LX1660/1661

PRODUCT DA T ABOOK 1996/1997

Copyright © 1998

Rev. 1.1

7/98

12

USING THE LX1660/61 DEVICES

Recommended sense resistor sizes are given in the following

table:

The dc/static tripping current I

I

Select L/R

than the static one. The dynamic tripping current I

I

General Guidelines for Selecting R

R

Select: R

and C

C

The above equation has taken into account the current-de-

pendency of the inductance. Typical values are: R

9k

Ω, C

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

C4 ERROR COMPARATOR INPUT BYPASS CAPACITOR

The LX1660/61 device has a unique topology which results in

extremely fast response to transient disturbances. Actual loop

closure is around a comparator. A capacitor should be placed

between the INV and NINV Error Comparator inputs to eliminate

jitter and noise. This capacitor value should be: C = ½ C

C

Refer to Capacitor C4 in the Applica-

tion Information section.

C7 HICCUP CAPACITOR SELECTION

The hiccup capacitor controls two time periods; the ON time

duration of 10% duty cycle mode, and the OFF-time duration

before re-try. The ON:OFF-time ratios will always be 1:10 due to

the current sources which charge (10I) and discharge (I) the

hiccup capacitor. Select C

Duration of reduced D operation = 100ms/µF

R

L

L/R

|T(j

ω)|

ω

1/R

R

FIGURE 9 — Sensor Gain

FIGURE 8 — Current Sense Circuit

R

L

L

R

S

C

S

V

CS

Current

Sense

Comparator

Load

R

S2

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

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

C

S

The voltage across the capacitor will be equal to the current

flowing through the resistor, i.e.

V

Since V

priate R

S

and C

voltage at the desired trip current.

Design Example

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

The gain of the sensor can be characterized as:

V

trip

L/(R

V

trip

R

L

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