LTC3413
8
3413fc
APPLICATIONS INFORMATION
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔI
L
increases with higher V
IN
or V
OUT
and
decreases with higher inductance.
Δ=
⎛
⎝
⎜
⎞
⎠
⎟
I
fL
V
V
V
L OUT
OUT
IN
1
1
()( )
–
Having a lower ripple current reduces the core losses in
the inductor, the ESR losses in the output capacitors and
the output voltage ripple. Highest effi ciency operation is
achieved at low frequency with small ripple current. This,
however, requires a large inductor.
A reasonable starting point for selecting the ripple current
is ΔI
L
= 0.4(I
MAX
). The largest ripple current occurs at the
highest V
IN
. To guarantee that the ripple current stays
below a specifi ed maximum, the inductor value should
be chosen according to the following equation:
L
V
fI
V
V
OUT
LMAX
OUT
IN MAX
=
Δ
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
() ()
–1
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fi xed inductor value, but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core losses and are used
often at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
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 and consequent output voltage ripple. Do
not allow the core to saturate!
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
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 shows some recommended surface mount induc-
tors for LTC3413 applications.
Table 1. Recommended Surface Mount Inductors
MANUFACTURER PART NUMBER
VALUE
(μH)
DCR
(mΩ)
Murata LQH55DNR47M01 0.47 13.0
Vishay/Dale IHLP252CZPJR47M01 0.47 4.2
Pulse P1166.681T 0.44 6.0
Cooper SD20-R47 0.47 20.0
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, is needed to fi lter the trapezoidal
wave current at the source of the top MOSFET. To prevent
large voltage transients from occurring, a low ESR input
capacitor sized for the maximum RMS current should be
used. The maximum RMS current is given by:
II
V
V
V
V
RMS OUT MAX
OUT
IN
IN
OUT
=
()
–1
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/2. This simple worst-case condition is commonly 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 derate 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.
The selection of C
OUT
is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
