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UCC5639

The UCC5639 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. A

LVD 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 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 a HVD

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 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 is single ended, between 0.7V and 1.9V is LVD

and above 2.2V is HVD.

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

110

Ω 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 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 systems.

Multi-layer boards need to adhere to the 120

Ω imped-

ance 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

Ω rather than

120

Ω differential systems.

Capacitance balance is critical for Ultra2 and Ultra3. The

balance capacitance standard is 0.5pF per line with the

balance between pairs of 2pF. The components are de-

signed 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 measured 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 UCC5639 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 loads,

allowing 150mV drop in the cable system. All Unitrode

LVD and multi-mode terminators are designed for 3.3V

systems, operating down to 2.7V.

APPLICATION INFORMATION