LX1669

PRODUCT DA T ABOOK 1996/1997

Copyright © 1999

Rev. 1.0

4/99

12

APPLICA TION INFORMA TION

There are several alternative types of sense resistor. The

surface-mount metal “staple” form of resistor has the advantage of

exposure to free air to dissipate heat and its value can be

controlled very tightly. Its main drawback, however, is cost. An

alternative is to construct the sense resistor using a copper PCB

trace. Although the resistance cannot be controlled as tightly, the

PCB trace is very low cost.

PCB Sense Resistor

A PCB sense resistor should be constructed as shown in Figure 7.

By attaching directly to the large pads for the capacitor and

inductor, heat is dissipated efficiently by the larger copper masses.

Connect the current sense lines as shown to avoid any errors.

Recommended sense resistor sizes are given in the following

table:

Copper

Copper

Desired Resistor

Dimensions (w x l)

Weight

Thickness

Value

mm

inches

68µm

2.5m

Ω

2.5 x 22

0.1 x 0.85

5m

Ω

2.5 x 43

0.1 x 1.7

TABLE 3 - PCB Sense Resistor Selection Guide

2.5m

Ω Sense Resistor

100mil Wide, 850mil Long

Output

Capacitor Pad

Inductor

Sense Lines

FIGURE 7 — Sense Resistor Construction Diagram

FIGURE 8 — Current Sense Circuit

R

L

L

R

S

C

S

V

CS

Current

Sense

Comparator

Load

R

S2

Loss-Less Current Sensing Using Resistance of Inductor

Any inductor has a parasitic resistance (R

voltage drop when current flows through the inductor. Figure 8

shows a sensor circuit comprising of a surface mount resistor, R

and capacitor, C

current sense resistor.

The current flowing through the inductor is a triangle wave. If

the sensor components are selected such that:

L/R

R

L

L/R

|T(j

ω)|

ω

1/R

R

FIGURE 9 — Sensor Gain

The voltage across the capacitor will be equal to the current

flowing through the resistor, i.e.

V

Since V

CS

reflects the inductor current, by selecting the

appropriate R

Design Example

(Pentium II circuit, with a maximum static current of 14.2A)

The gain of the sensor can be characterized as:

The dc/static tripping current I

I

Select L/R

the static one. The dynamic tripping current I

I

V

trip

L/(R

V

trip

R

L