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12/06/2005 Rev.3.01

SS8219G

ORDERING INFORMATION

SS8219GX XX

TR = tape and reel; TB = tubes

For example: SS8219GD TR

GD = RoHS-compliant TSSOP-28

=> SS8219 in RoHS-compliant TSSOP-28

GS = RoHS-compliant SSOP-36

supplied on tape and reel

APPLICATION INFORMATION

The SS8219G is used in multi-mode active-termination

applications where single ended (SE) and low voltage

differential (LVD) devices might coexist. The LVD termin-

ation section consists of 18 source/sink amplifiers (VTOP,

VBOT), biasing circuitry and nine precision resistor

strings (RTOP, RMID, RBOT). The SE termination

section consists of a 2.85V source/sink regulator with

nine precision 110

Ω resistors. The DIFFSENSE section

consists of a 1.3V, 5mA driver and a sensing circuit (Fig.1).

DIFFSENS is used to identify which types of SCSI

devices are present on the bus. If the voltage on

DIFFSENS is between 0V and 0.5V the bus is single-

ended; if it is between 0.7V and 1.9V, the bus is LVD;

and if it is greater than 2.4V, the bus is HVD.

The SS8219G DIFFB pin monitors the DIFFSENS line

to determine the proper operating mode of the device.

HVD Isolation Mode: The SS8219G identifies that there

is an HVD (high voltage differential) device on the SCSI

bus and isolates the termination pins from the bus.

When DISCNCT is pulled high, or during thermal

shutdown, the termination pins are isolated from the

SCSI bus and VREF is grounded. The DIFFSENSE

driver is shut down during either of these two events.

To ensure proper operation, the TPWR pin should be

connected to the SCSI bus TERMPWR line. As with all

analog circuitry, the TERMPWR lines should be by-

passed locally. A 2.2µF capacitor and a 0.01µF high

frequency capacitor are recommended between TPWR

and ground and placed as close as possible to the

SS8219G. The IC should be placed as close as possible

to the SCSI connector to minimize both signal and power

trace length, thereby reducing the input capacitance

and reflections which can degrade the bus signals.

The DIFFSENSE pin can drive the SCSI DIFFSENS

line (when M/S is pulled high) to determine the SCSI

bus operating mode. The SS8219G switches to the

appropriate termination mode for the bus based on

the value of the DIFFSENS voltage. These modes are:

LVD mode:

LVD termination is provided by a precision resistor

string with two amplifiers. This configuration yields a

105

Ω differential and 150Ω common mode impedance.

A fail safe bias of 112mV is maintained when no drivers

are connected to the SCSI bus.

SE mode:

When the external driver for a given signal line turns

off, the active terminator will pull that signal line

to 2.85 volts (quiescent state). When used with an

active-negation driver, the power amp can sink 22mA

per line while keeping the voltage reference in regulation.

The terminating resistors maintain their 110

Ω value.

To maintain the specified regulation, a 4.7µF capacitor

is required between the VREF pin and ground of each

SS8219G. A high frequency capacitor (0.1µF ceramic

recommended) can also be placed on the VREF pin

in applications that use fast rise/fall time drivers. A

typical SCSI bus configuration is shown in Figure 2.

An internal pull-down resistor assures that the SS8219G

will still terminate the bus if the DISCNCT pin is left floating.

DIFFSENS noise filtering

The SS8219G incorporates an internal digital filter to

remove the noise signal on the DIFFSENS control line,

thereby

eliminating

erroneous

switching

between

modes. An external filter may also be used in addition to

this internal digital filter.

DIFFSENS - refers to the SCSI bus signal.

DIFFSENSE - refers to the SS8219G pin name and internal circuitry related to differential sensing.

DIFFB - refers to the SS8219G pin name and internal circuitry related to monitoring the DIFFSENS line.

Please note for the following sections :