8
LTC1707
with the oscillator synchronized at its minimum frequency,
i.e., to a clock just above the oscillator free-running
frequency. The actual reduction in average current is less
than for peak current.
The basic LTC1707 application circuit is shown in Figure␣ 1a.
External component selection is driven by the load re-
quirement and begins with the selection of L followed by
C
IN
and C
OUT.
Inductor Value Calculation
The inductor selection will depend on the operating fre-
quency of the LTC1707. The internal preset frequency is
350kHz, but can be externally synchronized up to 550kHz.
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. However, oper-
ating at a higher frequency generally results in lower
efficiency because of increased internal gate charge losses.
The inductor value has a direct effect on ripple current. The
ripple current ∆I
L
decreases with higher inductance or
frequency and increases with higher V
IN
or V
OUT
.
∆I
fL
V
V
V
L OUT
OUT
IN
=
()()
−
1
1
(1)
Accepting larger values of ∆I
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 ∆I
L
= 0.4(I
MAX
).
The inductor value also has an effect on Burst Mode
operation. The transition to low current operation begins
when the inductor current peaks fall to approximately
200mA. Lower inductor values (higher ∆I
L
) will cause this
to occur at lower load currents, which can cause a dip in
efficiency in the upper range of low current operation. In
Burst Mode operation, lower inductance values will cause
the burst frequency to increase.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency 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 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 losses 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!
Kool Mµ (from Magnetics, Inc.) is a very good, low loss
core material for toroids with a “soft” saturation character-
istic. Molypermalloy is slightly more efficient at high
(>200kHz) switching frequencies but quite a bit more
expensive. Toroids are very space efficient, especially
when you can use several layers of wire, while inductors
wound on bobbins are generally easier to surface mount.
New designs for surface mount are available from
Coiltronics, Coilcraft and Sumida.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
OUT
/V
IN
. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
CI
VVV
V
IN MAX
OUT IN OUT
IN
required I
RMS
≅
−
()
[]
12/
This formula has a maximum at V
IN
= 2V
OUT
, where
I
RMS
= I
OUT
/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor, or
choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
Kool Mµ is a registered trademark of Magnetics, Inc.
APPLICATIO S I FOR ATIO
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