LTC3617_15 Datasheet, PDF (11/20 Pages) Linear Technology – 6A Monolithic Synchronous Step-Down Regulator for DDR Termination

LTC3617
Applications Information
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ∆I_L increases with higher V_IN and decreases
with higher inductance:
∆I_L
=


V_OUT ^
f_SW • L _
•


1–
V_OUT
V_IN


Having a lower ripple current reduces the core losses
in the inductor, the ESR losses in the output capacitors
and the output voltage ripple. A reasonable starting point
for selecting the ripple current is ∆I_L = 0.3 • I_OUT(MAX).
The largest ripple current occurs at the highest V_IN. To
guarantee that the ripple current stays below a specified
maximum, the inductor value should be chosen according
to the following equation:
L
=



f_SW
V_OUT
• ∆I_L(MAX
)



•

 ^1–

V_OUT
V_{IN(MAX )}



Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. Actual core loss is independent of core size for a
fixed inductor value, but it is very dependent on the induc-
tance selected. As the inductance increases, core losses de-
crease. 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,” meaning that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequently output voltage ripple. Do
not allow a ferrite core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price versus size requirements
and any radiated field/EMI requirements. Table 1 shows
some typical surface mount inductors that work well in
LTC3617 applications.
Input Capacitor (C_IN) Selection
In continuous mode, the source current of the top P-
channel MOSFET is a square wave of duty cycle V_OUT/
V_IN. To prevent large input voltage transients, a low ESR
capacitor sized for the maximum RMS current must be
used at V_IN.
The maximum RMS capacitor current is given by:
I_RMS = I_OUT(MAX) •
V_OUT
V_IN
•


V_IN
V_OUT

– 1_
This formula has a maximum at V_IN = 2 • V_OUT , where I_RMS =
I_OUT/2. This simple worst-case condition is commonly used
for design because even significant deviations do not offer
much relief. Note that ripple current ratings from capacitor
manufacturers are often based on only 2000 hours of life
which 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.
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