IRF7821 datasheet(13/18 Pages) RICHTEK

13 / 18 page

DS8298-01

November 2011

www.richtek.com

RT8298

is a registered trademark of Richtek Technology Corporation.

External N-MOSFET Selection

The RT8298 is designed to operate using an external low

side N-MOSFET. Important parameters for the power

MOSFETs are the breakdown voltage (BVDSS), threshold

voltage (VGS_TH), on-resistance (RDS(ON)), total gate charge

(Qg) and maximum current (ID(MAX)). The gate driver voltage

is from internal regulator (5V, VCC). Therefore logic level

N-MOSFET must be used in the RT8298 application. The

total gate charge (Qg) must be less than 50nC, lower Qg

characteristics results in lower power losses. Drain-source

on-resistance (RDS(ON)) should be as small as possible,

less than 30m

Ω is desirable. Lower RDS(ON) results in

higher efficiency.

Table 2. External N-MOSFET Selection

Part No.

Manufacture

Si7114

Vishay

A04474

ALPHA & OMEGA

FDS6670AS

Fairchild

IRF7821

International Rectifier

Inductor Selection

The inductor value and operating frequency determine the

ripple current according to a specific input and output

voltage. The ripple current

ΔIL increases with higher VIN

and decreases with higher inductance.

OUT

I =

⎡⎤ ⎡

⎤

Δ× −

⎢⎥ ⎢

⎥

⎣⎦ ⎣

⎦

Having a lower ripple current reduces not only the ESR

losses in the output capacitors but also the output voltage

ripple. High frequency with small ripple current can reduce

voltage. For the highest efficiency operation, however, it

requires a large inductor to achieve this goal.

OUT

L(MAX)

IN(MAX)

L =

⎡⎤ ⎡

⎤

×−

⎢⎥ ⎢

⎥

×Δ

⎣⎦ ⎣

⎦

The inductor's current rating (cause a 40

°C temperature

rising from 25

°C ambient) should be greater than the

maximum load current and its saturation current should

be greater than the short circuit peak current limit. Please

see Table 3 for the inductor selection reference.

Table 3. Suggested Inductors for Typical

Application Circuit

Component

Supplier

Series

Dimensions

(mm)

10 x 10 x 4

Zenithtek

ZPWM

6 x 6 x 3

74477

10 x 10 x 4

TAIYOYUDEN

NR8040

8 x 10 x 4

OUT

RMS

OUT(MAX)

OUT

= I

−

CIN and COUT Selection

The input capacitance, CIN, is needed to filter the

trapezoidal current at the source of the high side MOSFET.

To prevent large ripple current, a low ESR input capacitor

sized for the maximum RMS current should be used. The

approximate RMS current equation is given :

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT / 2. This simple worst case condition is

commonly used for design because even significant

deviations do not offer much relief.

Choose a capacitor rated at a higher temperature than

required. Several capacitors may also be paralleled to

meet size or height requirements in the design.

where fsw is the switching frequency, tON (min) is the

minimum switch on time (100ns). This equation shows

that the minimum duty cycle increases when the switching

frequency is increased. Therefore, slower switching

frequency is necessary to achieve high VIN/VOUT ratio

application.

For the ripple current selection, the value of

ΔIL= 0.24(IMAX)

will be a reasonable starting point. The largest ripple

current occurs at the highest VIN. To guarantee that the

ripple current stays below the specified maximum, the

inductor value should be chosen according to the following

equation :

IRF7821 датащи(PDF) 13 Page - Richtek Technology Corporation

IRF7821 датащи(HTML) 13 Page - Richtek Technology Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

датащи скачать

ссылки URL