AAT1275
Boost Converter with USB Power Switch
mizes this factor, but results may vary with package
size and among specific manufacturers.
USB Load Switch Application
Information
Use a 4.7µF 10V ceramic output capacitor to mini-
mize output ripple for the 5V output. Small 0805
sized ceramic capacitors are available which meet
these requirements.
Setting the Load Switch Current Limit
In most applications, the variation in ILIM must be
taken into account when determining RSET. The ILIM
variation is due to processing variations from part
to part, as well as variations in the voltages at
OUT1 and OUT2, plus the operating temperature.
The typical RSET value for a 300mA load is in the
range of 20 to 22kΩ.
Estimate the output capacitor required at the min-
imum switching frequency (FS) of 800kHz (worst-
case).
IOUT · DMAX
FS · ΔVOUT
COUT
=
Operation in Current Limit
When a heavy load is applied to OUT2 of the
AAT1275, the load current is limited to the value of
ILIM (determined by RSET) causing a drop in the
output voltage. This increases the AAT1275 power
dissipation and die temperature. When the die tem-
perature exceeds the over-temperature limit, the
AAT1275 shuts down until it has cooled sufficiently,
at which point it will start up again. The AAT1275
will continue to cycle on and off until the load is
removed, power is removed, or until a logic low
level is applied to the EN pin.
The boost converter input current flows during both
ON and OFF switching intervals. The input ripple
current is less than the output ripple and, as a
result, less input capacitance is required. A ceram-
ic output capacitor from 1µF to 4.7µF is recom-
mended. Minimum 6.3V rated capacitors are
required at the input. Ceramic capacitors sized as
small as 0603 are available which meet these
requirements.
Setting the Output Voltage
A fault flag indicates when the OUT2 pin load cur-
rent has exceeded the current limit level set by
RSET. The fault flag is an active low, open-drain pin
that requires 10kΩ pull-up to VIN. The fault signal
has a 4ms blanking time to prevent false over cur-
rent indicator during the charging of the USB bus
capacitor.
Program the output voltage through a resistive
divider located from the output to the FB pin to
ground. The internal error amplifier reference volt-
age is 0.6V. A 59.0kΩ programming resistor value
from VFB to GND with a 432kΩ resistor from FB to
the output will set the output voltage to 5V.
Steady-State Maximum Power
Dissipation
R2
⎛
⎝
⎞
⎠
VOUT = VREF · 1 +
R3
The maximum power dissipation for the AAT1275
occurs at the minimum input voltage, where it oper-
ates in continuous conduction mode (CCM). The
total power dissipation at full load is dominated by
the RDS(ON) losses of the power MOSFET. The dis-
sipation includes the losses in the input and output
switch, as well as both synchronous switches.
432kΩ
59.0kΩ
⎛
= 0.6V · 1 +
⎝
⎞
⎠
= 5.0V
12
1275.2007.01.1.3
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