14
Date: 1/21/05 SP6120 Low Voltage, AnyFET
TM
, Synchronous, Buck Controller © Copyright 2005 Sipex Corporation
Inductor Selection
There are many factors to consider in selecting
the inductor including cost, efficiency, size and
EMI. In a typical SP6120 circuit, the inductor is
chosen primarily for value, saturation current
and DC resistance. Increasing the inductor value
will decrease output voltage ripple, but degrade
transient response. Low inductor values pro-
vide the smallest size, but cause large ripple
currents, poor efficiency and more output ca-
pacitance to smooth out the larger ripple cur-
rent. The inductor must also be able to handle
the peak current at the switching frequency
without saturating, and the copper resistance in
the winding should be kept as low as possible to
minimize resistive power loss. A good compro-
mise between size, loss and cost is to set the
inductor ripple current to be within 20% to 40%
of the maximum output current.
The switching frequency and the inductor oper-
ating point determine the inductor value as
follows:
(max)(max )
(max)
)(
OUTrSIN
OUTINOUT
IKFV
VVV
L
−
=
where:
F
S
= switching frequency
K
r
= ratio of the ac inductor ripple current to
the maximum output current
The peak to peak inductor ripple current is:
LFV
VVV
I
SIN
OUTINOUT
PP
(max)
(max)
)( −
=
Once the required inductor value is selected, the
proper selection of core material is based on
peak inductor current and efficiency require-
ments.
The core material must be large enough not to
saturate at the peak inductor current
2
(max)
PP
OUTPEAK
I
II +=
and provide low core loss at the high switching
frequency. Low cost powdered iron cores have
a gradual saturation characteristic but can intro-
duce considerable ac core loss, especially when
the inductor value is relatively low and the
ripple current is high. Ferrite materials, on the
other hand, are more expensive and have an
abrupt saturation characteristic with the induc-
tance dropping sharply when the peak design
current is exceeded. Nevertheless, they are pre-
ferred at high switching frequencies because
they present very low core loss and the design
only needs to prevent saturation. In general,
ferrite or molypermalloy materials are better
choice for all but the most cost sensitive appli-
cations.
The power dissipated in the inductor is equal to
the sum of the core and copper losses. To mini-
mize copper losses, the winding resistance needs
to be minimized, but this usually comes at the
expense of a larger inductor. Core losses have a
more significant contribution at low output cur-
rent where the copper losses are at a minimum,
and can typically be neglected at higher output
currents where the copper losses dominate. Core
loss information is usually available from the
magnetic vendor.
The copper loss in the inductor can be calculated
using the following equation:
WINDINGRMSLCuL
RIP
2
)()(
=
where I
L(RMS)
is the RMS inductor current that
can be calculated as follows:
I
L(RMS)
= I
OUT(max)
1 +
1
(
I
PP
)
2
3 I
OUT(max)
Output Capacitor Selection
The required ESR (Equivalent Series Resis-
tance) and capacitance drive the selection of the
type and quantity of the output capacitors. The
ESR must be small enough that both the resis-
tive voltage deviation due to a step change in the
load current and the output ripple voltage do not
exceed the tolerance limits expected on the
output voltage. During an output load transient,
the output capacitor must supply all the addi-
tional current demanded by the load until the
APPLICATIONS INFORMATION
