LTC3633 LINER [Linear Technology], LTC3633 Datasheet - Page 13

• Manufacturer Part Number: LTC3633
• Description: Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
Ferrite designs exhibit very low core loss and are pre-
ferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard”, which means
that inductance collapses abruptly when the peak design
current is exceeded. This results in an abrupt increase in
inductor ripple current, so it is important to ensure that
the core will not saturate.
Different core materials and shapes will change the size/cur-
rent 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
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements
and any radiated fi eld/EMI requirements. Table 1 gives a
sampling of available surface mount inductors.
Table 1. Inductor Selection Table
Würth Electronik WE-HC 744312 Series
Vishay IHLP-2020BZ-01 Series
Toko FDV0620 Series
Coilcraft D01813H Series
TDK RLF7030 Series
INDUCTANCE
(μH)
0.25
0.47
0.72
0.22
0.33
0.47
0.68
0.20
0.47
0.33
0.56
1.0
1.5
1.0
1.2
1.0
1.5
1
(mΩ)
DCR
10.5
12.4
18.3
2.5
3.4
7.5
9.5
5.2
8.2
8.8
4.5
8.3
8.8
9.6
20
10
17
4
CURRENT
MAX
12.4
11.5
9.0
5.3
6.4
(A)
5.7
7.7
6.1
18
16
12
15
12
10
10
11
9
7
DIMENSIONS
5.2 × 5.5
6.9 × 7.3
6 × 8.9
7 × 7.7
7 × 7.7
(mm)
HEIGHT
(mm)
3.8
2.0
5.0
3.2
2
C
The input capacitance, C
dal wave current at the drain of the top power MOSFET.
To prevent large voltage transients from occurring, a low
ESR input capacitor sized for the maximum RMS current is
recommended. The maximum RMS current is given by:
This formula has a maximum at V
I
monly used for design because even signifi cant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life which makes it advisable to further de-
rate the capacitor, or choose a capacitor rated at a higher
temperature than required.
Several capacitors may also be paralleled to meet size or
height requirements in the design. For low input voltage
applications, suffi cient bulk input capacitance is needed
to minimize transient effects during output load changes.
Even though the LTC3633 design includes an overvoltage
protection circuit, care must always be taken to ensure
input voltage transients do not pose an overvoltage hazard
to the part.
The selection of C
resistance (ESR) that is required to minimize voltage ripple
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response. The output ripple, ΔV
approximated by:
When using low-ESR ceramic capacitors, it is more useful
to choose the output capacitor value to fulfi ll a charge stor-
age requirement. During a load step, the output capacitor
RMS
IN
I
RMS
and C
V
≅ I
OUT
OUT
=
OUT
I
< I
OUT MAX
/2. This simple worst case condition is com-
Selection
(
L
ESR +
OUT
)
is determined by the effective series
V
OUT
IN
8 • f • C
, is needed to fi lter the trapezoi-
(
V
V
1
IN
IN
OUT
−
V
OUT
IN
LTC3633
)
= 2V
OUT
, where
13
OUT
3633f
, is