Datasheet

LTC3677-3
31
36773f
operaTion
Low Supply Operation
An undervoltage lockout circuit on V
OUT
(V
OUT
UVLO)
shuts down the step-down switching regulators when V
OUT
drops below about 2.7V. It is recommended that the step-
down switching regulator input supplies (V
IN12
, V
IN3
) be
connected to the power path output (V
OUT
) directly. This
UVLO prevents the step-down switching regulators from
operating at low supply voltages where loss of regula-
tion or other undesirable operation may occur. If driving
the step-down switching regulator input supplies from
a voltage other than the V
OUT
pin, the regulators should
not be operated outside the specified operating range as
operation is not guaranteed beyond this range.
Inductor Selection
Many different sizes and shapes of inductors are available
from numerous manufacturers. Choosing the right inductor
from such a large selection of devices can be overwhelming,
but following a few basic guidelines will make the selection
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 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
Table 3. Recommended Inductors for Step-Down Switching Regulators
INDUCTOR TYPE L (μH) MAX I
DC
(A) MAX DCR (Ω) SIZE in mm (L × W × H) MANUFACTURER
DB318C
D312C
DE2812C
4.7
3.3
4.7
3.3
4.7
3.3
1.07
1.20
0.79
0.90
1.15
1.37
0.1
0.07
0.24
0.20
0.13*
0.105*
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
Toko
www.toko.com
CDRH3D16
CDRH2D11
CLS4D09
4.7
3.3
4.7
3.3
4.7
0.9
1.1
0.5
0.6
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
Sumida
www.sumida.com
SD3118
SD3112
SD12
SD10
4.7
3.3
4.7
3.3
4.7
3.3
4.7
3.3
1.3
1.59
0.8
0.97
1.29
1.42
1.08
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
Cooper
www.cooperet.com
LPS3015 4.7
3.3
1.1
1.3
0.2
0.13
3.0 × 3.0 × 1.5
3.0 × 3.0 × 1.5
Coil Craft
www.coilcraft.com
*Typical DCR