LTC3415 Datasheet, PDF (19/28 Pages) Linear Technology – 7A, PolyPhase Synchronous Step-Down Regulator

LTC3415
APPLICATIONS INFORMATION
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:

I_L
=


V_OUT
fL
_1–
V_OUT
V_IN



Lower ripple current reduces cores losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency, and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). Note that the largest ripple
current occurs at the highest V_IN. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tance should be chosen according to:
L

= _
V_OUT
f
I_L(MAX)

^1–
V_OUT
V_IN(MAX)


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 is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. 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 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!
Different core materials and shapes will change the size/cur-
rent 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. New designs for
surface mount inductors are available from Coiltronics,
Coilcraft, Toko, and Sumida.
Checking Transient Response
The OPTI-LOOP compensation allows the transient re-
sponse to be optimized for a wide range of loads and
output capacitors. The availability of the I_TH pin not only
allows optimization of the control loop behavior but also
provides a DC-coupled and AC filtered closed loop response
test point. The DC step, rise time and settling at this test
point truly reflects the closed loop response. Assuming a
predominantly second order system, phase margin and/or
damping factor can be estimated using the percentage of
overshoot seen at this pin. The bandwidth can also be
estimated by examining the rise time at the pin.
The I_TH external components shown in the Figure 12 circuit
will provide an adequate starting point for most applica-
tions. The series R-C filter sets the dominant pole-zero
loop compensation. The values can be modified slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the final PC layout is done and
the particular output capacitor type and value have been
determined. The output capacitors need to be selected
because their various types and values determine the
loop feedback factor gain and phase. An output current
pulse of 20% to 100% of full load current having a rise
time of 1μs to 10μs will produce output voltage and I_TH
pin waveforms that will give a sense of the overall loop
stability without breaking the feedback loop.
3415fa
19