ML6518CS MICRO-LINEAR [Micro Linear Corporation], ML6518CS Datasheet - Page 5

• Manufacturer Part Number: ML6518CS
• Description: 18 Line Hot-Insertable Active SCSI Terminator
• Manufacturer: MICRO-LINEAR [Micro Linear Corporation]

FUNCTIONAL DESCRIPTION
SCSI terminators are used to decrease the transmission line
effects of SCSI cable. Termination must be provided at the
beginning and end of the SCSI bus to ensure that data
errors due to reflections on the bus are eliminated. With
the increasing use of higher data rates and cable lengths in
SCSI subsystems, active termination has become
necessary. Active termination also minimizes power
dissipation and can be activated or deactivated under
software control, thus eliminating the need for end user
intervention. The V-I characteristics of popular SCSI
termination schemes are shown in Figure 1. Theoretically,
the desired V-I characteristics are the Boulay type for
signal assertion (high to low) and the ideal type for signal
negation (low to high). The ML6518 with its MOSFET-
based nonlinear termination element provides the most
optimum V-I characteristics for both signal assertion and
negation.
The ML6518 provides active termination for 18 signal
lines, thus accommodating basic SCSI which requires 18
lines to be terminated. When used with the ML6599, wide
SCSI, which requires 27, 36 or 45 lines to be terminated,
can also be accommodated. The ML6518 integrates an
accurate voltage reference (1V dropout voltage) and 18
MOSFET-based termination lines. A single internal resistor
is trimmed to tune the V-I characteristic of the MOSFETs.
The voltage reference circuit produces a precise 2.85V
level and is capable of sourcing 24mA into each of the
nine terminating lines when low (active). When the signal
line is negated (driver turns off), the terminator pulls the
signal line back to 2.85V. The regulator will source
400mA and sink 150mA while maintaining regulation of
2.85V.
TERMINATOR (SINK)
DRIVER (SOURCE)
Figure 1. V-1 Characteristics of Various SCSI Termination Schemes
12mA
24mA
8.5mA
17mA
7mA
14mA
0.2V
2.7V
2.5V
V
3.7V
3.6V
3.3V
TERMINATOR (SOURCE)
DRIVER (SINK)
The ML6518 SCSI terminator provides an active low
control signal (DISCNKT) which has an internal 200ký
pull-up resistor. The DISCNKT input isolates the ML6518
from the signal lines and effectively removes the
terminator from the SCSI bus with a disconnect mode
current of less than 100µA when pulled low. In addition,
the ML6518 provides for negative clamping of signal
transients and also supports current sink capability in
excess of 8.3mA per signal line to handle active negation
driver overshoot above 2.85V, a common occurrence with
SCSI transceivers.
Disconnect mode capacitance is a very critical parameter
in SCSI systems. The ML6518 provides a capacitance
contribution of only 5pF.
HOT INSERTABILITY
“Hot” insertion of a SCSI device refers to the act of
plugging a SCSI device which is initially unpowered into
a powered SCSI bus. The SCSI device subsequently draws
power from the TERMPWR line during its startup routine
and thereafter. “Hot” removal refers to the act of removing
a powered SCSI device from a powered SCSI bus. A
device which performs both tasks with no physical
damage to itself or other devices on the bus, nor which
alters the existing state of the bus by drawing excessive
currents, is termed “hot-swappable.”
The ML6518 hot-insertable SCSI terminator typically
draws 1µA from any given output line (L1–L18) during a
hot insertion/removal procedure, thereby protecting itself
and preserving the state of the bus. The low insertion
current is achieved by effectively shorting the gate to drain
of the output PMOS device until the 2.85V reference
(V
Schottky diode is used as the shorting bypass device. After
V
off and the part operates normally.
REF
REF
) has powered up. A second PMOS in series with a
reaches a sufficient level, the bypass device is turned
2.85V
ML6518
20mA
40mA
IDEAL
24mA
48mA
I
ML6518
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