G220 Datasheet, PDF (2/9 Pages) Global Mixed-mode Technology Inc – Ultra320 Multi-mode LVD/SE SCSI Terminator

Global Mixed-mode Technology Inc.
G220
Description
The G220 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 G220 DIFFB pin monitors the DIFFSENS line to
determine the proper operating mode of the device.
HVD Isolation Mode The G220 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 VREF is
grounded. 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 is recommended between TPWR
and ground and placed as close as possible to the
G220. The G220 should be placed as close as possi-
ble 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 G220 switches to the termi-
nation 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
G220. 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 G220
will be terminating the bus if the DISCNCT pin is left
floating.
DIFFSENS noise filtering
The G220 incorporates a internal digital filter to re-
move the noise signal on the DIFFSENS control line,
thereby eliminating erroneous switching between
modes. The external filter may be used in addition to
the internal digital filter.
Ver: 0.1 Preliminary
Apr 03, 2003
TEL: [phone]
http://www.gmt.com.tw
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