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
per line. Current limiting and thermal shutdown protection
are also included.
FEATURES
s
s
s
s
s
s
s
s
s
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
*This Part Is End Of Life As Of August 1, 2000
BLOCK DIAGRAM
TERMPWR
V
REF
DISCNKT
2.85V
LINEAR
REGULATOR
1V DROPOUT
2.85V
GND
R
TRIM
V
REF
CONTROL
BLOCK
MOSFETs WITH I
MAX
= 24mA
...
NCLAMP
...
...
...
NCLAMP
NCLAMP
L1
L2
...
L18
18 TERMINATION LINES
NCLAMP = Negative Clamp
Circuit design patent pending.
1
ML6518
PIN CONFIGURATIONS
ML6518
28-Pin SOIC (S28)
ML6518
28-Pin SOIC (S28)
ISENSE
H1
H2
H3
RSCS
CT
VREF
VSPEED
L1
L2
L3
ILIMIT
VCO OUT
VDD
1
2
3
4
5
6
7
8
9
10
11
12
13
14
28
27
26
25
24
23
22
21
20
19
18
17
16
15
GND
RREF
COS
BRAKE
FB 3
FB 2
FB 1
CRAMP
VCO IN
CEN
FAULT
CRST
RVCO
CVCO
TOP VIEW
ML6518
32-Pin TQFP (H32-7)
ML6518
32-Pin TQFP (H32-7)
CLK IN
DGND
AGND
VOUT
GAIN
VREF
VCC
XXX
CLK IN
GAIN
DGND
AGND
VOUT
VREF
VCC
XXX
32 31 30 29 28 27 26 25
24
1
2
3
4
5
6
7
8
9
23
22
21
20
19
18
17
10 11 12 13 14 15 16
CLK IN
GAIN
DGND
AGND
VOUT
VREF
VCC
XXX
CLK IN
GAIN
DGND
AGND
VOUT
VREF
VCC
TOP VIEW
2
XXX
ML6518
PIN DESCRIPTION
TQFP
PIN#
SOIC
PIN#
NAME
FUNCTION
TQFP
PIN#
SOIC
PIN#
NAME
FUNCTION
1
2
3
4
5
6
7
8
9
25
26
27
28
1
2
3
4
6
L1
L2
L3
L4
L5
L6
L7
L8
L9
Signal termination for SCSI
bus line 1
Signal termination for SCSI
bus line 2
Signal termination for SCSI
bus line 3
Signal termination for SCSI
bus line 4
Signal termination for SCSI
bus line 5
Signal termination for SCSI
bus line 6
Signal termination for SCSI
bus line 7
Signal termination for SCSI
bus line 8
Signal termination for SCSI
bus line 9
Ground
16
17
18
19
20
21
22
23
24
25, 26
11
12
13
14
15
16
17
18
19
L10
L11
L12
L13
L14
L15
L16
L17
L18
Signal termination for SCSI
bus line 10
Signal termination for SCSI
bus line 11
Signal termination for SCSI
bus line 12
Signal termination for SCSI
bus line 13
Signal termination for SCSI
bus line 14
Signal termination for SCSI
bus line 15
Signal termination for SCSI
bus line 16
Signal termination for SCSI
bus line 17
Signal termination for SCSI
bus line 18
11–14, 7, 22, GND
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 200k½ internal pull-up
resistor connected to
TERMPWR for use with a
mechanical switch
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
24
V
REF
2.85V V
REF
output. External
decoupling with a 10µF
tantalum in parallel with a
0.1µF ceramic capacitor is
recommended as shown in the
application diagram
31, 32
3
ML6518
ABSOLUTE MAXIMUM RATINGS
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
Regulator Output Current ................................... ±500mA
TERMPWR Voltage .......................................... –0.3 to 7V
Junction Temperature ............................................. 150°C
Storage Temperature Range ...................... –65°C to 150°C
Lead Temperature (Soldering 10 sec) .................... +260°C
Thermal Resistance (q
JA
)
SOIC Package .................................................. 75°C/W
TQFP Package .................................................. 65°C/W
OPERATING CONDITIONS
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, T
A
= Operating Temperature Range (Note 1)
PARAMETER
Supply
TERMPWR Supply Current
L1-L18 open,
DISCNKT
open
L1-L18 = 0.2V,
DISCNKT
open
Disconnect Mode Current
DISCNKT
Input Low Voltage
Input High Voltage
Output
Output High Voltage
Output Current (Normal Mode)
Hot Insertion Peak Current
Output Clamp Voltage
Sinking Current (per line)
Output Capacitance
(Micro Linear method)
Output Capacitance
(X3T9.2/855D method)
Regulator
Output Voltage
Sourcing 0-400mA
Sinking 0-150mA
Sinking Current
Short Circuit Current
V
LINE
= 3.5
V
REF
= 0V
V
REF
= 5V
Dropout Voltage
Thermal Shutdown
Note 1:
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
CONDITIONS
MIN
TYP
MAX
UNITS
5
450
75
7
500
100
mA
mA
µA
DISCNKT
= 0V
1.0
TERMPWR
–1.0
V
V
Each line measured with other 17
lines high
V
OUT
= 0.2V, each line measured
with other 17 lines high
TERMPWR = 0V, V
REF
= 0V,
Any or all signal lines = 2.85V
I
OUT
= –30mA
V
OUT
= 3.3V
DISCNKT
= 0V, 2V
P-P
100kHz square
wave biased at 1V applied to the
output
DISCNKT
= 0V, 0.4V
P-P
1MHz square
wave biased at 0.5V applied to the
output
2.8
20
2.85
2.9
24
V
mA
µA
mV
mA
1
–150
10
12
4
2
150
5
pF
6
7
pF
2.8
2.8
240
2.85
2.85
300
300
600
1.0
170
2.9
2.9
V
V
mA
mA
mA
L1–L18 = 0.2V
1.2
V
°C
4
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 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.
V
3.7V
3.6V
The ML6518 SCSI terminator provides an active low
control signal (DISCNKT) which has an internal 200k½
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
REF
) has powered up. A second PMOS in series with a
Schottky diode is used as the shorting bypass device. After
V
REF
reaches a sufficient level, the bypass device is turned
off and the part operates normally.
3.3V
2.85V
2.7V
2.5V
IDEAL
ML6518
BOULAY
220/330
0.2V
12mA 8.5mA 7mA
24mA 17mA 14mA
I
TERMINATOR (SINK)
DRIVER (SOURCE)
TERMINATOR (SOURCE)
DRIVER (SINK)
20mA
40mA
24mA
48mA
Figure 1. V-1 Characteristics of Various SCSI Termination Schemes
5
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