Datasheet

LTC3634
14
3634fb
For more information www.linear.com/LTC3634
applicaTions inForMaTion
core loss, greater ESR loss in the output capacitor, and
larger output voltage ripple. Generally, highest efficiency
operation is obtained at low operating frequency with
small ripple current.
A reasonable starting point is to choose a ripple current
somewhere between 600mA and 1.2A peak-to-peak. Note
that the largest ripple current occurs at the highest V
IN
.
Exceeding 1.8A is not recommended in order to minimize
output voltage ripple. To guarantee that ripple current does
not exceed a specified maximum, the inductance should
be chosen according to:
L =
V
OUT
f • ∆I
L(MAX)






1−
V
OUT
V
IN(MAX)






Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value, but is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire, leading to increased DCR
and copper loss.
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 field/EMI requirements. Table 1 gives a
sampling of available surface mount inductors.
Table 1. Inductor Selection Table
INDUCTANCE
(µH)
DCR
(mΩ)
MAX
CURRENT (A)
DIMENSIONS
(mm)
HEIGHT
(mm)
Würth Electronik WE-HC 744310 Series
0.24
0.55
0.95
1.15
2.00
2.1
3.8
6.4
9.0
14.0
18.0
14.0
11.0
8.5
6.5
7
× 7 3.3
Vishay IHLP-2020BZ-01 Series
0.22
0.33
0.47
0.68
1
5.2
8.2
8.8
12.4
20
15
12
11.5
10
7
5.2 × 5.5
2
Toko FDV0620 Series
0.20
0.47
1.0
4.5
8.3
18.3
12.4
9.0
5.7
7 × 7.7
2.0
Coilcraft D01813H Series
0.33
0.56
1.2
4
10
17
10
7.7
5.3
6 × 8.9
5.0
TDK RLF7030 Series
1.0
1.5
8.8
9.6
6.4
6.1
6.9 × 7.3
3.2
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, is needed to filter the trapezoi-
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 for a single
regulator is given by:
I
RMS
= I
OUT(MAX)
V
OUT
V
IN
− V
OUT
( )
V
IN
When both regulators are active, the input current wave-
form is significantly different. Furthermore, the input RMS
current varies depending on each output’s load current as
well as whether V
TT
is sinking or sourcing current.