LTC3622_15 Datasheet, PDF (13/24 Pages) Linear Technology – 17V, Dual 1A Synchronous Step-Down Regulator with Ultralow Quiescent Current

LTC3622/
LTC3622-2/LTC3622-23/5
Applications Information
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. Typically, five cycles are required to
respond to a load step, but only in the first cycle does the
output voltage drop linearly. The output droop, V_DROOP, is
usually about three times the linear drop of the first cycle.
Thus, a good place to start with the output capacitor value
is approximately:
C_OUT
=
3
ƒ_O
ΔI_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 10µF ceramic capacitor is usually enough
for these conditions. Place this input capacitor as close
to the V_IN1 and V_IN2 pins as possible.
Output Power Good
When the LTC3622’s output voltages are within the ±7.5%
window of the regulation point, the output voltages are good
and the PGOOD pins are pulled high with external resistors.
Otherwise, internal open-drain pull-down devices (275Ω)
will pull the PGOOD pins low. To prevent unwanted PGOOD
glitches during transients or dynamic V_OUT changes, the
LTC3622’s PGOOD falling edge includes a blanking delay
of approximately 32 switching cycles.
Frequency Synchronization Capability
The LTC3622 has the capability to synchronize to a ±50%
range of the internal programmed frequency. It takes
several cycles of external clock to engage the sync mode,
and roughly 2μs for the part to detect the absence of the
external clock signal. Once engaged in sync, the LTC3622
immediately runs at the external clock frequency.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple current:
ΔI_L=
V_OUT
ƒ •L

^1–
V_OUT
V_{IN(M AX )}


Lower ripple current reduces power 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). To guarantee that ripple
current does not exceed a specified maximum, the induc-
tance should be chosen according to:
L
=
ƒ
•
V_OUT
ΔI_{L(M AX )}

^1–
V_OUT
V_{IN(M AX )}


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 or frequency in-
creases, core loss decreases. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses 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
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