Datasheet

LTC3103
11
3103f
APPLICATIONS INFORMATION
The basic LTC3103 application circuit is shown as the
Typical Application on the front page of this data sheet.
The external component selection is determined by the
desired output voltage, output current, desired noise im-
munity and ripple voltage requirements for each particular
application. However, basic guidelines and considerations
for the design process are provided in this section.
Inductor Selection
The choice of inductor value influences both the efficiency
and the magnitude of the output voltage ripple. Larger
inductance values will reduce inductor current ripple
and will therefore lead to lower output voltage ripple. For
a fixed DC resistance, a larger value inductor will yield
higher efficiency by lowering the peak current to be closer
to the average. However, a larger value inductor within the
same family will generally have a greater series resistance,
thereby offsetting this efficiency advantage. Given a desired
peak-to-peak current ripple, ∆I
L
(A), the required inductance
can be calculated via the following expression:
L ≥
V
OUT
1.2 • ∆I
L
• 1–
V
OUT
V
IN






µH
( )
A reasonable choice for ripple current is ∆I
L
= 120mA
which represents 40% of the maximum 300mA load
current. The DC current rating of the inductor should be
at least equal to the maximum load current plus half the
ripple current in order to prevent core saturation and loss
of efficiency during operation. To optimize efficiency the
inductor should have a low series resistance. In particularly
space restricted applications it may be advantageous to
use a much smaller value inductor at the expense of larger
ripple current. In such cases, the converter will operate
in discontinuous conduction for a wider range of output
loads and efficiency will be reduced. In addition, there is a
minimum inductor value required to maintain stability of the
current loop (given the fixed internal slope compensation).
Specifically, if the buck converter is going to be utilized at
duty cycles greater than 40%, the inductance value must
be at least 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 using standard induc-
tor values.
Table 1. Minimum Inductance
OUTPUT VOLTAGE (V) MINIMUM INDUCTANCE (µH)
0.8 2.2
1.2 3.3
2.0 5.6
2.7 6.8
3.3 8.3
5.0 15
A large variety of low ESR, power inductors are available that
are well suited to the LTC3103 converter applications. The
trade-off generally involves PCB area, application height,
required output current and efficiency. Table 2 provides a
representative sampling of small surface mount inductors
that are well suited for use with the LTC3103. The induc-
tor specifications listed are 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 fixed inductor size), the DC resistance generally
increases and the maximum current generally decreases
with increased inductance.