6

UCC5672

UNITRODE CORPORATION

7 CONTINENTAL BLVD. • MERRIMACK, NH 03054

TEL. (603) 424-2410 • FAX (603) 424-3460

with allowance for the 3.3V supply tolerance (+/- 10%), a

unidirectional fusing device and cable drop. In 3.3V

TRMPWR systems, the UCC3918 is recommended in

place of the fuse and diode. The UCC3918's lower volt-

age drop allows additional margin over the fuse and di-

ode, for the far end terminator.

Layout is critical for Ultra2 and Ultra3/Ultra160 systems.

The SPI-2 standard for capacitance loading is 10pF max-

imum from each positive and negative signal line to

ground, and a maximum of 5pF between the positive and

negative signal lines of each pair is allowed. These maxi-

mum capacitances apply to differential bus termination

circuitry that is not part of a SCSI device, (e.g. a cable

terminator). If the termination circuitry is included as part

of a SCSI device, (e.g., a host adaptor, disk or tape

drive), then the corresponding requirements are 30pF

maximum from each positive and negative signal line to

ground and 15pF maximum between the positive and

negative signal lines of each pair.

The SPI-2 standard for capacitance balance of each pair

and balance between pairs is more stringent. The stan-

dard is 0.75pF maximum difference from the positive and

negative signal lines of each pair to ground. An additional

requirement is a maximum difference of 2pF when com-

paring pair to pair. These requirements apply to differen-

tial bus termination circuitry that is not part of a SCSI

device. If the termination circuitry is included as part of a

device, then the corresponding balance requirements are

2.25pF maximum difference within a pair, and 3pF from

pair to pair.

Feed-throughs,

through-hole

connections,

and

etch

lengths

need

to

be

carefully

balanced.

Standard

multi-layer power and ground plane spacing add 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 will reduce the capacitance.

Similarly, opening up the power and ground planes under

the

connector

will

reduce

the

capacitance

for

through-hole connector applications. Capacitance

will

also be affected by components, in close proximity,

above and below the circuit board.

Unitrode multi-mode terminators are designed with very

tight balance, typically 0.1pF between pins in a pair and

0.3pF between pairs. At each L+ pin, a ground driver

drives the pin to ground, while in single ended mode. The

ground driver is specially designed to not effect the ca-

pacitive balance of the bus when the device is in LVD

SCSI or disconnect mode.

Multi-layer boards need to adhere to the 120

Ω imped-

ance standard, including the connectors and feed-

throughs. This is normally done on the outer layers with

4 mil etch and 4 mil spacing between runs within a pair,

and a minimum of 8 mil spacing to the adjacent pairs to

reduce crosstalk. Microstrip technology is normally too

low of impedance and should not be used. It is designed

for 50

Ω rather than 120Ω differential systems. Careful

consideration must be given to the issue of heat manage-

ment. A multi-mode terminator, operating in SE mode,

will dissipate as much as 130mW of instantaneous power

per active line with TRMPWR = 5.25V. The UCC5672 is

offered in a 28 pin TSSOP. This package includes two

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 tem-

perature. Both of the HS/GND pins need to be connected

to etch area or four feed-through per pin connecting to

the ground plane layer on a multi-layer board.

APPLICATION INFORMATION (cont.)