Datasheet

LTC3101
22
3101fb
APPLICATIONS INFORMATION
buck converter is going to be utilized at duty cycles over
40%, the inductance value must be at least equal to L
MIN
as given by the following equation:
L
MIN
= 2.5 • V
OUT
(µH)
Table 1 depicts the minimum required inductance for
several common output voltages.
Table 1. Buck Minimum Inductance
OUTPUT VOLTAGE MINIMUM INDUCTANCE
0.8V 2.0H
1.2V 3.0H
1.8V 4.7H
2.0V 5.0H
2.7V 6.8H
A large variety of low ESR, high current power inductors
are available that are well suited to LTC3101 buck converter
applications. The tradeoff generally involves PCB area,
application height, required output current and effi ciency.
Table 2 provides a representative sampling of small sur-
face mount inductors that are well suited for use with
the LTC3101 buck converters. All inductor specifi cations
are listed at an inductor value of 4.7µH for comparison
purposes but other values within these inductor families
are generally well suited to this application as well. Within
each family (i.e., at a fi xed inductor size), the DC resistance
generally increases and the maximum current generally
decreases with increased inductance.
Buck Output Capacitor Selection
A low ESR output capacitor should be utilized at the buck
converter output in order to minimize output voltage ripple.
Multilayer ceramic capacitors are an excellent choice as
they have low ESR and are available in small footprints. In
addition to controlling the ripple magnitude, the value of
the output capacitor also sets the loop crossover frequency
and therefore can impact loop stability. In general, there is
both a minimum and maximum capacitance value required
to ensure stability of the loop. If the output capacitance is
too small, the loop crossover frequency will increase to
the point where switching delay and the high frequency
parasitic poles of the error amplifi er will degrade the phase
Table 2. Representative Buck Inductors
PART NUMBER
VALUE
(H)
DCR
()
MAX DC
CURRENT (A)
SIZE (mm)
W × L × H
Coilcraft
LPS3015
EPL2014
EPL2010
LPS4018
4.7
4.7
4.7
4.7
0.20
0.23
0.43
0.125
1.2
0.88
0.65
1.9
3.0 × 3.0 × 1.5
2.0 × 2.0 × 1.4
2.0 × 2.0 × 1.0
4.0 × 4.0 × 1.8
Cooper-Bussmann
SD3118
SD3112
SD3110
SD10
4.7
4.7
4.7
4.7
0.162
0.246
0.285
0.154
1.31
0.80
0.68
1.08
3.1 × 3.1 × 1.8
3.1 × 3.1 × 1.2
3.1 × 3.1 × 1.0
5.2 × 5.2 × 1.0
Murata
LQH3NP
LQM31PN
LQH32CN
4.7
4.7
4.7
0.26
0.30
0.15
0.80
0.70
0.65
3.0 × 3.0 × 0.9
3.2 × 1.6 × 0.85
3.2 × 2.5 × 2.0
Panasonic
ELLVEG
ELL4G
ELL4LG
4.7
4.7
4.7
0.24
0.16
0.09
0.70
0.86
1.10
3.0 × 3.0 × 1.0
3.8 × 3.8 × 1.1
3.8 × 3.8 × 1.8
Sumida
CDRH2D09
CDRH3D16/LD
CDRH2D09B
4.7
4.7
4.7
0.167
0.081
0.218
0.42
0.62
0.70
3.2 × 3.2 × 1.0
3.2 × 3.2 × 1.8
3.0 × 2.8 × 1.0
Taiyo-Yuden
CBC2518
CBC3225T
NR3010T
4.7
4.7
4.7
0.2
0.1
0.19
0.68
1.01
0.75
2.5 × 1.8 × 1.8
3.2 × 2.5 × 2.5
3.0 × 3.0 × 1.0
TOKO
DE2812C
D310F
DB3015C
4.7
4.7
4.7
0.13
0.26
0.09
1.2
0.9
0.86
3.0 × 3.2 × 1.2
3.8 × 3.8 × 1.0
3.2 × 3.2 × 1.8
Wurth
744028004
744032004
744029003
4.7
4.7
4.7
0.265
0.280
0.170
0.90
0.49
0.80
2.8 × 2.8 × 1.1
3.2 × 2.5 × 2.0
2.8 × 2.8 × 1.35
margin. In addition, the wider bandwidth produced by a
small output capacitor will make the loop more susceptible
to switching noise. Table 3 depicts the minimum recom-
mended output capacitance for several typical output
voltages. At the other extreme, if the output capacitor is
too large, the crossover frequency can decrease too far
below the compensation zero and also lead to degraded
phase margin. In such cases, the phase margin and tran-
sient performance can be improved by simply increasing
the size of the feedforward capacitor in parallel with the
upper resistor divider resistor. (See Buck Output Voltage
Programming section for more details).