LTC3600_15 Datasheet, PDF (12/28 Pages) Linear Technology – 15V, 1.5A Synchronous Rail-to-Rail Single Resistor Step-Down Regulator

LTC3600
Applications Information
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoi-
dal wave current at the drain of the top power MOSFET.
To 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_{R M S}
=
I_{O U T(M A X )}


V_{O U T}
V_IN


V_{IN − 1}
V_{O U T}
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.
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, ΔV_OUT , is
determined by:
∆V_{O U T}
≈
∆I_L
8 • f_SW • C_OUT
+ ∆I_L
• R_ESR
The output ripple is highest at maximum input voltage
since ΔI_L increases with input voltage. Multiple capacitors
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.
Their relatively low value of bulk capacitance may require
multiples in parallel.
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, three to four 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 two to three 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 • ∆I_OUT
f_SW • V_DROOP

More capacitance may be required depending on the duty
cycle and load step requirements.
3600fc
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