LTC3547B
9
3547bfb
APPLICATIO S I FOR ATIO
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Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter
is a square wave with a duty cycle of approximately
V
OUT
/V
IN
. To prevent large voltage transients, a low equiv-
alent series resistance (ESR) input capacitor sized for
the maximum RMS current must be used. The max-
imum RMS capacitor current is given by:
I
RMS
≈ I
MAX
V
OUT
(V
IN
− V
OUT
)
V
IN
(2)
Where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case is commonly used to
design because even signifi cant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This 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
the size or height requirements of the design. An addi-
tional 0.1μF to 1μF ceramic capacitor is also recommended
on V
IN
for high frequency decoupling when not using an
all-ceramic capacitor solution.
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C
OUT
has been met, the RMS current rating
generally far exceeds the I
RIPPLE(P-P)
requirement. The
output ripple ΔV
OUT
is determined by:
V
OUT
I
L
ESR+
1
8fC
OUT
(3)
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. For a fi xed output
voltage, the output ripple is highest at maximum input
voltage since ΔI
L
increases with input voltage.
If tantalum capacitors are used, it is critical that the capaci-
tors are surge tested for use in switching power supplies.
An excellent choice is the AVX TPS series of surface mount
tantalum. These are specially constructed and tested for low
ESR so they give the lowest ESR for a given volume. Other
capacitor types include Sanyo POSCAP, Kemet T510 and
T495 series, and Sprague 593D and 595D series. Consult
the manufacturer for other specifi c recommendations.
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. Because the
LTC3547B control loop does not depend on the output
capacitor’s ESR for stable operation, ceramic capacitors
can be used freely to achieve very low output ripple and
small circuit size.
However, care must be taken when ceramic capacitors are
used at the input. 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 input, V
IN
. 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. For more information, see Application Note 88.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage charac-
teristics of all the ceramics for a given value and size.
