February 1997
ML6599
*
Hot-Insertable Active SCSI Terminator
GENERAL DESCRIPTION
The ML6599 BiCMOS 9 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 ML6599 provides support for hot-insertability on the
SCSI bus.
The ML6599 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 ML6599 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.
*Some Packages Are Obsolete
BLOCK DIAGRAM
NCLAMP = Negative Clamp
TERMPWR
LINEAR
REGULATOR
1V DROPOUT
2.85V
DISCNKT
CONTROL
BLOCK
V
REF
R
TRIM
MOSFETs WITH I
MAX
= 24mA
NCLAMP
9 TERMINATION LINES
. . .
. . .
. . .
NCLAMP
NCLAMP
. . .
. . .
GND
L2
L9
L1
V
REF
2.85V
1
FEATURES
s
Fully monolithic IC solution providing active
termination for 9 lines of the SCSI bus
s
Provides on board support for hot-insertability on the
SCSI bus
s
Low dropout voltage (1V) linear regulator, trimmed for
accurate termination current
s
Output capacitance typically < 5pF
s
Disconnect mode -- logic pin to disconnect terminator
from the SCSI bus, <100
A
s
Current sinking -- can sink current in excess of 10mA
per line to handle active negation driver overshoots
above 2.85V
s
Negative clamping on all lines to handle signal
assertion transients
s
Regulator can source 200mA and sink 100mA while
maintaining regulation
s
Current limit & thermal shutdown protection
2
ML6599
PIN CONFIGURATION
20-Pin TSSOP
16-Pin SOIC
TERMPWR
NC
NC
GND
DISCNKT
NC
L1
L2
L9
L8
L7
V
REF
L6
L5
L4
L3
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
TERMPWR
HS
NC
GND
NC
DISCNKT
NC
HS
L1
L2
L9
L8
HS
L7
V
REF
L6
L5
L4
HS
L3
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
NOTE :
The
DISCNKT line has a 200k
internal pullup resistor connected to the
supply. This pin should be left floating for normal operation and should
be connected to ground to enable the function.
PIN DESCRIPTION
NAME
DESCRIPTION
TERMPWR
Termination Power. Should be connected
to the SCSI TERMPWR line. A 10
F
tantalum local bypass capacitor is
recommended per system, as shown in
the application diagram
L1
Signal Termination 1. SCSI Bus line 1
L2
Signal Termination 2. SCSI Bus line 2
L3
Signal Termination 3. SCSI Bus line 3
L4
Signal Termination 4. SCSI Bus line 4
L5
Signal Termination 5. SCSI Bus line 5
L6
Signal Termination 6. SCSI Bus line 6
L7
Signal Termination 7. SCSI Bus line 7
L8
Signal Termination 8. SCSI Bus line 8
L9
Signal Termination 9. SCSI Bus line 9
NAME
DESCRIPTION
V
REF
2.85V
REF
Output. External decoupling
with a 10
F tantalum in parallel with a
0.1
F ceramic capacitor is
recommended, as shown in the
application diagram.
DISCNKT
Disconnect Terminator. Logic input to
disconnect the terminator from the bus
when the SCSI device no longer needs
termination due to not being the last
device on the bus or otherwise. Active
low input.
GND
Ground. Signal ground (0V)
HS
Heat Sink Ground. Should be
connected to GND.
3
ML6599
ABSOLUTE MAXIMUM RATINGS
Signal Line Voltage .................. 0.3 to TERMPWR + 0.3V
Regulator Output Current ......................... 100 to 300mA
TERMPWR Voltage ........................................... 0.3 to 7V
Storage Temperature ................................. 65
C to 150
C
Soldering Temperature ................................ 260
C for 10s
Thermal Impedance (
JA
)
SOIC ................................................................ 95
C/W
TSSOP ............................................................ 110
C/W
OPERATING CONDITIONS
TERMPWR Voltage ........................................ 4V to 5.25V
Operating Temperature ................................. 0
C to 70
C
ELECTRICAL CHARACTERISTICS
Unless otherwise stated, these specifications apply for 4V
TERMPWR
5.25V, and T
A
= 0
C to 70
C (Note 1)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Supply
TERMPWR Supply Current
L1L9 = open,
DISCNKT = open
4.5
5.5
mA
L1L9 = 0.2 V,
DISCNKT = open
225
250
mA
Disconnect Mode Current
DISCNKT = 0 (active)
75
100
A
DISCNKT
Input Low Voltage
1.0
V
Input High Voltage
TERMPWR 1.0
V
Output
Output High Voltage
Measuring each signal line
2.8
2.85
2.9
V
while other eight are high
Output Current
V
OUT
= 0.2V, Measuring each signal
20
24
mA
(Normal Mode)
line while the other eight are high
Hot Insertion Peak Current
TERMPWR = 0V, V
REF
= 0V
1
2
A
Any signal line (L1L9) at 2.85V
Output Clamp Level
I
OUT
= 30mA (Note 2)
0.15
0
0.15
V
Sinking Current (per line)
V
OUT
= 3.3V (per line)
10
12
mA
Output Capacitance
L1 thru L9,
DISCNKT = 0
4
5
pF
(Micro Linear Method)
2V
P-P
100kHz square wave
applied biased at 1V D.C.
Output Capacitance
L1 thru L9,
DISCNKT = 0
6
7
pF
(X3T9.2/855D method)
0.4V
P-P
, 1MHz square wave
applied biased at 0.5V D.C.
Regulator
Output Voltage
Sourcing 0-200mA
2.8
2.85
2.9
V
Sinking 0-100mA
2.8
2.85
2.9
V
Sinking Current
V = 3.5V
125
150
mA
Short Circuit Current
V
REF
= 0V
150
mA
V
REF
= 5V
300
mA
Dropout Voltage
L1L9 = 0.2V
1.0
1.2
V
Thermal Shutdown
170
C
Note 1:
Limits are guaranteed by 100% testing, sampling, or correlation with worst case test conditions.
4
ML6599
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 ML6599 with its MOSFET-
based nonlinear termination element provides the most
optimum V-I characteristics for both signal assertion and
negation.
IDEAL
ML6599
BOULAY
220/330
2.85V
2.7V
2.5V
0.2V
TERMINATOR (SOURCE)
DRIVER (SINK)
20mA
40mA
24mA
48mA
V
I
Figure 1. V-1 Characteristics of Various
SCSI Termination Schemes
The ML6599 provides active termination for 9 signal lines,
thus accommodating basic SCSI which requires 9 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 ML6599 integrates an
accurate voltage reference (1V dropout voltage) and 9
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
200mA and sink 100mA while maintaining regulation of
2.85V.
The ML6599 SCSI terminator provides an active low
control signal (
DISCNKT) which has an internal 200k
pull-up resistor. The
DISCNKT input isolates the ML6599
from the signal lines and effectively removes the
terminator from the SCSI bus with a disconnect mode
current of less than 100
A when pulled low. In addition,
the ML6599 provides for negative clamping of signal
transients and also supports current sink capability in
excess of 10mA 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 ML6599 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 ML6599 hot-insertable SCSI terminator typically
draws 1
A from any given output line (L1L9) 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. 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. 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
5
ML6599
being hot-inserted/removed should be inactive
(accomplished by grounding the
DISCNKT pin in the case
of the ML6599). 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 ML6599 requires parallel 0.1
F
and 10
F capacitors connected between the V
REF
and
GND pins and the TERMPOWER line needs a 10
F
bypass capacitor at each node in the system.
In an 8-bit wide SCSI bus arrangement ("A" Cable),
two ML6599s would be needed at each end of the SCSI
cable in order to terminate the 9 active signal lines.
16-bit wide SCSI would use three ML6599s, while 32-bit
wide SCSI bus would require five ML6599s.
In a typical SCSI subsystem, the open collector driver in
the SCSI transceiver pulls low when asserted. The
Figure 2. Application Diagram Showing Typical SCSI Bus Configuration with the ML6599
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
STEP
). With higher SCSI data rates,
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 I
O
is as close
as possible to the SCSI maximum current specification.
V
STEP
is defined as:
V
STEP
= V
OL
+ (I
O
Z
O
)
where V
OL
is the driver output low voltage, I
O
is the
current from the receiving terminator, and Z
O
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.
TERMPWR LINE
ML6599
TERMPWR
V
REF
GND
L9
L2 L1
DISCNKT
0.1F
10F
TERMPWR
V
REF
GND
L9
L1 L2
ML6599
DISCNKT
SCSI XCVR
SCSI XCVR
SCSI XCVR
V
REG
V
STEP
V
REG
t
D
t
D
LINE ASSERTED LINE NEGATED
0.1F
10F
. . .
. . .
. . .
. . .
V
OL
SCSI CABLE
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