LTC1877 Linear Technology, LTC1877 Datasheet - Page 9

• Manufacturer Part Number: LTC1877
• Description: High Efficiency Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

APPLICATIONS
Accepting larger values of I
inductance, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is I
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
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 M
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 M (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
Kool M is a registered trademark of Magnetics, Inc.
I
L
f L
®
1
cores. Actual core loss is independent of core
V
OUT
L
U
= 0.4(I
1
INFORMATION
V
U
V
OUT
MAX
IN
).
L
allows the use of low
W
L
) will cause this
U
(1)
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 continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum
RMS capacitor current is given by:
This formula has a maximum at V
I
monly used for design because even significant deviations
do not offer much relief. Note 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
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple V
where f = operating frequency, C
and I
is highest at maximum input voltage since I
with input voltage. For the LTC1877, the general rule for
proper operation is:
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 volt-
age. The I
RMS
IN
C
C
and C
OUT
IN
V
= I
OUT
L
required I
OUT
= ripple current in the inductor. The output ripple
required ESR < 0.25
OUT
TH
/2. This simple worst-case condition is com-
pin compensation components can be opti-
Selection
I ESR
L
RMS
OUT
OUT
is driven by the required effective
I
OMAX
8
is determined by:
fC
1
OUT
V
OUT
OUT
OUT
/V
= output capacitance
IN
IN
V
IN
. To prevent large
= 2V
V
LTC1877
IN
V
OUT
OUT
L
increases
, where
1 2 /
9