LX1681/1682

PRODUCT DA T ABOOK 1996/1997

7

Copyright © 1999

Rev. 1.0 5/99

APPLICA TION INFORMA TION

capacitor current plus the load current. The soft-start capacitor

should be selected so that the overall inductor current does not

exceed it maximum.

The capacitor current to follow the SS-pin voltage is:

where C

should be in the range of 0.1 to 0.2µF.

During the soft-start interval the load current from a micro-

processor is negligible; therefore, the capacitor current is ap-

proximately the required inductor current.

OVER-CURRENT PROTECTION

Current limiting occurs at current level I

detected by the current sense comparator is greater than the

current sense comparator threshold, V

I

So,

R

=

Example:

For 10A current limit, using IRL3303 MOSFET (26m

Ω R

R

= 3.1k

Ω

Current Sensing Using Sense Resistor

The method of current sensing using the R

MOSFET is economical, but can have a large tolerance, since the

R

A more accurate alterna-

tive is to use an external sense resistor (R

Since one input

to the current sense comparator is the supply voltage to the IC

(V

pin 8), the sense resistor could be a PCB trace (for

construction details, see Application Note AN-10 or LX1668 data

sheet).

The over-current trip point is calculated as in the equations

above, replacing R

Example:

For 10A current limit, using a 5m

Ω sense resistor:

R

=

= 7.8k

Ω

V

I

SET

I

=

* e

-(t/RssCss )

dV

dt

C

OUT

C

SS

400mV - I

45µA

0.4 - 10 * 0.026

45 * 10

-6

OUTPUT ENABLE

The LX1681/82 FET driver outputs are driven to ground by

pulling the soft-start pin below 0.3V.

PROGRAMMING THE OUTPUT VOLTAGE

The output voltage is sensed by the feedback pin (V

a 1.25V reference. The output voltage can be set to any voltage

above 1.25V (and lower than the input voltage) by means of a

resistor divider (see Product Highlight).

V

Note: Keep 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

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.

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 1 - 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 /

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.

V

I

SET

0.4 - 10 * 0.005