LX1669

PRODUCT DA T ABOOK 1996/1997

13

Copyright © 1999

Rev. 1.0

4/99

APPLICA TION INFORMA TION

General Guidelines for Selecting R

R

Select: R

and C

C

The above equation has taken into account the current-

dependency of the inductance.

Typical values are: R

2.5µH at 0A current.

In cases where R

be lower than the desired short-circuit current limit, a resistor (R

can be put in parallel with C

of components is as follows:

=

C

=

*

Again, select (R

more information.

OUTPUT ENABLE

The LX1669 FET driver outputs are driven to ground by pulling

the soft-start pin below 0.5V.

PROGRAMMING THE OUTPUT VOLTAGE

The output voltage is set by the DAC with a 5-bit digital voltage-

identification (VID) code input (see Table 1). The DAC input is

designed to be compatible with digital circuits. The VID code

may be hard-wired into the package of the processor [as in the

case of a Pentium II or Pentium Pro processor]. If the processor

does not have a VID code, the output voltage can be set by

means of a DIP-switch, jumpers or TTL-compatible digital

circuits. When using a DIP-switch or jumpers, connect the VID

pin to ground (DIP-switch ON) for a low or “0” signal and leave

the VID pin open (DIP-switch OFF) for a high or “1” signal.

FET SELECTION

To insure reliable operation, the operating junction temperature

of the FET switches must be kept below certain limits. The Intel

specification states that 115°C maximum junction temperature

should be maintained with an ambient of 50°C. This is achieved

by properly derating the part, and by adequate heat sinking. One

of the most critical parameters for FET selection is the R

DS(ON)

resistance.

This parameter directly contributes to the power

dissipation of the FET devices, and thus impacts heat sink design,

mechanical layout, and reliability.

In general, the larger the

current handling capability of the FET, the lower the R

be, since more die area is available.

The recommended solution is to use IRL3102 for the high side

and IRL3303 for the low side FET, for the best combination of cost

and performance. Alternative FET’s from any manufacturer could

be used, provided they meet the same criteria for R

Heat Dissipated In Upper MOSFET

The heat dissipated in the top MOSFET will be:

P

Where t

and f

For the IRL3102 (13m

Ω R

will result in typical heat dissipation of 1.92W.

Synchronous Rectification – Lower MOSFET

The lower pass element can be either a MOSFET or a Schottky

diode. The use of a MOSFET (synchronous rectification) will result

in higher efficiency, but at higher cost than using a Schottky diode

(non-synchronous).

Power dissipated in the bottom MOSFET will be:

P

[IRL3303 or 1.76W for the IRL3102]

Non-Synchronous Operation - Schottky Diode

A typical Schottky diode with a forward drop of 0.6V will dissipate

0.6 x 15 * (1-2/5) = 5.4W (compared to the 1.8 to 3.5W dissipated

by a MOSFET under the same conditions).

This power loss

becomes much more significant at lower duty cycles – synchro-

nous rectification is recommended. The use of a dual Schottky

diode in a single TO-220 package (e.g. the MBR2535) helps

improve thermal dissipation.

Device

R

Max. Break-

10V (m

Ω

Ω

Ω

Ω

Ω)T

down Voltage

IRL3803

6

83

30

IRL22203N

7

71

30

IRL3103

14

40

30

IRL3102

13

56

20

IRL3303

26

24

30

IRL2703

40

17

30

TABLE 4 - FET Selection Guide

This table gives selection of suitable FETs from International Rectifier.

All devices in TO-220 package. For surface mount devices (TO-263 /

D

V

trip

I

trip,S

L

n

R

R

L (Required)

R

L (Actual)

R

S2

R

L

R

L

R

L (Actual)

R

R