LX1663CD datasheet(13/15 Pages) MICROSEMI

13 / 15 page

SINGLE-CHIP PROGRAMMABLE PWM CONTROLLERS WITH 5-BIT DAC

LX1662/62A, LX1663/63A

PRODUCT DA T ABOOK 1996/1997

Rev. 1.1 11/99

P RODUCTION D ATA S HEET

USING THE LX1662/63 DEVICES

CURRENT LIMIT (continued)

PCB Sense Resistor

A PCB sense resistor should be constructed as shown in Figure

10. 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.

Copper

Desired Resistor

Dimensions (w x l)

Weight Thickness

Value

inches

2 oz/ft2

68µm

2.5mΩ

2.5 x 22

0.1 x 0.85

5mΩ

2.5 x 43

0.1 x 1.7

TABLE 3 - PCB Sense Resistor Selection Guide

Recommended sense resistor sizes are given in the following

table:

CURRENT LIMIT (continued)

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

sensor components are selected such that:

L/R

L = RS * CS

The voltage across the capacitor will be equal to the current

flowing through the resistor, i.e.

CS = ILRL

Since V

reflects the inductor current, by selecting the

appropriate R

S and CS, VCS can be made to reach the comparator

voltage (60mV for LX166xA or 100mV for the LX166x) 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:

Loss-Less Current Sensing Using Resistance of Inductor

Any inductor has a parasitic resistance, R

L, which causes a DC

voltage drop when current flows through the inductor. Figure 11

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

and capacitor, C

S, in parallel with the inductor, eliminating the

current sense resistor.

2.5m

9 Sense Resistor

100mil Wide, 850mil Long

2.5mm x 22mm (2 oz/ft2 copper)

Output

Capacitor Pad

Inductor

Sense Lines

FIGURE 10 — Sense Resistor Construction Diagram

L/R

SCS

|T(

j M )|

1/R

SCS

L/L

FIGURE 12 — Sensor Gain

FIGURE 11 — Current Sense Circuit

Current

Sense

Comparator

Load

The dc/static tripping current I

trip,S satisfies:

trip,S =

Select L/R

SCS ≤ RL to have higher dynamic tripping current

than the static one. The dynamic tripping current I

trip,d satisfies:

trip,d =

General Guidelines for Selecting R

S , CS , and RL

L =

Select: R

S ≤ 10 kΩ

and C

S according to:

Sn =

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

pendency of the inductance.

The test circuit (Figure 6) used the following parameters:

L = 3mΩ, RS = 9kΩ, CS = 0.1µF, and L is 2.5µH at 0A current.

trip

L/(R

SCS)

trip

trip,S

L RS

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