February 1997

Active SCSI Terminator

1

GENERAL DESCRIPTION

The ML6509 BiCMOS SCSI terminator provides active

termination in a SCSI system with single ended drivers and

receivers, in full compliance with the SCSI-1, SCSI-2 and

SCSI-3 recommendations. It provides a 2.85V reference

through an internal 1V dropout linear regulator. Active

SCSI termination helps the system designer to effectively

control analog transmission line effects like ringing, noise,

crosstalk, ground bounce, etc. In addition it provides

greater immunity to voltage drops on the TERMPWR line

of the SCSI bus. The desired V-I characteristics for signal

negation requires that the terminator source 0–24mA

while maintaining 2.85V and for signal assertion

preferably follow a linear slope of 110

W. The ML6509

attempts to provide a V-I characteristic optimized to

minimize the transmission line effects during both signal

assertion and negation, using a MOSFET based

architecture. The desired V-I characteristic is achieved by

trimming one resistor in the control block. It provides

negative clamping for signal assertion transients and

current sink capability, to handle active negation driver

overshoots above 2.85V, which is currently accomplished

with external components in SCSI subsystems today. It

provides a disconnect mode, where the terminator is

completely disconnected from the SCSI

bus and the output capacitance is < 5pF, typically.

FEATURES

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Fully monolithic IC solution providing active

termination for 9 lines of the SCSI bus

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Low dropout voltage (1V) linear regulator, trimmed for

accurate termination current, with 300mA current

source capability

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Output capacitance typically <5pF

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Disconnect mode — logic pin to disconnect terminator

from the SCSI bus, <100µA

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Lowpower mode — for power conscious, portable

system & peripheral applications, using less than 6"

cables. (Equivalent to a 1mA current drive with a

2.5k

W termination)

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Current sinking — can sink current >10mA per line to

handle active negation driver overshoots above 2.85V

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Negative clamping on all lines to handle signal

assertion transients

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Regulator can source 200mA and sink 50mA while

maintaining regulation

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Current limit & thermal shutdown protection

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Small and low profile package options; 16-pin SOIC

(300 mil), 20-pin TSSOP (1 mil height)

* Some Packages Are End Of Life As Of August 1, 2000

BLOCK DIAGRAM

NCLAMP = Negative Clamp

TERMPWR

LINEAR

REGULATOR

1V DROPOUT

2.85V

216mA

(MAX)

2.85V

DISCNKT

LPWR

CONTROL

BLOCK

NORMAL MODE

NCLAMP

LOW POWER MODE

9 TERMINATION LINES

. . .

. . .

. . .

NCLAMP

NCLAMP

GND