G218A Datasheet, PDF (2/9 Pages) Global Mixed-mode Technology Inc – Multi-mode LVD/SE SCSI Terminator

Global Mixed-mode Technology Inc.
Ordering Information
PART NUMBER
TEMP. RANGE
G218AT-S4
0ºC-70ºC
G218AR-S4
0ºC-70ºC
G218AT-S5
0ºC-70ºC
G218AR-S5
0ºC-70ºC
G218AT-D3
0ºC-70ºC
G218AR-D3
0ºC-70ºC
PIN-PACKAGE
SSOP-28 pin / Tube
SSOP-28 pin / Tape & Reel
SSOP-36 pin / Tube
SSOP-36 pin / Tape & Reel
TSSOP-28 pin / Tube
TSSOP-28 pin / Tape & Reel
G218A
FEATURE
DISCNCT does not disable VREF
DISCNCT does not disable VREF
DISCNCT does not disable VREF
DISCNCT does not disable VREF
DISCNCT does not disable VREF
DISCNCT does not disable VREF
Functional Description
The G218A combines LVD and SE termination with
DIFFSENS sourcing and detection. The LVD termina-
tion section consists of two source/sink amplifiers
(VTOP, VBOT), biasing circuitry and nine precision
resistor strings (RTOP, RMID, RBOT). The SE termi-
nation section consists of a 2.85V source/sink regula-
tor with 9 precision 110ohm resistor. The DIFFSENSE
section consists of a 1.3V, 5mA driver and a sensing
circuit (Figure 1)
DIFFSENSE is used by multi-mode transceiver
(LVD/SE) to determine if the device is connected to a
single-ended, LVD or HVD SCSI bus. If the voltage on
DIFFSENSE is between 0-0.5V the bus is sin-
gle-ended; if it is between 0.7-1.9V the bus is LVD;
and if it is greater than 2.4V, the bus is HVD.
The G218A DIFFB pin monitors the DIFFSENS line to
determine the proper operating mode of the device;
this mode is indicated by the SE/LVD/HVD outputs.
HVD Isolation Mode The G218A identifies that there
is an HVD (high voltage differential) device on the
SCSI bus and isolates the termination pins from the
bus.
When DISCNCT=1, the termination pins are isolated
from the SCSI bus, and the bus mode indicators
(SE/LVD/HVD) as well as VREF remains active. Dur-
ing thermal shutdown, the termination pins are isolated
from the SCSI bus and VREF becomes high imped-
ance. The DIFFSENSE driver is shut down during ei-
ther 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 and VDD lines should
be bypassed locally. A 2.2µF capacitor and a 0.01µF
high frequency capacitor is recommended between
TPWR and ground and placed as close as possible to
the G218A. The G218A should be placed as close as
possible to the SCSI connector to minimize signal and
The DIFFSENSE pin can also drive the SCSI
DIFFSENS line (when M/S=1) to determine the SCSI
bus operating mode. The G218A switches to the ter-
mination mode that is appropriate 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 105Ω 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 110Ω value.
power trace length, thereby resulting in less input ca-
pacitance and reflections which can degrade the bus
signals.
To maintain the specified regulation, a 4.7µF capacitor
is required between the VREF pin and ground of each
G218A. A high frequency cap (0.1µF ceramic recom-
mended) can also be placed on the VREF pin in ap-
plications that use fast rise/fall time drivers. A typical
SCSI bus configuration is shown in Figure2.
An internal pull down resistor assures that the G218A
will be terminating the bus if the DISCNCT pin is left
floating.
Note:
DIFFSENSRefers to the SCSI bus signal.
DIFFSENSERefers to the G218A pin name and in-
ternal circuitry relating to differential sensing.
DIFFBRefers to the G218A pin name and internal
circuitry relating to monitoring the DIFFSENS line.
Ver 3.3
Sep 11, 2001
TEL: [phone]
http://www.gmt.com.tw
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