UCC5628

SLUS302C − DECEMBER 1994 − REVISED MARCH 2004

5

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APPLICATION INFORMATION

The UCC5628 is a multimode active terminator with selectable single ended (SE) and low voltage differential

(LVD) SCSI termination integrated into a monolithic component. 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 multimode 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 terminator through a 50-Hz noise filter for SPI−2 and a 100-ms

to 300-ms filter for SPI−3 and SPI−4 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 0.5 V is single ended, between 0.7 V and 1.9

V is LVD and above 2.4 V is HVD.

In the single ended mode, a multimode terminator has a 110-

Ω terminating resistor connected to a 2.7-V

termination voltage regulator. The 2.7-V regulator is used on all unitrode terminators designed for 3.3-V

systems. This requires the terminator to operate in specification down to 2.7-V TRMPWR voltage to allow for

the 3.3-V supply tolerance, 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 specially 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

1.5-pF 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. Multilayer boards need to adhere to the

120-

Ω 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 1

pF to each plane. Each feed-through will add about 2.5 pF to 3.5 pF. 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

Ω rather than 120-Ω differential

systems.

Capacitance balance is critical for Ultra2, Ultra3, Ultra160 and Ultra320. The balance capacitance standard is

0.5 pF per line with the balance between pairs of 2 pF. The components are designed with very tight balance,

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

Multimode terminators need to consider power dissipation; the UCC5628 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 multilayer board.

In 3.3-V 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 terminator. 3.3-V battery systems normally have a 10%

tolerance. The UCC3912 is 150-mV drop under LVD loads, allowing 150-mV drop in the cable system. All

Unitrode LVD and multimode terminators are designed for 3.3-V systems, operating down to 2.7 V.