LTC3605_15 Datasheet, PDF (11/22 Pages) Linear Technology – 15V, 5A Synchronous Step-Down Regulator

LTC3605
Operation
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input
and the power is supplied by a wall adapter through long
wires, a load step at the output can induce ringing at the
V_IN input. At best, this ringing can couple to the output and
be mistaken as loop instability. At worst, a sudden inrush
of current through the long wires can potentially cause
a voltage spike at V_IN large enough to damage the part.
When choosing the input and output ceramic capacitors,
choose the X5R and X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acteristics of all the ceramics for a given value and size.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation and the
output capacitor size. Typically, 3 to 4 cycles are required
to respond to a load step, but only in the first cycle does
the output drop linearly. The output droop, V_DROOP , is
usually about 2 to 3 times the linear drop of the first cycle.
Thus, a good place to start with the output capacitor value
is approximately:
C_OUT
≈
2.5
f_O
DI_OUT
• V_DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance to
the supply is very low. A 22µF ceramic capacitor is usually
enough for these conditions. Place this input capacitor as
close to the PV_IN pins as possible.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple current:
DI_L
=
V_OUT
f •L
⎛
⎝⎜⎜^1–
V_OUT
V_{IN(MAX )}
⎞
⎠⎟⎟
Lower ripple current reduces core 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 trade-off between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 50% of I_OUT(MAX). This is especially impor-
tant at low V_OUT operation where V_OUT is 1.8V or below.
Care must be given to choose an inductance value that
will generate a big enough current ripple (40% to 50%)
so that the chip’s valley current comparator has enough
signal-to-noise ratio to force constant switching frequency.
Meanwhile, also note that the largest ripple current occurs
at the highest V_IN. To guarantee that ripple current does
not exceed a specified maximum, the inductance should
be chosen according to:
L
=
f
•
V_OUT
DI_{L(MAX )}
•
⎛
⎝⎜⎜^1–
V_OUT
V_{IN(MAX )}
⎞
⎠⎟⎟
However, the inductor ripple must not be so large that its
valley current level can exceed the negative current limit,
which can be as low as –3.5A. If the negative current limit
is exceeded in forced continuous mode of operation, V_OUT
can get charged to above the regulation level until the
inductor current no longer exceeds the negative current
limit. In such instances, choose a larger inductor value
to reduce the inductor ripple current. The alternative is
to reduce the R_T resistor value to increase the switching
frequency in order to reduce the inductor ripple current.
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
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