LTC3553 Linear Technology Corporation, LTC3553 Datasheet - Page 22

• Manufacturer Part Number: LTC3553
• Description: Micropower USB Power Manager
• Manufacturer: Linear Technology Corporation

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OPERATION
LTC3553
Inductor value should be chosen based on the desired
output voltage. See Table 1. Table 3 shows several induc-
tors that work well with the step-down switching buck
regulator. These inductors offer a good compromise in
current rating, DCR and physical size. Consult each manu-
facturer for detailed information on their entire selection
of inductors.
Larger value inductors reduce ripple current, which
improves output ripple voltage. Lower value inductors
result in higher ripple current and improved transient
response time, but will reduce the available output cur-
rent. To maximize effi ciency, choose an inductor with a
low DC resistance.
Choose an inductor with a DC current rating at least 1.5
times larger than the maximum load current to ensure that
the inductor does not saturate during normal operation.
If output short circuit is a possible condition, the induc-
tor should be rated to handle the maximum peak current
specifi ed for the buck converter.
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or permalloy
Table 1. Choosing the Inductor Value
DESIRED OUTPUT VOLTAGE
1.8V or Less
1.8V to 2.5V
2.5V to 3.3V
Table 3. Recommended Inductors for the Buck Regulator
INDUCTOR PART NO.
1117AS-4R7M
1117AS-6R8M
1117AS-100M
CDRH2D11BNP-4R7N
CDRH2D11BNP-6R8N
CDRH2D11BNP-100N
SD3112-4R7-R
SD3112-6R8-R
SD3112-100-R
EPL2014-472ML_
EPL2014-682ML_
EPL2014-103ML_
* = Typical DCR
22
RECOMMENDED INDUCTOR VALUE
10μH
6.8μH
4.7μH
L (μH)
4.7
6.8
4.7
6.8
4.7
6.8
4.7
6.8
10
10
10
10
MAX I
0.64
0.54
0.45
0.48
0.68
0.55
0.88
0.7
0.6
0.8
0.8
0.6
DC
(A)
MAX DCR (Ω)
Table 2. Ceramic Capacitor Manufacturers
AVX
Murata
Taiyo Yuden
Vishay Siliconix
TDK
materials are small and do not radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. Inductors that are
very thin or have a very small volume typically have much
higher core and DCR losses, and will not give the best ef-
fi ciency. The choice of which style inductor to use often
depends more on the price versus size, performance and
any radiated EMI requirements than on what the buck
requires to operate.
The inductor value also has an effect on Burst Mode
operation. Lower inductor values will cause Burst Mode
switching frequency to increase.
Input/Output Capacitor Selection
Low ESR (equivalent series resistance) ceramic capaci-
tors should be used at the buck output as well as at the
buck input supply. Only X5R or X7R ceramic capacitors
should be used because they retain their capacitance over
wider voltage and temperature ranges than other ceramic
types. For good transient response and stability the output
capacitor should retain at least 4μF of capacitance over
operating temperature and bias voltage. Generally, a good
starting point is to use a 10μF output capacitor.
0.250*
0.380*
0.246*
0.291*
0.446*
0.18*
0.248
0.284
0.428
0.254
0.316
0.459
SIZE (L × W × H) (mm) MANUFACTURER
3.0 × 2.8 × 1.0
3.0 × 3.0 × 1.2
3.1 × 3.1 × 1.2
2.0 × 1.8 × 1.4
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
Toko
www.toko.com
Sumida
www.sumida.com
Cooper
www.cooperet.com
Coilcraft
www.coilcraft.com
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