16
LTC1553
Solving this equation with our typical 5V to 2.8V applica-
tion with a 2µH inductor, we get:
22 056
300 2
2
..
()( )
()()
=
kHz H
A
µ
P-P
Peak inductor current at 11.2A load:
11 2
2
2
12 2..A
A
A+=
The ripple current should generally be between 10% and
40% of the output current. The inductor must be able to
withstand this peak current without saturating, and the
copper resistance in the winding should be kept as low as
possible to minimize resistive power loss. Note that in
circuits not employing the current limit function, the
current in the inductor may rise above this maximum
under short circuit or fault conditions; the inductor should
be sized accordingly to withstand this additional current.
Inductors with gradual saturation characteristics are often
the best choice.
Input and Output Capacitors
A typical LTC1553 design puts significant demands on
both the input and the output capacitors. During constant
load operation, a buck converter like the LTC1553 draws
square waves of current from the input supply at the
switching frequency. The peak current value is equal to the
output load current plus 1/2 peak-to-peak ripple current,
and the minimum value is zero. Most of this current is
supplied by the input bypass capacitor. The resulting RMS
current flow in the input capacitor will heat it up, causing
premature capacitor failure in extreme cases. Maximum
RMS current occurs with 50% PWM duty cycle, giving an
RMS current value equal to I
OUT
/2. A low ESR input
capacitor with an adequate ripple current rating must be
used to ensure reliable operation.
Note that capacitor manufacturers’ ripple current ratings
are often based on only 2000 hours (three months)
APPLICATIONS INFORMATION
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lifetime at rated temperature. Further derating of the input
capacitor ripple current beyond the manufacturer’s speci-
fication is recommended to extend the useful life of the
circuit. Lower operating temperature will have the largest
effect on capacitor longevity.
The output capacitor in a buck converter sees much less
ripple current under steady-state conditions than the input
capacitor. Peak-to-peak current is equal to that in the
inductor, usually 10% to 40% of the total load current.
Output capacitor duty places a premium not on power
dissipation but on ESR. During an output load transient,
the output capacitor must supply all of the additional load
current demanded by the load until the LTC1553 can
adjust the inductor current to the new value. Output
capacitor ESR results in a step in the output voltage equal
to the ESR value multiplied by the change in load current.
An 11A load step with a 0.05Ω ESR output capacitor will
result in a 550mV output voltage shift; this is 19.6% of the
output voltage for a 2.8V supply! Because of the strong
relationship between output capacitor ESR and output
load transient response, the output capacitor is usually
chosen for ESR, not for capacitance value; a capacitor with
suitable ESR will usually have a larger capacitance value
than is needed for energy storage.
Electrolytic capacitors rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC1553 applications. OS-CON
electrolytic capacitors from SANYO and other manufac-
turers give excellent performance and have a very high
performance/size ratio for electrolytic capacitors. Surface
mount applications can use either electrolytic or dry
tantalum capacitors. Tantalum capacitors must be surge
tested and specified for use in switching power supplies.
Low cost, generic tantalums are known to have very short
lives followed by explosive deaths in switching power
supply applications. AVX TPS series surface mount
devices are popular surge tested tantalum capacitors that
work well in LTC1553 applications.
A common way to lower ESR and raise ripple current
capability is to parallel several capacitors. A typical LTC1553
