7447786004 datasheet(21/34 Pages) MICROCHIP

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DS22285A-page 21

MCP16321/2

5.0.7

BOOST CAPACITOR

The boost capacitor is used to supply current for the

internal high-side drive circuitry that is above the input

voltage. The boost capacitor must store enough energy

to completely drive the high-side switch on and off. A

22 nF X5R or X7R capacitor is recommended for all

applications. The boost capacitor maximum voltage is

5.5V, so a 6.3V or 10V rated capacitor is

recommended.

5.0.8

THERMAL CALCULATIONS

The MCP16321/2 is available in a 3x3 QFN-16

package. By calculating the power dissipation and

applying the package thermal resistance (θJA), the

junction temperature is estimated. The maximum

continuous junction temperature rating for the

MCP16321/2 is +125°C.

To quickly estimate the internal power dissipation for

the switching step-down regulator, an empirical

calculation using measured efficiency can be used.

Given the measured efficiency, the internal power

dissipation is estimated in Equation 5-4. This power

dissipation

includes

all

internal

and

external

component losses. For a quick internal estimate,

subtract the estimated inductor ESR loss from the PDIS

calculation in Equation 5-4.

EQUATION 5-4:

TOTAL POWER

DISSIPATION ESTIMATE

The difference between the first term, input power, and

the second term, power delivered, is the total system

power dissipation. The inductor losses are estimated

by PL =IOUT2 xLESR.

EXAMPLE 5-5:

POWER DISSIPATION –

MCP16321

EXAMPLE 5-6:

POWER DISSIPATION –

MCP16322

DIS

OUT

Efficency

-------------------------------V

OUT

()

–

VIN =12V

VOUT =5.0V

IOUT =1A

Efficiency = 92.5%

Total System Dissipation = 405 mW

LESR =0.04 Ω

PL =40 mW

MCP16321

internal power dissipation estimate:

PDIS –PL =365 mW

θJA =38.5°C/W

Estimated Junction = +14.052°C

Note 1:

θJA = 38.5°C/W for a 4-layer FR4 Printed

Circuit Board with a 13.5 in2, 1 oz internal

copper ground plane.

A smaller ground plane will result in a

larger

θJA temperature rise.

VIN =12V

VOUT =3.3V

IOUT =2A

Efficiency = 87.5%

Total System Dissipation = 943 mW

LESR =0.04 Ω

PL = 160 mW

MCP16322

internal power dissipation estimate:

PDIS –PL =783 mW

θJA =38.5°C/W

Estimated Junction = +30.14°C

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