SP6121CN-L Datasheet, PDF (8/42 Pages) Sipex Corporation – Low Voltage, Synchronous Step Down PWM Controller

Inductor Selection
There are many factors to consider in selecting
the inductor including cost, efficiency, size and
EMI. In a typical SP6121 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 induc-
tor 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 oper-
ating point determine the inductor value as fol-
lows:
L = ^{VOUT (V IN (max) −VOUT )}
V F K I _{IN (max) S r OUT ( max)}
where;
F_S = switching frequency
K_r = ratio of the peak to peak inductor ripple
current to the maximum output current
The peak to peak inductor ripple current is:
I _PP
= ^{VOUT (VIN (max) −VOUT )}
V_IN(max) F_S L
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
I _PEAK
=
I _{OUT (max)}
+ ^IPP
2
and provide low core loss at the high switching
frequency. Low cost powdered iron cores have
a gradual saturation characteristic but can intro-
APPLICATION INFORMATION
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.
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:
P = I R _L(Cu)
2
L ( RMS ) WINDING
where I_L(RMS) is the RMS inductor current that
can be calculated as follows:
( ) ^{IL(RMS) = IOUT(max)}
1
+
1
3
I_PP ²
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
SP6121 adjusts the inductor current to the new
value. Therefore the capacitance must be large
enough so that the output voltage is held up
Date: 11/29/04
SP6121 Low Voltage, Synchronous Step Down PWM Controller
8
© Copyright 2004 Sipex Corporation