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參數(shù)資料
| 型號： | AAT1276 |
| 廠商： | Advanced Analogic Technologies, Inc. |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | Boost Converter with USB Power Switch |
| 中文描述： | 加強(qiáng)與USB電源開關(guān)轉(zhuǎn)換器 |
| 文件頁數(shù)： | 11/18頁 |
| 文件大?。?/td> | 363K |
| 代理商： | AAT1276 |
AAT1276
Boost Converter with USB Power Switch
SwitchReg
TM
PRODUCT DATASHEET
1276.2007.11.1.1
11
w w w . a n a l o g i c t e c h . c o m
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 1.5
μ
H to 2.7
μ
H 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:
V
O
- V
IN(MIN)
V
O
D
MAX
=
V
IN(MIN)
· D
L · F
S
I
PP
=
I
O
1 - D
I
P
=
I
PP
2
I
PK
= I
P
+
I
V
= I
P
- I
PP
I
RMS
=
I
PK
2
+ I
PK
· I
V
+ I
V
3
2
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.
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 4.7
μ
F 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).
I
OUT
· D
MAX
F
S
·
Δ
V
OUT
C
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
1
μ
F to 4.7
μ
F 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.
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