LTC3624_15 Datasheet, PDF (11/20 Pages) Linear Technology – 17V, 2A Synchronous Step-Down Regulator with 3.5A Quiescent Current

LTC3624/LTC3624-2
Applications Information
prevent large voltage transients from occurring, a low
ESR input capacitor sized for the maximum RMS current
should be used. The maximum RMS current is given by:
I_RMS
≅ I_OUT(MAX)
V_OUT
V_IN
V_{IN – 1}
V_OUT
This formula has a maximum at V_IN = 2V_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. For low input
voltage applications, sufficient bulk input capacitance is
needed to minimize transient effects during output load
changes.
Output Capacitor (C_OUT) Selection
The selection of C_OUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response. The output ripple, ∆V_OUT, is
determined by:
∆V_OUT
<
∆I_L

8 •
1
f •C_OUT
+ESR^
The output ripple is highest at maximum input voltage
since ∆I_L increases with input voltage. Multiple capaci-
tors placed in parallel may be needed to meet the ESR
and RMS current handling requirements. Dry tantalum,
special polymer, aluminum electrolytic, and ceramic
capacitors are all available in surface mount packages.
Special polymer capacitors are very low ESR but have
lower capacitance density than other types. Tantalum
capacitors have the highest capacitance density but it is
important to only use types that have been surge tested
for use in switching power supplies. Aluminum electrolytic
capacitors have significantly higher ESR, but can be used
in cost-sensitive applications provided that consideration
is given to ripple current ratings and long-term reliability.
Ceramic capacitors have excellent low ESR characteristics
and small footprints.
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. 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
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