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202061B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • March 19, 2013

11

AAT1276

DATA SHEET

Boost Converter with USB Power Switch

Selecting the Boost Inductor

The AAT1276 boost controller utilizes hysteretic control

and the switching frequency varies with output load and

input voltage. The value of the inductor determines the

maximum switching frequency of the boost converter.

Increasing output inductance decreases the switching

frequency, resulting in higher peak currents and

increased output voltage ripple. To maintain the 2MHz

switching frequency 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 satu-

ration characteristics.

Measure the inductor current at full load and high ambi-

ent temperature to ensure that the inductor does not

saturate or exhibit excessive temperature rise. Select

the output inductor (L) to avoid saturation at the mini-

mum input voltage and maximum load. The RMS current

flowing through the boost inductor is equal to the DC

plus AC ripple components. 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:

1 - D

2

3

At light load and low output voltage, the controller

reduces the operating frequency to maintain maximum

efficiency. As a result, further reduction in output 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 manufactur-

er’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.

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 compared to shielded, wound varieties.

Selecting the Step-Up

Converter Capacitors

The high output ripple inherent in the boost converter

necessitates low impedance output filtering. Multi-layer

ceramic (MLC) capacitors provide small size, adequate

capacitance, with low parasitic equivalent series resis-

tance (ESR) and equivalent series inductance (ESL). This

makes them well suited for use with the AAT1276. MLC

capacitors of type X7R or X5R ensure good capacitance

stability 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 minimizes this factor, but

results may vary with package size and among specific

manufacturers.

Use a 4.7μF 10V ceramic output capacitor to minimize

output ripple for the 5V output. Small 0805 sized ceram-

ic capacitors are available which meet these require-

ments.

Estimate the output capacitor required at the minimum

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 ceramic output capacitor from

1μF to 4.7μF is recommended. Minimum 6.3V rated

capacitors are required at the input. Ceramic capacitors

sized as small as 0603 are available which meet these

requirements.