Datasheet

LTC3553
22
3553fb
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
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 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 effi 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 4F of capacitance over
operating temperature and bias voltage. Generally, a good
starting point is to use a 10F output capacitor.
OPERATION
Table 3. Recommended Inductors for the Buck Regulator
INDUCTOR PART NO. L (µH) MAX I
DC
(A) MAX DCR () SIZE (L × W × H) (mm) MANUFACTURER
1117AS-4R7M
1117AS-6R8M
1117AS-100M
4.7
6.8
10
0.64
0.54
0.45
0.18*
0.250*
0.380*
3.0 × 2.8 × 1.0
Toko
www.toko.com
CDRH2D11BNP-4R7N
CDRH2D11BNP-6R8N
CDRH2D11BNP-100N
4.7
6.8
10
0.7
0.6
0.48
0.248
0.284
0.428
3.0 × 3.0 × 1.2
Sumida
www.sumida.com
SD3112-4R7-R
SD3112-6R8-R
SD3112-100-R
4.7
6.8
10
0.8
0.68
0.55
0.246*
0.291*
0.446*
3.1 × 3.1 × 1.2
Cooper
www.cooperet.com
EPL2014-472ML_
EPL2014-682ML_
EPL2014-103ML_
4.7
6.8
10
0.88
0.8
0.6
0.254
0.316
0.459
2.0 × 1.8 × 1.4
Coilcraft
www.coilcraft.com
* = 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
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