Global Mixed-mode Technology Inc.
Ordering Information
ORDER
NUMBER
G218BT-S4
G218BR-S4
G218BT-S5
G218BR-S5
G218BT-D3
G218BR-D3
G218B
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
ORDER NUMBER
(Pb free)
G218BT-S4f
G218BR-S4f
G218BT-S5f
G218BR-S5f
G218BT-D3f
G218BR-D3f
TEMP.
RANGE
0ºC-70ºC
0ºC-70ºC
0ºC-70ºC
0ºC-70ºC
0ºC-70ºC
0ºC-70ºC
PACKAGE
SSOP-28 / Tube
SSOP-28 / Tape & Reel
SSOP-36 / Tube
SSOP-36 / Tape & Reel
TSSOP-28 / Tube
TSSOP-28 / Tape & Reel
Note:
DIFFSENS⎯Refers to the SCSI bus signal.
DIFFSENSE⎯Refers to the G218B pin name and internal circuitry relating to differential sensing.
DIFFB⎯Refers to the G218B pin name and internal circuitry relating to monitoring the DIFFSENS line.
Functional Description
The G218B is used in multi-mode active termination
applications, where single ended (SE) and low voltage
differential (LVD) devices might coexist. The LVD ter-
mination section consists of 18 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)
DIFFSENS is used to identify which types of SCSI
devices are present on the bus. If the voltage on
DIFFSENS is between 0-0.5V the bus is single-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 G218B 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 G218B 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, the termination pins
are isolated from the SCSI bus, and the bus mode
indicators (SE/LVD/HVD) as well as VREF remains
active. During thermal shutdown, the termination pins
are isolated from the SCSI bus and VREF becomes
high impedance. 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 is recommended between TPWR
and ground and placed as close as possible to the
G218B. The G218B should be placed as close as
possible to the SCSI connector to minimize signal and
power trace length, thereby resulting in less input ca-
pacitance 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 G218B 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 regula-
tion. The terminating resistors maintain their 110Ω
value.
To maintain the specified regulation, a 4.7µF capacitor
is required between the VREF pin and ground of each
G218B. 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 G218B
will be terminating the bus if the DISCNCT pin is left
floating.
Ver 1.4
Apr 07, 2005
TEL: 886-3-5788833
http://www.gmt.com.tw
2
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
