MCP16331 datasheet(22/48 Pages) MICROCHIP

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MCP16331

DS20005308D-page 22

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5.7

Freewheeling Diode

The freewheeling diode creates a path for inductor

current flow after the internal switch is turned off. The

average diode current is dependent upon the output

load current and the duty cycle (D). The efficiency of

the converter is a function of the forward drop and

speed of the freewheeling diode. A low forward drop

Schottky diode is recommended. The current rating

and voltage rating of the diode is application-depen-

dent. The diode voltage rating should be a minimum of

VIN plus margin. The average diode current can be cal-

culated using Equation 5-6.

EQUATION 5-6:

DIODE AVERAGE

CURRENT

EXAMPLE 5-4:

In case of the aforementioned example, the usage of a

0.5A to 1A diode is suggested and a list of

recommended freewheeling diodes is shown in

Table 5-5, below.

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 should exceed 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 the 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.

In case of a noise-sensitive application, an additional

resistor, connected in series with the boost capacitor,

that will reduce the high-frequency noise associated

with switching power supplies can be added. A typical

value for the resistor is 82

.

5.10

Thermal Calculations

The MCP16331 device is available in the 6-lead

SOT-23 and 8-lead TDFN packages. By calculating the

power dissipation and applying the package thermal

resistance (

JA), the junction temperature can be

estimated.

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 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 it by the diode forward drop.

The

inductor

losses

are

estimated

PL = IOUT2 x LDCR.

EQUATION 5-8:

DIODE POWER

DISSIPATION ESTIMATE

TABLE 5-5:

FREEWHEELING DIODES

App

Mfr.

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

IDAVG

–

 I

OUT



IOUT =0.5A

VIN =15V

VOUT =5V

D = 5/15

IDAVG =333 mA

VOUT IOUT



Efficiency

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





VOUT IOUT





–

PDis

PDiode

–

 I

OUT







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