LTC3731
14
3731fc
APPLICATIONS INFORMATION
this basic trade-off, the effect of inductor value on ripple
current and low current operation must also be considered.
The PolyPhase approach reduces both input and output
ripple currents while optimizing individual output stages to
run at a lower fundamental frequency, enhancing efficiency.
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I
L
per individual section,
N, decreases with higher inductance or frequency and
increases with higher V
IN
or V
OUT
:
∆I
L
=
V
OUT
fL
1−
V
OUT
V
IN
where f is the individual output stage operating frequency.
In a PolyPhase converter, the net ripple current seen by
the output capacitor is much smaller than the individual
inductor ripple currents due to the ripple cancellation. The
details on how to calculate the net output ripple current
can be found in Application Note 77.
Figure 4 shows the net ripple current seen by the output
capacitors for the different phase configurations. The
output ripple current is plotted for a fixed output voltage
as the duty factor is varied between 10% and 90% on the
x-axis. The output ripple current is normalized against
the inductor ripple current at zero duty factor. The graph
can be used in place of tedious calculations. As shown in
Figure 4, the zero output ripple current is obtained when:
V
OUT
V
IN
=
k
N
where k = 1, 2,...,N –1
So the number of phases used can be selected to minimize
the output ripple current and therefore the output ripple
voltage at the given input and output voltages. In appli-
cations having a highly varying input voltage, additional
phases will produce the best results.
Accepting larger values of ∆I
L
allows the use of low in-
ductances but can result in higher output voltage ripple.
A reasonable starting point for setting ripple current is
∆I
L
= 0.4(I
OUT
)/N, where N is the number of channels and
I
OUT
is the total load current. Remember, the maximum
∆I
L
occurs at the maximum input voltage. The individual
inductor ripple currents are constant, determined by the
input and output voltages and the inductance.
Inductor Core Selection
Once the value for L1 to L3 is determined, the type of induc-
tor must be selected. High efficiency converters generally
cannot afford the core loss found in low cost powdered
iron cores, forcing the use of ferrite, molypermalloy or
Kool Mµ cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
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!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mµ. Toroids are very space effi-
cient, especially when you can use several layers of wire.
Because they lack a bobbin, mounting is more difficult.
However, designs for surface mount are available which
do not increase the height significantly.
DUTY FACTOR (V
OUT
/V
IN
)
0.1 0.2 0.3 0.4
0.5 0.6 0.7 0.8 0.9
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
3731 F04
6-PHASE
12-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
O(P-P)
V
O
/fL
Figure 4. Normalized Peak Output Current
vs Duty Factor [I
RMS
= 0.3(I
O(P-P)
]
