LTC3577_15 Datasheet, PDF (34/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
Table 3. Recommended Inductors for Step-Down Switching Regulators
INDUCTOR TYPE
DB318C
D312C
DE2812C
L (μH)
4.7
3.3
4.7
3.3
4.7
3.3
MAX I_DC (A)
1.07
1.20
0.79
0.90
1.15
1.37
MAX DCR (Ω)
0.1
0.07
0.24
0.20
0.13*
0.105*
SIZE in mm (L × W × H)
3.8 × 3.8 × 1.8
3.8 × 3.8 × 1.8
3.6 × 3.6 × 1.2
3.6 × 3.6 × 1.2
3.0 × 2.8 × 1.2
3.0 × 2.8 × 1.2
CDRH3D16
CDRH2D11
CLS4D09
4.7
0.9
3.3
1.1
4.7
0.5
3.3
0.6
4.7
0.75
0.11
0.085
0.17
0.123
0.19
4 × 4 × 1.8
4 × 4 × 1.8
3.2 × 3.2 × 1.2
3.2 × 3.2 × 1.2
4.9 × 4.9 × 1
SD3118
SD3112
SD12
SD10
4.7
1.3
3.3
1.59
4.7
0.8
3.3
0.97
4.7
1.29
3.3
1.42
4.7
1.08
3.3
1.31
0.162
0.113
0.246
0.165
0.117*
0.104*
0.153*
0.108*
3.1 × 3.1 × 1.8
3.1 × 3.1 × 1.8
3.1 × 3.1 × 1.2
3.1 × 3.1 × 1.2
5.2 × 5.2 × 1.2
5.2 × 5.2 × 1.2
5.2 × 5.2 × 1.0
5.2 × 5.2 × 1.0
LPS3015
4.7
1.1
0.2
3.3
1.3
0.13
3.0 × 3.0 × 1.5
3.0 × 3.0 × 1.5
*Typical DCR
MANUFACTURER
Toko
www.toko.com
Sumida
www.sumida.com
Cooper
www.cooperet.com
Coil Craft
www.coilcraft.com
process much simpler. The step-down switching regula-
tors are designed to work with inductors in the range of
2.2μH to 10μH. For most applications a 4.7μH inductor is
suggested for step-down switching regulators providing
up to 500mA of output current while a 3.3μH inductor is
suggested for step-down switching regulators providing
up to 800mA. Larger value inductors reduce ripple current,
which improves output ripple voltage. Lower value induc-
tors 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. For a 1.2V output, efficiency is reduced about
2% for 100mΩ series resistance at 400mA load current,
and about 2% for 300mΩ series resistance at 100mA load
current. 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 inductor should be rated to handle the maximum peak
current specified for the step-down converters. Different
core materials and shapes will change the size/current
and price/current relationship of an inductor. Toroid or
shielded pot cores in ferrite or Permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
34
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 step-down switching
regulators 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.
Table 3 shows several inductors that work well with the
step-down switching regulators. These inductors offer a
good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
Input/Output Capacitor Selection
Low ESR (equivalent series resistance) ceramic capacitors
should be used at both step-down switching regulator
outputs as well as at each step-down switching regulator
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.
A 10μF output capacitor is sufficient for the step-down
switching regulator outputs. For good transient response
3577fa