LX1662/62A, LX1663/63A

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.1 11/99

14

Device

R

I

Max. Break-

10V (mΩ

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 /

USING THE LX1662/63 DEVICES

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

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

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.

For the IRL3102 (13m

Ω R

will result in typical heat dissipation of 1.48W.

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 1.12W for the IRL3102]

Catch Diode – Lower MOSFET

A low-power Schottky diode, such as a 1N5817, is recommended

to be connected between the gate and source of the lower

MOSFET when operating from a 12V-power supply (see Figure 9).

This will help protect the controller IC against latch-up due to the

inductor voltage going negative. Although latch-up is unlikely, the

use of such a catch diode will improve reliability and is highly

recommended.

Non-Synchronous Operation - Schottky Diode

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

0.6* 14* [1 – 2.8/5] = 3.7W (compared to the 1.1 to 2.2W dissipated

by a MOSFET under the same conditions). This power loss

becomes much more significant at lower duty cycles – synchro-

nous rectification is recommended especially when a 12V-power

input is used. The use of a dual Schottky diode in a single TO-220

package (e.g. the MBR2535) helps improve thermal dissipation.

MOSFET GATE BIAS

The power MOSFETs can be biased by one of two methods:

charge pump or 12V supply connected to V

1) Charge Pump (Bootstrap)

When 12V is supplied to the drain of the MOSFET, as in

Figure 9, the gate drive needs to be higher than 12V in order

to turn the MOSFET on. Capacitor C

are used as a charge pump voltage doubling circuit to raise

the voltage of V

high enough voltage to turn on Q

The 12V supply must

always be connected to V

itself, as well as gate drive for the bottom MOSFET.

2) 12V Supply

When 5V is supplied to the drain of Q

be connected to both V

R

L (Required)

R

L (Actual)

R

S2

R

L

R

L

R

L (Actual)

R

R

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