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UCC5638 Datasheet(PDF) 4 Page - Texas Instruments

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Part No. UCC5638
Description  Multimode SCSI 15 Line Terminator
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com

UCC5638 Datasheet(HTML) 4 Page - Texas Instruments

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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-
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
The diff sense line is driven and monitored by the termi-
nator through a 50Hz noise filter at the DIFFB input pin.
A set of comparators, that allow for ground shifts, deter-
mine the bus status as follows. Any diff sense signal be-
low 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
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 and Ultra3/Ultra160 sys-
tems. Multi-layer boards need to adhere to the 120
pedance 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 ca-
pacitance for through hole connector applications. Mi-
crostrip technology is normally too low of impedance and
should not be used. It is designed for 50
rather than
differential systems.
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.

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