AAT1275
Boost Converter with USB Power Switch
Selecting the Boost Inductor
2
IPK2 + IPK · IV + IV
IRMS
=
The AAT1275 boost controller utilizes hysteretic
control and the switching frequency varies with out-
put load and input voltage. The value of the induc-
tor determines the maximum switching frequency
of the boost converter. Increasing output induc-
tance decreases the switching frequency, resulting
in higher peak currents and increased output volt-
age ripple. To maintain the 2MHz switching fre-
quency and stable operation, an output inductor
sized from 1.5µH to 2.7µH is recommended.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
peak inductor current rating, which is a function of
the saturation characteristics.
3
PLOSS(INDUCTOR) = I2
· DCR
RMS
At light load and low output voltage, the controller
reduces the operating frequency to maintain maxi-
mum efficiency. As a result, further reduction in out-
put load does not reduce the peak current. The
minimum peak current ranges from 0.5A to 0.75A.
Compare the RMS current values with the manu-
facturer's temperature rise, or thermal derating
guidelines. For a given inductor type, smaller
inductor size leads to an increase in DCR winding
resistance and, in most cases, increased thermal
impedance. Winding resistance degrades boost
converter efficiency and increases the inductor's
operating temperature.
Measure the inductor current at full load and high
ambient temperature to ensure that the inductor
does not saturate or exhibit excessive temperature
rise. Select the output inductor (L) to avoid satura-
tion at the minimum input voltage and maximum
load. The RMS current flowing through the boost
inductor is equal to the DC plus AC ripple compo-
nents. The maximum inductor RMS current occurs
at the minimum input voltage and the maximum
load. Use the following equations to calculate the
maximum peak and RMS current:
Shielded inductors provide decreased EMI and
may be required in noise sensitive applications.
Unshielded chip inductors provide significant space
savings at a reduced cost compared to shielded
inductors. In general, chip-type inductors have
increased winding resistance (DCR) when com-
pared to shielded, wound varieties.
VO - VIN(MIN)
VO
DMAX
=
Selecting the Step-Up Converter
Capacitors
The high output ripple inherent in the boost con-
verter necessitates low impedance output filtering.
Multi-layer ceramic (MLC) capacitors provide small
size, adequate capacitance, with low parasitic
equivalent series resistance (ESR) and equivalent
series inductance (ESL). This makes them well
suited for use with the AAT1275. MLC capacitors of
type X7R or X5R ensure good capacitance stabili-
ty over the full operating range. MLC capacitors
exhibit significant capacitance reduction with an
applied DC voltage. Output ripple measurements
can confirm that the capacitance used meets the
specific ripple requirements. Voltage derating mini-
VIN(MIN) · D
L · FS
IPP
=
IO
1 - D
IP =
IPP
2
IPK = IP +
IV = IP - IPP
1275.2007.01.1.3
11
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