LX5218

PRODUCT DA T ABOOK 1996/1997

Copyright © 1997

Rev. 1.7

12/97

4

UltraMAX

BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

THERMAL

LIMITING

CIRCUIT

TERM POWER

CURRENT

BIASING

CIRCUIT

24mA CURRENT

LIMITING CIRCUIT

DATA OUTPUT

PIN DB(0)

2.85V

1 OF 9 CHANNELS

DISC1

DISC2

Cable transmission theory suggests to optimize signal speed

and quality, the termination should act both as an ideal voltage

reference when the line is released (deasserted) and as an

ideal current source when the line is active (asserted).

Common active terminators,

which consist of Linear Regulators

in series with resistors (typically

110

Ω), are a compromise. As the

line voltage increases, the amount

of current decreases linearly by the

equation V = I * R. The LX5218,

with its unique new architecture

applies the maximum amount of

current regardless of line voltage

until the termination high thresh-

old (2.85V) is reached.

Acting as a near ideal line ter-

minator, the LX5218 closely reproduces the optimum case

when the device is enabled. To enable the device the DISC1

and DISC2 Pins must be pulled logic High, Open, or any

combination of both High and Low. During this mode of

operation, quiescent current is 6mA and the device will

respond to line demands by delivering 24mA on assertion and

by imposing 2.85V on deassertion.

In order to disable the

device, the DISC1 and DISC2 pins must be driven logic Low.

This mode of operation places the device in a sleep state where

a meager 275µA of quiescent cur-

rent is consumed. Additionally, all

outputs are in a Hi-Z (impedance)

state. Sleep mode can be used for

power conservation or to com-

pletely eliminate the terminator

from the SCSI chain. In the second

case, termination node capacitance

is important to consider. The ter-

minator will appear as a parasitic

distributed capacitance on the line,

which can detract from bus per-

formance.

For this reason, the

LX5218 has been optimized to have only 3pF of capacitance

per output in the sleep state.

An additional feature of the LX5218 is its compatibility with

active negation drivers.

The device handles up to 60mA of

sink current for drivers which exceed the 2.85V output high.

Outputs

Quiescent

Current

H

H

Enabled

6mA

H

L

Enabled

6mA

L

H

Enabled

6mA

L

L

Disabled

275µA

Open

Open

Enabled

6mA

DISC1

DISC2