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UCC5631A

UNITRODE CORPORATION

7 CONTINENTAL BLVD. • MERRIMACK, NH 03054

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

The DIFSENS line is monitored by each terminator

through a 50Hz noise filter at the DIFFB input pin. A set

of comparators detect and select the appropriate termi-

nation for the bus as follows. If the DIFSENS signal is be-

low 0.5V, the termination network is SE. Between 0.7V

and 1.9V, the termination network switches to LVD, and

above 2.4V is HVD, causing the terminators to discon-

nect from the bus. The thresholds accommodate differ-

ences in ground potential that can occur with long lines.

Three UCC5631A multi-mode parts are required at each

end of the bus to terminate 27 (18 data, plus 9 control)

lines. Each part includes a DIFSENS driver, but only one

is necessary to drive the line. A MSTR/SLV input pin is

provided to disable the other two. The "master" part must

have its' MSTR/SLV pin connected to TRMPWR and the

two "slave" parts must have the MSTR/SLV inputs

grounded. Only the "master" is connected directly to the

SCSI bus DIFSENS line. The DIFFB inputs on all three

parts are connected together, allowing them to share the

same 50Hz noise filter. This multi-mode terminator oper-

ates in full specification down to 2.7V TRMPWR voltage.

This accommodates 3.3V systems, with allowance for the

3.3V supply tolerance (+/- 10%), a unidirectional fusing

device and cable drop. In 3.3V TRMPWR systems, the

UCC3912 is recommended in place of the fuse and di-

ode. The UCC3912's lower voltage drop allows additional

margin over the fuse and diode, for the far end termina-

tor.

Layout is critical for Ultra2 and Ultra3 systems. The SPI-2

standard for capacitance loading is 10pF maximum from

each positive and negative signal line to ground, and a

maximum of 5pF between the positive and negative sig-

nal lines of each pair is allowed. These maximum capaci-

tances 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 de-

vice, (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 or

disconnect mode.

Multi-layer boards need to adhere to the 120

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

rather than 120

differential systems. Careful

consideration must be given to the issue of heat man-

agement. A multi-mode terminator, operating in SE

mode, will dissipate as much as 130mW of instanta-

neous power per active line with TRMPWR = 5.25V. The

UCC5631A is offered in a 36 pin SSOP and a 48 lead

LFQP. Both packages include 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. All of the

HS/GND 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.

APPLICATION INFORMATION (cont.)