LTC3553-2_15 Datasheet, PDF (22/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
Inductor value should be chosen based on the desired
output voltage. See Table 1. Table 3 shows several inductors
that work well with the step-down switching buck regulator.
These inductors offer a good compromise in current rat-
ing, DCR and physical size. Consult each manufacturer for
detailed information on their entire selection of inductors.
Larger value inductors reduce ripple current, which im-
proves output ripple voltage. Lower value inductors result in
higher ripple current and improved transient response time,
but will reduce the available output current. To maximize
efficiency, 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
specified for the buck converter.
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. To-
roid or shielded pot cores in ferrite or permalloy 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 efficiency. 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.
Table 1. Choosing the Inductor Value
DESIRED OUTPUT VOLTAGE RECOMMENDED INDUCTOR VALUE
1.8V or Less
10μH
1.8V to 2.5V
6.8μH
2.5V to 3.3V
4.7μH
Table 3. Recommended Inductors for the Buck Regulator
INDUCTOR PART NO.
L (μH)
MAX I_DC (A)
1117AS-4R7M
1117AS-6R8M
1117AS-100M
4.7
0.64
6.8
0.54
10
0.45
CDRH2D11BNP-4R7N
4.7
0.7
CDRH2D11BNP-6R8N
6.8
0.6
CDRH2D11BNP-100N
10
0.48
SD3112-4R7-R
SD3112-6R8-R
SD3112-100-R
4.7
0.8
6.8
0.68
10
0.55
EPL2014-472ML_
EPL2014-682ML_
EPL2014-103ML_
4.7
0.88
6.8
0.8
10
0.6
* = Typical DCR
Table 2. Ceramic Capacitor Manufacturers
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
MAX DCR (Ω)
0.18*
0.250*
0.380*
0.248
0.284
0.428
0.246*
0.291*
0.446*
0.254
0.316
0.459
SIZE (L × W × H) (mm) MANUFACTURER
3.0 × 2.8 × 1.0
Toko
www.toko.com
3.0 × 3.0 × 1.2
Sumida
www.sumida.com
3.1 × 3.1 × 1.2
Cooper
www.cooperet.com
2.0 × 1.8 × 1.4
Coilcraft
www.coilcraft.com
35532f
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