Page 1
August 2000
ML6518*
18 Line Hot-Insertable Active SCSI Terminator
GENERAL DESCRIPTION
The ML6518 BiCMOS 18 line SCSI terminator provides
active termination in SCSI systems using single-ended
drivers and receivers. Active SCSI termination helps to
effectively control analog transmission line effects such as
ringing, noise, crosstalk, and ground bounce. In addition,
the ML6518 provides support for hot insertability on the
SCSI bus.
The ML6518 provides a V-I characteristic optimized to
minimize transmission line effects during both signal
negation and assertion using a MOSFET-based
architecture. The desired V-I characteristic is achieved by
trimming one resistor in the control block. Internal
clamping controls signal assertion transients and provides
current sink capability to handle active negation driver
overshoots above 2.85V. It provides a 2.85V reference
through an internal low dropout (1V) linear regulator.
The ML6518 also provides a disconnect function which
effectively removes the terminator from the SCSI bus. The
disconnect mode capacitance is typically less than 5pF
FEATURES
■ Fully monolithic IC solution providing active
termination for 18 lines of the SCSI bus
■ Provides onboard support for hot-insertability on the
SCSI bus
■ Low dropout voltage (1V) linear regulator, trimmed for
accurate termination current
■ Low disconnect capacitance (typically < 5pF)
■ Logic pin with active pull-up to disconnect terminator
from the SCSI bus
■ Current sinking capability in excess of 8.3mA per line to
handle active negation driver overshoots above 2.85V
■ Negative clamping on all lines to handle signal
assertion transients
■ Regulator can source 400mA and sink 150mA while
maintaining regulation
■ Current limit and thermal shutdown protection
per line. Current limiting and thermal shutdown protection
are also included. *This Part Is End Of Life As Of August 1, 2000
BLOCK DIAGRAM
GND
NCLAMP
. . .
NCLAMP
NCLAMP
TERMPWR
LINEAR
REGULATOR
1V DROPOUT
MOSFETs WITH I
18 TERMINATION LINES
. . .
L2 L18L1
2.85V
MAX
. . .
= 24mA
V
REF
R
TRIM
DISCNKT
2.85V
V
REF
CONTROL
BLOCK
. . .
. . .
NCLAMP = Negative Clamp
Circuit design patent pending.
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ML6518
PIN CONFIGURATIONS
I
SENSE
H1
H2
H3
RSCS
CT
VREF
V
SPEED
ILIMIT
VCO OUT
VDD
28-Pin SOIC (S28)
28-Pin SOIC (S28)
1
2
3
4
5
6
7
8
9
L1
10
L2
11
L3
12
13
14
ML6518
ML6518
TOP VIEW
28
GND
27
R
REF
26
C
OS
25
BRAKE
24
FB 3
23
FB 2
22
FB 1
21
C
RAMP
20
VCO IN
19
C
EN
18
FAULT
17
C
RST
16
R
VCO
15
C
VCO
CLK IN
GAIN
DGND
AGND
VOUT
VREF
VCC
XXX
ML6518
32-Pin TQFP (H32-7)
ML6518
32-Pin TQFP (H32-7)
CLK IN
GAIN
DGND
AGND
VOUT
VREF
VCC
32 31 30 29 28 27 26 25
1
2
3
4
5
6
7
8
9 10111213141516
VCC
AGND
VOUT
VREF
GAIN
CLK IN
DGND
TOP VIEW
24
23
22
21
20
19
18
17
XXX
CLK IN
GAIN
DGND
AGND
VOUT
VREF
VCC
XXX
XXX
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PIN DESCRIPTION
TQFP SOIC
PIN# PIN# NAME FUNCTION
1 25 L1 Signal termination for SCSI
bus line 1
2 26 L2 Signal termination for SCSI
bus line 2
3 27 L3 Signal termination for SCSI
bus line 3
4 28 L4 Signal termination for SCSI
bus line 4
5 1 L5 Signal termination for SCSI
bus line 5
6 2 L6 Signal termination for SCSI
bus line 6
7 3 L7 Signal termination for SCSI
bus line 7
8 4 L8 Signal termination for SCSI
bus line 8
9 6 L9 Signal termination for SCSI
bus line 9
11–14, 7, 22, GND Ground
27–30 23
15 8 DISCNKT Terminator disconnect. Logic
input to disconnect the
terminator from the bus when
the SCSI device no longer
needs termination. DISCNKT
has a 200ký internal pull-up
resistor connected to
TERMPWR for use with a
mechanical switch
ML6518
TQFP SOIC
PIN# PIN# NAME FUNCTION
16 11 L10 Signal termination for SCSI
bus line 10
17 12 L11 Signal termination for SCSI
bus line 11
18 13 L12 Signal termination for SCSI
bus line 12
19 14 L13 Signal termination for SCSI
bus line 13
20 15 L14 Signal termination for SCSI
bus line 14
21 16 L15 Signal termination for SCSI
bus line 15
22 17 L16 Signal termination for SCSI
bus line 16
23 18 L17 Signal termination for SCSI
bus line 17
24 19 L18 Signal termination for SCSI
bus line 18
25, 26 20, 21 TERMPWR Power should be connected
to the SCSI TERMPWR line.
A 22µF tantalum bypass
capacitor is recommended
as shown in the application
diagram
31, 32 24 V
REF
2.85V V
decoupling with a 10µF
tantalum in parallel with a
0.1µF ceramic capacitor is
recommended as shown in the
application diagram
output. External
REF
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ML6518
ABSOLUTE MAXIMUM RATINGS
Junction Temperature ............................................. 150°C
Storage Temperature Range ...................... –65°C to 150°C
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
Signal Line Voltage ...... GND – 0.3V to TERMPWR + 0.3V
Lead Temperature (Soldering 10 sec) .................... +260°C
Thermal Resistance (qJA)
SOIC Package .................................................. 75°C/W
TQFP Package.................................................. 65°C/W
OPERATING CONDITIONS
Regulator Output Current ................................... ±500mA
TERMPWR Voltage .......................................... –0.3 to 7V
Temperature Range........................................ 0°C to 70°C
TERMPWR Voltage Range ........................... 4.0V to 5.25V
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, TERMPWR = 4V to 5.25V, TA = Operating Temperature Range (Note 1)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply
TERMPWR Supply Current L1-L18 open, DISCNKT open 5 7 mA
L1-L18 = 0.2V, DISCNKT open 450 500 mA
Disconnect Mode Current DISCNKT = 0V 75 100 µA
DISCNKT
Input Low Voltage 1.0 V
Input High Voltage TERMPWR V
–1.0
Output
Output High Voltage Each line measured with other 17 2.8 2.85 2.9 V
lines high
Output Current (Normal Mode) V
Hot Insertion Peak Current TERMPWR = 0V, V
Output Clamp Voltage I
Sinking Current (per line) V
Output Capacitance DISCNKT = 0V, 2V
(Micro Linear method) wave biased at 1V applied to the
Output Capacitance DISCNKT = 0V, 0.4V
(X3T9.2/855D method) wave biased at 0.5V applied to the
Regulator
Output Voltage Sourcing 0-400mA 2.8 2.85 2.9 V
Sinking Current V
Short Circuit Current V
Dropout Voltage L1–L18 = 0.2V 1.0 1.2 V
= 0.2V, each line measured 20 24 mA
OUT
with other 17 lines high
= 0V, 1 2 µA
Any or all signal lines = 2.85V
= –30mA –150 150 mV
OUT
= 3.3V 10 12 mA
OUT
output
output
Sinking 0-150mA 2.8 2.85 2.9 V
= 3.5 240 300 mA
LINE
= 0V 300 mA
REF
V
= 5V 600 mA
REF
REF
100kHz square 4 5 pF
P-P
1MHz square 6 7 pF
P-P
Thermal Shutdown 170 °C
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
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ML6518
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 MOSFETbased 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.
V
3.7V
3.6V
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
) has powered up. A second PMOS in series with a
REF
Schottky diode is used as the shorting bypass device. After
V
reaches a sufficient level, the bypass device is turned
REF
off and the part operates normally.
TERMINATOR (SINK)
DRIVER (SOURCE)
Figure 1. V-1 Characteristics of Various SCSI Termination Schemes
12mA
24mA
8.5mA
17mA
7mA
14mA
3.3V
2.7V
2.5V
0.2V
TERMINATOR (SOURCE)
DRIVER (SINK)
2.85V
220/330
ML6518
BOULAY
20mA
40mA
IDEAL
24mA
48mA
I
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ML6518
As outlined in Annex G of the ANSI SCSI-3 Parallel
Interface Specification (X3T9.2/855D), “The SCSI bus
termination shall be external to the device being inserted
or removed.” In other words, any terminator connected to
a device being hot inserted/removed should be inactive
(accomplished by grounding the DISCNKT pin in the case
of the ML6518). If the terminator being inserted/removed
were in the active state, at some point in time the bus
would be terminated by either 1 or 3 terminators. In either
case, data integrity on the bus will be compromised.
Figure 2 gives an application diagram showing a typical
SCSI bus configuration. To ensure proper operation, the
TERMPWR pin must be connected to the SCSI
TERMPOWER line. Each ML6518 requires parallel 0.1µF
and 10µF capacitors connected between the V
REF
and
GND pins and the TERMPOWER line needs a 10µF
bypass capacitor at one node in the system.
In an 8-bit wide SCSI bus arrangement (“A” Cable),
a single ML6518 would be needed at each end of the SCSI
cable in order to terminate the 18 active signal lines.
16-bit wide SCSI would use one ML6518 and one
ML6599, while 32-bit wide SCSI bus would require two
ML6518s and one ML6599.
In a typical SCSI subsystem, the open collector driver in
the SCSI transceiver pulls low when asserted. The
termination resistance serves as the pull-up when negated.
Figure 2 also shows a typical cable response to a pulse.
The receiving end of the cable will exhibit a single time
delay. When negated, the initial step will reach an
intermediate level (V
). With higher SCSI data rates,
STEP
sampling could occur during this step portion. In order to
get the most noise margin, the step needs to be as high as
possible to prevent false triggering. For this reason the
regulator voltage and the resistor defining the MOSFET
characteristic are trimmed to ensure that the IO is as close
as possible to the SCSI maximum current specification.
V
is defined as:
STEP
V
= VOL + (IO ¥ ZO)
STEP
where VOL is the driver output low voltage, IO is the
current from the receiving terminator, and ZO is the
characteristic impedance of the cable.
This is a very important characteristic that the terminator
helps to overcome by increasing the noise margin and
boosting the step as high as possible.
0.1µF
10µF
V
GND
L18 L2 L1
SCSI XCVR
SCSI XCVR
REF
. . .
ML6518
TERMPWR
DISCNKT
. . .
TERMPWR LINE
V
REG
SCSI CABLE
10µF
V
V
REG
STEP
ML6518
TERMPWR
DISCNKT
. . .
V
REF
GND
L18L1 L2
SCSI XCVR
. . .
0.1µF
10µF
V
OL
t
LINE ASSERTED LINE NEGATED
D
t
D
Figure 2. Application Diagram Showing Typical SCSI Bus Configuration with the ML6518
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PHYSICAL DIMENSIONS inches (millimeters)
Package: H32-7
32-Pin (7 x 7 x 1mm) TQFP
0.354 BSC
(9.00 BSC)
0.276 BSC
(7.00 BSC)
1
PIN 1 ID
25
ML6518
0º - 8º
0.003 - 0.008
(0.09 - 0.20)
0.354 BSC
(9.00 BSC)
0.037 - 0.041
0.048 MAX
(1.20 MAX)
(0.95 - 1.05)
0.018 - 0.030
(0.45 - 0.75)
SEATING PLANE
9
0.032 BSC
(0.8 BSC)
0.276 BSC
(7.00 BSC)
17
0.012 - 0.018
(0.29 - 0.45)
Package: S28
28-Pin SOIC
0.699 - 0.713
28
(17.75 - 18.11)
0.024 - 0.034
(0.61 - 0.86)
(4 PLACES)
0.090 - 0.094
(2.28 - 2.39)
0.291 - 0.301
(7.39 - 7.65)
PIN 1 ID
1
0.050 BSC
(1.27 BSC)
0.012 - 0.020
(0.30 - 0.51)
0.095 - 0.107
(2.41 - 2.72)
SEATING PLANE
0.398 - 0.412
(10.11 - 10.47)
0.005 - 0.013
(0.13 - 0.33)
0º - 8º
0.022 - 0.042
(0.56 - 1.07)
0.009 - 0.013
(0.22 - 0.33)
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ML6518
ORDERING INFORMATION
PART NUMBER TEMPERATURE RANGE PACKAGE
ML6518CH0°C to 70°C32-pin TQFP (H32-7) (End Of Life)
ML6518CS0°C to 70°C28-pin SOIC (S28) (End Of Life)
© Micro Linear 1997 is a registered trademark of Micro Linear Corporation
Products described in this document may be covered by one or more of the following patents, U.S.: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940;
5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; Japan: 2598946; 2619299. Other patents are pending.
Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design.
Micro Linear does not assume any liability arising out of the application or use of any product described herein,
neither does it convey any license under its patent right nor the rights of others. The circuits contained in this
data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to
whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility
or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel
before deciding on a particular application.
8
2092 Concourse Drive
San Jose, CA 95131
Tel: 408/433-5200
Fax: 408/432-0295
DS6518-01
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