provides a list
of some suggested inductor manufacturers and
their part numbers.
Start-up and Inrush Current Limit
The start-up sequence of the AAT1265 varies
depending upon whether the input voltage is
greater than or less than 2V. Above 1V, a start-up
oscillator operating at 25% duty cycle controls the
synchronous power stage and charges the output
to 2V. For an input voltage greater than 2V, the out-
put is charged at a constant current of 300mA until
the output reaches a voltage level just below the
input voltage. The step-up converter then takes
control and continues to charge the output to the
steady-state voltage. The step-up converter N-
channel switch current is limited to 1A and the typ-
ical start-up time is 2.5ms.
Output and Input Capacitor Selection
Surface mount X5R or X7R ceramic capacitors are
suggested for both the output and the input. For the
output capacitor (C1 in Figure 4) a 4.7µF, 10V, X5R
ceramic capacitor is necessary for stability, tran-
sient response, and ripple performance. The same
0805 sized capacitor is used for the input (C2 of
Figure 4). If desired, a smaller, 0603 sized, 4.7µF,
6.3V, X5R ceramic capacitor can be substituted for
the input capacitor (C2). Suggested ceramic
capacitor suppliers are listed in Table 1.
Shutdown and Output Disconnect
A typical synchronous step-up (boost) converter
has a conduction path from the input to the output
via the body diode of the P-channel MOSFET. The
AAT1265 design disconnects this body diode from
the output and eliminates this conduction path.
This enables the AAT1265 to provide true load dis-
connect during shutdown and inrush current limit at
turn-on.
PCB Layout Guidelines
Figures 1, 2, and 3 display the evaluation board
layout and suggested component placement. Due
to the high switching speed of the AAT1265, it is
important that the evaluation board layout be fol-
lowed. Even the best switch-mode power supply
design cannot overcome the problems that can be
caused by a poor layout.
For the AAT1265, it is necessary that C1 and C2 be
placed as close as possible to the IC with a good
low impedance path to the GND pins of the IC. It
is also good practice to minimize the length of the
trace from the OUT pin to the output inductor. This
prevents switching noise from radiating into other
high noise sensitive, high impedance circuits.
Short-Circuit Protection
The P-channel synchronous MOSFET body diode
disconnect feature also gives the AAT1265 the
ability to provide output short-circuit current limit
protection.
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