AAT1265_07 Datasheet, PDF (10/13 Pages) Advanced Analogic Technologies – Low Voltage 2MHz Step-Up DC/DC Converter in SC70JW Package

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PRODUCT DATASHEET
AAT1265
Low Voltage 2MHz Step-Up DC/DC Converter in SC70JW Package
frequency (Light Load) mode to reduce the dominant
switching losses. In addition to Light Load operation, a
zero current comparator blocks reverse current in the
P-channel synchronous MOSFET, forcing DCM operation at
light load. These controls, along with very low quiescent
current, help to maintain high efficiency over the com-
plete load range without the compromise of increased
output voltage ripple during light load conditions.
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 output is charged at a constant cur-
rent 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 typical start-up
time is 2.5ms.
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 disconnect during shut-
down and inrush current limit at turn-on.
Short-Circuit Protection
The P-channel synchronous MOSFET body diode discon-
nect feature also gives the AAT1265 the ability to pro-
vide output short-circuit current limit protection.
Applications Information
Inductor Selection
The AAT1265 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 AAT1265, 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 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 switch-
ing noise from radiating into other high noise sensitive,
high impedance circuits.
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