LTC1624 Datasheet, PDF (7/28 Pages) Linear Technology – High Efficiency SO-8 N-Channel Switching Regulator Controller

LTC1624
APPLICATIONS INFORMATION
The LTC1624 can be used in a wide variety of switching
regulator applications, the most common being the step-
down converter. Other switching regulator architectures
include step-up, SEPIC and positive-to-negative converters.
The basic LTC1624 step-down application circuit is shown
in Figure 1 on the first page. External component selection
is driven by the load requirement and begins with the
selection of R_SENSE. Once R_SENSE is known, the inductor
can be chosen. Next, the power MOSFET and D1 are
selected. Finally, C_IN and C_OUT are selected. The circuit
shown in Figure 1 can be configured for operation up to an
input voltage of 28V (limited by the external MOSFETs).
Step-Down Converter: R_SENSE Selection for
Output Current
R_SENSE is chosen based on the required output current.
The LTC1624 current comparator has a maximum thresh-
old of 160mV/R_SENSE. The current comparator threshold
sets the peak of the inductor current, yielding a maximum
average output current I_MAX equal to the peak value less
half the peak-to-peak ripple current, ∆I_L.
Allowing a margin for variations in the LTC1624 and
external component values yields:
R_SENSE
=
100mV
I_MAX
The LTC1624 works well with values of R_SENSE from
0.005Ω to 0.5Ω.
Step-Down Converter: Inductor Value Calculation
With the operating frequency fixed at 200kHz smaller
inductor values are favored. Operating at higher frequen-
cies generally results in lower efficiency 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 ∆I_L decreases with higher induc-
tance and increases with higher V_IN or V_OUT:
( )( ) ^∆IL
=
V_IN
−
f
V_OUT
L


V_OUT + V_D^
V_IN + V_D 
where V_D is the output Schottky diode forward drop.
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). Remember, the
maximum ∆I_L occurs at the maximum input voltage.
The inductor value also has an effect on low current
operation. Lower inductor values (higher ∆I_L) will cause
Burst Mode operation to begin at higher 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
decrease. In general, inductor values from 5µH to 68µH
are typical depending on the maximum input voltage and
output current. See also Modifying Burst Mode Operation
section.
Step-Down Converter: 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 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 manu-
facturer is Kool Mµ. 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, designs for surface mount that do not
increase the height significantly are available.
Kool Mu is a registered trademark of Magnetics, Inc.
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