AAT2265
Low Voltage 2MHz Step-Up DC/DC Converter in SC70JW PackageSwitchRegTM
PRODUCT DATASHEET
2265.2009.05.1.1 9
www.analogictech.com
Start-up and Inrush Current Limit
The start-up sequence of the AAT2265 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 output 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 con-
trol 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 typical start-up time is
approximately 1ms dependant upon load conditions.
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 AAT2265
design disconnects this body diode from the output and
eliminates this conduction path. This enables the
AAT2265 to provide true load disconnect during shut-
down and inrush current limit at turn-on.
Short-Circuit Protection
The AAT2265 is designed to function from a single AA
cell battery which is typically in the range of 1.5V to 0.8V
during discharge. The part is guaranteed to operate at
0.8V with less than 50mA load current because the con-
trol circuitry is powered off the output of the boost. This
unique mode of operation allows the device to run down
to very low input voltages while maintaining control of
regulation, temperature and short circuit protection. The
P-channel synchronous MOSFET body diode disconnect
feature also gives the AAT2265 the ability to provide
limited output short-circuit current protection. However,
when the output is shorted directly to ground, there is a
loss of control circuitry, so the short circuit protection is
limited by the current source of the battery. For single
cell AA, the device can withstand a dead short. But
since the control circuitry is not functional during the
short circuit event, the part can be damaged if a source
greater than an AA battery is used to power the device.
A resistive short where the output does not go below
approximately 2V will be short circuit protected even at
higher than 1.5V input voltage levels.
Applications Information
Inductor Selection
The AAT2265 is designed to operate with a 2.2μH induc-
tor for all input/output voltage combinations. The induc-
tor saturation current rating should be greater than the
NMOS current limit specification listed in the Electrical
Characteristics table. If necessary, the peak inductor cur-
rent can exceed the saturation level by a small amount
with no significant effect on performance. Table 1 pro-
vides a list of some suggested inductor manufacturers
and their part numbers.
Output and Input Capacitor Selection
Surface mount X5R or X7R ceramic capacitors are sug-
gested 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, transient 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.
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 AAT2265, it is important that the
evaluation board layout be followed. Even the best
switch-mode power supply design cannot overcome the
problems that can be caused by a poor layout.
For the AAT2265, 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 switch-
ing noise from radiating into other high noise sensitive,
high impedance circuits.