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UCC5638

The UCC5638 is a Multi-mode active terminator with se-

lectable single ended (SE) and low voltage differential

(LVD) SCSI termination integrated into a monolithic com-

ponent.

Mode selection is accomplished with the “diff

sense" signal.

The diff sense signal is a three level signal, which is

driven at each end of the bus by one active terminator.

An LVD SCSI or multi-mode terminator drives the diff

sense line to 1.3 V. If diff sense is at 1.3 V, then bus is in

LVD SCSI mode. If a single ended SCSI device is

plugged into the bus, the diff sense line is shorted to

ground. With diff sense shorted to ground, the terminator

changes to single ended mode to accommodate the SE

device. If an HVD SCSI device is plugged in to the bus,

the diff sense line is pulled high and the terminator shuts

down.

The diff sense line is driven and monitored by the termi-

nator through a 100ms to 300ms noise filter at the DIFFB

input pin. A set of comparators, that allow for ground

shifts, determine the bus status as follows. Any diff sense

signal below 0.5V indicates single ended, between 0.7V

and 1.9V is LVD SCSI and above 2.2V is HVD SCSI.

In the single ended mode, a multi-mode terminator has a

110

terminating resistor connected to a 2.7V termina-

tion voltage regulator. The 2.7V regulator is used on all

Unitrode terminators designed for 3.3V systems. This re-

quires the terminator to operate in specification down to

2.7V TRMPWR voltage to allow for the 3.3V supply toler-

ance, an unidirectional fusing device and cable drop. At

each L+ pin, a ground driver drives the pin to ground,

while in single ended mode. The ground driver is spe-

cially designed so it will not effect the capacitive balance

of the bus when the device is in LVD SCSI or disconnect

mode. The device requirements call for 0.5pF balance on

the lines of a differential pair. The terminator capacitance

has to be a small part of the capacitance imbalance.

Layout is very critical for Ultra2, Ultra3, Ultra160 and

Ultra320 systems. Multi-layer boards need to adhere to

the 120

impedance standard, including connector and

feed-through. This is normally done on the outer layers

with 4 mil etch and 4 mil spacing between the runs within

a pair, and a minimum of 8 mil spacing to the next pair.

This spacing between the pairs reduces potential

crosstalk. Beware of feed-throughs and each through

hole connection adds a lot of capacitance. Standard

power and ground plane spacing yields about 1pF to

each plane. Each feed-through will add about 2.5pF to

3.5pF. Enlarging the clearance holes on both power and

ground planes can reduce the capacitance and opening

up the power and ground planes under the connector

can reduce the capacitance for through hole connector

applications. Microstrip technology is normally too low of

impedance and should not be used. It is designed for

50

rather than 120

differential systems.

Capacitance

balance

is

critical

for

Ultra2

and

Ultra3/Ultra160. The balance capacitance standard is

0.5pF per line with the balance between pairs of 2pF.

The components are designed with very tight balance,

typically 0.1pF between pins in a pair and 0.3pF between

pairs. Layout balance is critical, feed-throughs and etch

length must be balanced, preferably no feed-throughs

would be used. Capacitance for devices should be meas-

ured in the typical application, material and components

above and below the circuit board effect the capacitance.

Multi-mode terminators need to consider power dissipa-

tion; the UCC5638 is offered in a power package with

heat sink ground pins. These heat sink/ground pins are

directly connected to the die mount paddle under the die

and conduct heat from the die to reduce the junction

temperature. These pins need to be connected to etch

area or a feed-through per pin connecting to the ground

plane layer on a multi-layer board.

In 3.3V TRMPWR systems, the UCC3912 should be

used to replace the fuse and diode. This reduces the

voltage drop, allowing for cable drop to the far end termi-

nator. 3.3V battery systems normally have a 10% toler-

ance. The UCC3912 is 150mV drop under LVD SCSI

loads, allowing 150mV drop in the cable system. All Uni-

trode LVD SCSI and multi-mode terminators are de-

signed for 3.3V systems, operating down to 2.7V.

APPLICATION INFORMATION