LX5219

PRODUCT DA T ABOOK 1996/1997

Copyright © 1998

Rev. 1.8

3/98

4

UltraMAX

BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

THERMAL

LIMITING

CIRCUIT

TERM POWER

CURRENT

BIASING

CIRCUIT

1.4V

DISABLE

PIN

24mA CURRENT

LIMITING CIRCUIT

DATA OUTPUT

PIN DB(0)

2.85V

1 OF 9 CHANNELS

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 LX5219,

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 terminator, the LX5219 closely

reproduces the optimum case when the device is enabled.

To enable the device the Disable Pin must be pulled logic

High or left Open. 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 Disable pin

must be driven logic Low.

This mode of operation places

the device in a sleep state where a meager 275µA of quiescent

current is consumed. Additionally, all outputs are in a Hi-Z

(impedance) state. Sleep mode can

be used for power conservation or

to completely eliminate the termi-

nator from the SCSI chain. In the

second case, termination node ca-

pacitance is important to consider.

The terminator will appear as a

parasitic distributed capacitance on

the line, which can detract from bus

performance. For this reason, the

LX5219 has been optimized to have only 3pF of capacitance

per output in the sleep state.

An additional feature of the LX5219 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.

Disable

Outputs

Quiescent

(LX5219)

Current

H

Enabled

6mA

L

HI Z

275µA

Open

Enabled

6mA