APPLICATIONS INFORMATION
Inductor Selection
IPP
There are many factors to consider in selecting
the inductor including core material, inductance
vs. frequency, current handling capability, effi-
ciency, size and EMI. In a typical SP7653 cir-
cuit, the inductor is chosen primarily for value,
saturationcurrentandDCresistance. Increasing
the inductor value will decrease output voltage
ripple, but degrade transient response. Low in-
ductor values provide the smallest size, but
cause large ripple currents, poor efficiency and
moreoutputcapacitancetosmoothoutthelarger
ripple current. The inductor must be able to
handle the peak current at the switching fre-
quency without saturating, and the copper resis-
tance in the winding should be kept as low as
possible to minimize resistive power loss. A
good compromise between size, loss and cost is
to set the inductor ripple current to be within
20% to 40% of the maximum output current.
IPEAK = IOUT (max)
+
2
and provide low core loss at the high switching
frequency. Low cost powdered iron cores have
a gradual saturation characteristic but can intro-
duceconsiderableACcoreloss,especiallywhen
the inductor value is relatively low and the
ripplecurrentishigh.Ferritematerials,although
more expensive, have an abrupt saturation char-
acteristic with the inductance dropping sharply
when the peak design current is exceeded. Nev-
ertheless, they are preferred at high switching
frequencies because they present very low core
loss while the designer is only required to
prevent saturation. In general, ferrite or
molypermalloy materials are a better choice for
all but the most cost sensitive applications.
The switching frequency and the inductor oper-
ating point determine the inductor value as fol-
lows:
Optimizing Efficiency
The power dissipated in the inductor is equal to
the sum of the core and copper losses. To mini-
mizecopperlosses,thewindingresistanceneeds
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
currentswherethecopperlossesdominate.Core
loss information is usually available from the
magnetic vendor. Proper inductor selection can
affect the resulting power supply efficiency by
more than 15%!
VOUT (VIN (max) −VOUT
)
L =
VIN (max) FS Kr IOUT (max)
where:
fS = switching frequency
Kr = ratio of the AC inductor ripple current to
the maximum output current
The peak to peak inductor ripple current is:
Thecopperlossintheinductorcanbecalculated
using the following equation:
VOUT (VIN (max) −VOUT
)
PL(Cu) = IL2(RMS) RWINDING
IPP
=
VIN(max) FS L
where IL(RMS) is the RMS inductor current that
can be calculated as follows:
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 must be large enough not to
saturate at the peak inductor current
2
1
3
IPP
IOUT(max)
IL(RMS) = IOUT(max) 1 +
(
)
Date: 03/25/05
SP7653 Wide Input Voltage Range, 1.3MHz, Buck Regulator
© Copyright 2005 Sipex Corporation
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