AAT1276 Datasheet, PDF (11/18 Pages) Advanced Analogic Technologies – Boost Converter with USB Power Switch

SwitchReg^TM
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:
D_MAX
= ^VO
-
V_IN(MIN)
V_O
I_PP
=
V_IN(MIN) · D
L · F_S
I_P
=
I_O
1-D
I_PK = I_P +
I_PP
2
I_V = I_P - I_PP
I_RMS =
I_PK2 + I_PK · I_V + I_V2
3
P_{LOSS(INDUCTOR)} = I^2RMS · DCR
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.
PRODUCT DATASHEET
AAT1276
Boost Converter with USB Power Switch
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
switching frequency (F_S) of 800kHz (worst-case).
C_OUT =
I_OUT · D_MAX
F_S · ΔV_OUT
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.
1276.2007.11.1.1
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