Datasheet

 2014 Microchip Technology Inc. DS20005308B-page 19
MCP16331
5.4 Input Capacitor Selection
The step-down converter input capacitor must filter the
high-input ripple current, as a result of pulsing or
chopping the input voltage. The MCP16331 input
voltage pin is used to supply voltage for the power train
and as a source for internal bias. A low Equivalent
Series Resistance (ESR), preferably a ceramic
capacitor, is recommended. The necessary
capacitance is dependent upon the maximum load
current and source impedance. Three capacitor
parameters to keep in mind are the voltage rating,
equivalent series resistance and the temperature
rating. For wide temperature-range applications, a
multi-layer X7R dielectric is mandatory, while for
applications with limited temperature range, a multi-
layer X5R dielectric is acceptable. Typically, input
capacitance between 4.7
µF and 20 µF is sufficient for
most applications.
The input capacitor voltage rating should be a minimum
of V
IN
plus margin. Table 5-2 contains the
recommended range for the input capacitor value.
5.5 Output Capacitor Selection
The output capacitor helps in providing a stable output
voltage during sudden load transients, and reduces the
output voltage ripple. As with the input capacitor, X5R
and X7R ceramic capacitors are well suited for this
application.
The amount and type of output capacitance and equiv-
alent series resistance will have a significant effect on
the output ripple voltage and system stability. The
range of the output capacitance is limited due to the
integrated compensation of the MCP16331.
The output voltage capacitor voltage rating should be a
minimum of V
OUT
, plus margin.
Table 5-2 contains the recommended range for the
input and output capacitor value:
5.6 Inductor Selection
The MCP16331 is designed to be used with small
surface mount inductors. Several specifications should
be considered prior to selecting an inductor. To
optimize system performance, the inductance value is
determined by the output voltage (Tabl e 5-1) so the
inductor ripple current is somewhat constant over the
output voltage range.
EQUATION 5-4: INDUCTOR RIPPLE
CURRENT
EXAMPLE 5-3:
EQUATION 5-5: INDUCTOR PEAK
CURRENT
For the example above, an inductor saturation rating of
minimum 660
mA is recommended. Low DCR
inductors result in higher system efficiency. A trade-off
between size, cost and efficiency is made to achieve
the desired results.
TABLE 5-2: CAPACITOR VALUE RANGE
Parameter Min. Max.
C
IN
4.7 µF none
C
OUT
20 µF —

I
L
V
IN
V
OUT
–
L
--------------------------- -t
ON
 =
V
IN
=12V
V
OUT
=3.3V
I
OUT
=500 mA
I
LPK
 I
L
2
-------- I
OUT
+=
Inductor ripple current = 319 mA
Inductor peak current = 660 mA