MCP16331 datasheet(21/40 Pages) MICROCHIP

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 2014 Microchip Technology Inc.

DS20005308B-page 21

MCP16331

EXAMPLE 5-4:

A 0.5A to 1A Diode is recommended.

5.8

Boost Diode

The boost diode is used to provide a charging path from

the low-voltage gate drive source while the switch node

is low. The boost diode blocks the high voltage of the

switch node from feeding back into the output voltage

when the switch is turned on, forcing the switch node

high.

A standard 1N4148 ultra-fast diode is recommended

for its recovery speed, high voltage blocking capability,

availability and cost. The voltage rating required for the

boost diode is VIN.

For low-boost voltage applications, a small Schottky

diode with the appropriately rated voltage can be used

to lower the forward drop increasing the boost supply

for gate drive.

5.9

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

0.1 µF 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.

5.10

Thermal Calculations

The MCP16331 is available in 6-lead SOT-23 and 8-

lead TDFN packages. By calculating the power

dissipation and applying the package thermal

resistance (

JA), the junction temperature is estimated.

To quickly estimate the internal power dissipation for

the switching step-down regulator, an empirical calcu-

lation using measured efficiency can be used. Given

the measured efficiency, the internal power dissipation

is estimated by Equation 5-7. This power dissipation

includes all internal and external component losses.

For a quick internal estimate, subtract the estimated

Schottky diode loss and inductor DCR loss from the

PDIS calculation in Equation 5-7.

EQUATION 5-7:

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 freewheeling Schottky diode

losses are determined by calculating the average diode

current and multiplying by the diode forward drop. The

inductor losses are estimated by PL = IOUT2 x LDCR.

EQUATION 5-8:

DIODE POWER

DISSIPATION ESTIMATE

EXAMPLE 5-5:

TABLE 5-5:

FREEWHEELING DIODES

App

Manufacturer

Part

Number

Rating

12 VIN

500 mA

Diodes



Inc.

DFLS120L-7

20V, 1A

24 VIN

100 mA

Diodes



Inc.

B0540Ws-7

40V, 0.5A

18 VIN

500 mA

Diodes



Inc.

B130L-13-F

30V, 1A

48 VIN

500 mA

Diodes



Inc.

B1100

100V, 1A

IOUT

=0.5A

VIN

= 15V

VOUT

=5V

D= 5/15

IDAVG

= 333 mA

OUT



Efficiency

-------------------------------





OUT





–

Dis

Diode

–

 I

OUT







VIN

=10V

VOUT

=5.0V

IOUT

=0.4A

Efficiency

= 90%

Total System Dissipation

= 222 mW

LDCR

=0.15



=24 mW

Diode VF

=0.50

D= 50%

PDiode

=125 mW

MCP16331 internal power dissipation estimate:

PDIS - PL - PDIODE = 73 mW

JA

=198°C/W

Estimated Junction



Temperature Rise

=+14.5°C

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