S P613 4
600kHz Synchronous PWM Step Down Controller
APPLICATIONS INFORMATION
I
NDUCTOR
S
ELECTION
There are many factors to consider in selecting
the inductor including cost, efficiency, size and
EMI. In a typical SP6134 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 provide the smallest size, but
cause large ripple currents, poor efficiency and
more output capacitance to smooth out the
larger ripple current. 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 compromise 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
operating point determine the inductor value
as follows:
requirements. The core must be large enough
not to saturate at the peak inductor current
and provide low core loss at the high switching
frequency. Low cost powdered iron cores have
a gradual saturation characteristic but can
introduce 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
inductance dropping sharply when the peak
design current is exceeded. Nevertheless, they
are preferred 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 applications. The power dissipated in
the inductor is equal to the sum of the core
and copper losses. To minimize 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 current
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:
L
=
V
OUT
(
V
IN
(
max
)
−
V
OUT
)
V
IN
(
max
)
F
S
K
r
I
OUT
(
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:
P
L
(
Cu
)
=
I
2
L
(
RMS
)
R
WINDING
where IL(RMS) is the RMS inductor current
that can be calculated as follows:
L
=
V
OUT
(
V
IN
(
max
)
−
V
OUT
)
V
IN
(
max
)
F
S
L
I
L
(
RMS
)
−
I
OUT
(
max
)
1
⎛
I
PP
⎞
⎟
1
+ ⎜
⎜
I
3
⎝
OUT
(
max
)
⎟
⎠
2
O
UTPUT
C
APACITOR
S
ELECTION
The
required
ESR
(Equivalent
Series
Resistance)
and
capacitance
drive
the
selection of the type and quantity of the
output capacitors. The ESR must be small
enough that both the resistive voltage
deviation due to a step change in the load
current and the output ripple voltage do not
8/15
Rev. 2.0.0
I
PEAK
I
=
I
OUT
(
max
)
+
PP
2
Once the required inductor value is selected,
the proper selection of core material is based
on peak inductor current and efficiency
© 2008 Exar Corporation
EXAR [ EXAR CORPORATION ]
