MSS7341 datasheet(18/38 Pages) MICROCHIP

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MCP16301

DS25004A-page 18

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 MCP16301 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

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 recommended, while for

applications with limited temperature range, a multi-

layer X5R dielectric is acceptable. Typically, input

capacitance between 4.7 µF and 10 µF is sufficient for

most applications. For applications with 100 mA to

200 mA load, a 1 µF X7R capacitor can be used,

depending on the input source and its impedance.

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 MCP16301 is internally compensated, so the

output capacitance range is limited. See Table 5-2 for

the recommended output capacitor range.

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 MCP16301.

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 MCP16301 is designed to be used with small sur-

face mount inductors. Several specifications should be

considered prior to selecting an inductor. To optimize

system performance, the inductance value is deter-

mined 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

An inductor saturation rating minimum of 760 mA is

recommended. Low ESR 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

2.2 µF

none

COUT

20 µF

none

------t

VIN =12V

VOUT =3.3V

IOUT =600 mA

LPK

ΔI

--------

OUT

Inductor ripple current = 319 mA

Inductor peak current = 760 mA

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