MCP16311SLASH2 datasheet(20/40 Pages) MICROCHIP

20 / 40 page

MCP16311/2

DS20005255C-page 20

 2013-2019 Microchip Technology Inc.

5.4

Input Capacitor Selection

The step-down converter input capacitor must filter the

high-input ripple current that results from pulsing or

chopping the input voltage. The MCP16311/2 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 recommended, while for

applications with limited temperature range, a

multi-layer X5R dielectric is acceptable. Typically, input

capacitance between 10 µF and 20 µF is sufficient for

most applications. For applications with 100 mA to

200 mA load, a 4.7 µF to 2.2 µF X7R capacitor can be

used, depending on the input source and its

impedance. In case of an application with high

variations of the input voltage, a higher capacitor value

is recommended. The input capacitor voltage rating

must be VIN plus margin.

Table 5-2 contains the recommended range for the

input capacitor value.

5.5

Output Capacitor Selection

The output capacitor provides 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. For typical applications, the output

capacitance can be as low as 10 µF ceramic and as

high as 100 µF electrolytic. In a typical application, a

20 µF output capacitance usage will result in a 10 mV

output ripple.

The amount and type of output capacitance and

equivalent 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 MCP16311/2. See

Table 5-2 for the recommended output capacitor range.

The output voltage capacitor rating should be a

minimum of VOUT plus margin.

5.6

Inductor Selection

The MCP16311/2 is designed to be used with small

surface-mount inductors. Several specifications should

be considered prior to selecting an inductor. To

optimize system performance, low DCR inductors

should be used.

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 5-4:

EQUATION 5-5:

INDUCTOR PEAK

CURRENT

For this example, an inductor with a current saturation

rating of minimum 960 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

2.2 µF

None

COUT

20 µF

None

VIN =12V

VOUT =3.3V

IOUT =800 mA

TABLE 5-3:

MCP16311/2 RECOMMENDED

3.3V VOUT INDUCTORS

Part Number

Size

WxLxH

(mm)

Coilcraft

XAL4040

0.109

2.8

4.0x4.0x2.1

LPS6235

0.125 2.00

6.0x6.0x3.5

MSS6132

0.135 1.56

6.1x6.1x3.2

XAL6060

0.057 1.78 6.36x6.5x6.1

MSS7341

0.057 1.78

7.3x7.3x4.1



VIN VOUT

–

----------------------------tON



ILPK

I

--------

IOUT

Where:

Inductor ripple current = 319 mA

Inductor peak current = 960 mA

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