LTC3404 Datasheet, PDF (9/16 Pages) Linear Technology – 1.4MHz High Efficiency Monolithic Synchronous Step-Down Regulator

LTC3404
APPLICATIO S I FOR ATIO
∆I_L
=
1
(^f)(^L)
V_OUT


1−
V_OUT
V_IN


(1)
Accepting larger values of ∆I_L allows the use of smaller
inductors, 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
250mA. 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 pre-
ferred 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 cur-
rent 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 inductors are available
from Coiltronics, Coilcraft, Dale 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:
[ ^{( )}] _{CIN required IRMS ≅ IOMAX}
V_OUT V_IN − V_OUT
V_IN
1/ 2
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 the 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 size or height requirements in the
design. Always consult the manufacturer if there is any
question.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple ∆V_OUT is determined by:
∆V_OUT
≅

∆I_{L }ESR
+
1
8fC_OUT


where f = operating frequency, C_OUT = output capacitance
and ∆I_L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I_L increases
with input voltage. For the LTC3404, the general rule for
proper operation is:
C_OUT required ESR < 0.25Ω
The choice of using a smaller output capacitance
increases the output ripple voltage due to the frequency
dependent term but can be compensated for by using
capacitor(s) of very low ESR to maintain low ripple
Kool Mµ is a registered trademark of Magnetics, Inc.
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