Datasheet

7
LTC1779
A smaller value than L
MIN
could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
R
SENSE
Selection for Output Current
The selection of R
SENSE
determines the output current
limit, the maximum possible output current before the
internal current limit threshold is reached. I
OUT(MAX)
, the
maximum specified output current in a design, must be
less than I
CL
. With the current comparator monitoring the
voltage developed across R
SENSE
, the threshold of the
comparator determines the inductor’s peak current. The
maximum output current, I
CL
, the LTC1779 can provide is
given by:
IM
SF V
R
I
CL
SENSE
RIPPLE
=






+Ω






100
012
22
.
–
where SF and M are as defined in the previous section,
Figures 2 and 3. Typically, R
SENSE
is chosen between 0Ω
and 20Ω. Current limit is at a minimum at minimum input
voltage and maximum at maximum input voltage. Both
conditions should be considered in a design where current
limit is important.
To calculate several current limit conditions and choose
the best sense resistor for your design, first use minimum
input voltage. Calculate the duty cycle at minimum input
voltage.
DC
V
V
OUT
IN MIN
=
()
Choose the slope factor, SF, from Figure 2 based on the
duty cycle. The ripple current calculated at minimum input
voltage and the chosen L should be used in the current
limit equation (see Inductor Value Calculation). Figure 3
provides several values of R
SENSE
and their corresponding
M values at different input voltages. Select the minimum
input voltage and calculate the resulting minimum current
limit settings.
The process must be repeated for maximum current limit
using duty cycle, slope factor, ripple current and mirror
ratio based on maximum input voltage in order to choose
the best sense resistor for a particular design and to
understand how it is going to work over the entire input
voltage range.
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 Mu
®
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 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!
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 manu-
facturer is Kool Mu. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
Output Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
internal P-channel switch duty cycle. At high input volt-
ages the diode conducts most of the time. As V
IN
ap-
proaches V
OUT
the diode conducts only a small fraction of
the time. The most stressful condition for the diode is
when the output is short-circuited. Under this condition
the diode must safely handle I
PK
at close to 100% duty
cycle. Therefore, it is important to adequately specify the
diode peak current and average power dissipation so as
not to exceed the diode ratings.
Kool Mu is a registered trademark of Magnetics, Inc.
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