LX1660/1661

PRODUCT DA T ABOOK 1996/1997

13

Copyright © 1998

Rev. 1.1

7/98

USING THE LX1660/61 DEVICES

The resulting time is the duration allowed for the 10% duty

cycle drive to be applied to the bottom switch (the top switch is

OFF during current limit). The OFF-time will be fixed at ten times

(10x) this number, and determines the time interval the supply

remains completely OFF until re-try.

If the short has been

removed, the supply will resume normal operation. No power

cycling is necessary to reset the VRM module after a current limit

event.

As an example, if a 0.1µF hiccup capacitor is chosen, the

bottom switch drive will pulse at (approximately) a 10% duty cycle

for 10mA, when current limit is reached. It will then shut OFF for

100ms, at which time a re-try cycle is attempted, which will result

in either normal operation or another 10% duty cycle burst.

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.

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]

Non-Synchronous Operation - Schottky Diode

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

pate 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 – syn-

chronous 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 dis-

sipation.

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 5 (option), the gate drive needs to be higher than 12V

in order to turn the MOSFET on. Capacitor C20 and diodes

D1 & D2 are used as a charge pump voltage doubling circuit

to raise the voltage of V

provides a high enough voltage to turn on Q1. The 12V

supply must always be connected to V

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

2) 12V Supply

When 5V is supplied to the drain of Q1, a 12V supply should

be connected to both V

CURRENT SHARE APPLICATION

Synchronous rectifier stages should not be paralleled unless they

are locked in at the same frequency, or undesirable current

sourcing/sinking could occur. If synchronization is not practical,

the next best alternative is to disable the synchronous (bottom)

switch. This is easily accomplished with the LX1660/61 by pulling

the SYNCEN pin HIGH. In most applications, a 5 to 6% reduction

in efficiency will result when the synchronous driver is disabled.

A Schottky diode of the proper voltage and current ratings should

be installed across the inactive FET to conduct the inductor

current.

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 /

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