MCP16331 datasheet(19/40 Pages) MICROCHIP

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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,

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

VIN 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 VOUT, 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 (Table 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.

CIN

4.7 µF

none

COUT

20 µF

—



OUT

–

----------------------------t



VIN =12V

VOUT =3.3V

IOUT =500 mA

LPK

I

--------

OUT

Inductor ripple current = 319 mA

Inductor peak current = 660 mA

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