LTC3707
14
3707fb
APPLICATIONS INFORMATION
Selection of Operating Frequency
The LTC3707 uses a constant frequency architecture with
the frequency determined by an internal oscillator capacitor.
This internal capacitor is charged by a fi xed current plus
an additional current that is proportional to the voltage
applied to the FREQSET pin.
A graph for the voltage applied to the FREQSET pin vs
frequency is given in Figure 5. As the operating frequency
is increased the gate charge losses will be higher, reducing
effi ciency (see Effi ciency Considerations). The maximum
switching frequency is approximately 310kHz.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. So why would
anyone ever choose to operate at lower frequencies with
larger components? The answer is effi ciency. A higher
frequency generally results in lower effi ciency because
of MOSFET gate charge losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ΔI
L
decreases with higher
inductance or frequency and increases with higher V
IN
:
ΔI
L
=
1
(f)(L)
V
OUT
1–
V
OUT
V
IN
⎛
⎝
⎜
⎞
⎠
⎟
OPERATING FREQUENCY (kHz)
120 170 220 270 320
FREQSET PIN VOLTAGE (V)
3707 F05
2.5
2.0
1.5
1.0
0.5
0
Figure 5. FREQSET Pin Voltage vs Frequency
Accepting larger values of ΔI
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI = 30% • I
OUT(MAX)
or higher for
good load transient response and suffi cient ripple current
signal in the current loop. Remember, the maximum ΔI
L
occurs at the maximum input voltage.
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
25% of the current limit determined by R
SENSE
. Lower
inductor values (higher ΔI
L
) will cause this to occur at
lower load currents, which can cause a dip in effi ciency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High effi ciency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy,
or Kool Mµ
®
cores. Actual core loss is independent of core
size for a fi xed 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 con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance 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 generally lack a bobbin, mounting is more diffi cult.
