DS21T06
SCSI Terminator
www.dalsemi.com
FEATURES
Fully compliant with SCSI-1, Fast SCSI and
Ultra SCSI
Functional drop in replacement for the
DS2106
Complimentary part to DS21T07 for wide
SCSI
Provides active termination for 18 signal lines
5% tolerance on termination resistors and
voltage regulator
5% tolerance on termination current
Low power down capacitance of 3 pF
Onboard thermal shutdown circuitry
SCSI bus hot plug compatible
Fully supports actively negated SCSI signals
PIN ASSIGNMENT
PD
R1
R2
R3
R4
R5
HS-GND
HS-GND
NC
R6
R7
R8
R9
TPWR
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
R18
R17
R16
R15
R14
NC
HS-GND
NC
R13
R12
R11
R10
V
REF
DS21T06S 28-Pin SOIC (300-mil)
DESCRIPTION
Fast SCSI and Ultra SCSI require the use of active terminations at both ends of every cable segment in a
SCSI system with single-ended drivers and receivers. Ultra SCSI requires support of active negation and
hot plugging. The DS21T06 is fully compliant with these standards.
The DS21T06 integrates a low dropout regulator and 18 precise switched 110 ohms termination resistors
into a 28-pin 300-mil SOIC package.
Active termination provides greater immunity to voltage drops on the TERMPWR (TERMination
PoWeR) line, enhanced high-level noise immunity, intrinsic TERMPWR decoupling, and very low
quiescent current consumption. One DS21T06 can be used to terminate the Narrow SCSI bus, or a
DS21T07 can be used with a DS21T06 for termination of a Wide SCSI bus.
REFERENCE DOCUMENTS
SCSI-2 (X3.131-1994)
SCSI-3 Parallel Interface (X3T10/855D)
SCSI-3 Fast–20 (X3T10/1071D)
Available from: Global Engineering Documents
15 Inverness Way East
Englewood, CO 80112-5704
Phone: (800) 854-7179,(303) 792-2181
Fax: (303) 792-2192
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112099
DS21T06
FUNCTION DESCRIPTION
The DS21T06 is designed to be a single chip termination subsystem for use in Narrow SCSI systems. The
DS21T06 consists of a bandgap reference, buffer amplifier, and 18 termination resistors (Figure 1). The
voltage regulator circuitry (bandgap reference and class AB power amplifier) produces a precise laser
trimmed 2.85 volt level and is capable of sourcing 25 mA into each of the terminating resistors when the
signal line is low (active). 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 22 mA per line while keeping the voltage reference in regulation; the terminating
resistors maintain their 110 ohms value over the entire voltage range. When all lines settle in the
quiescent state, the regulator will consume about 10 mA.
When the DS21T06 is put into power-down mode by bringing
PD
low, the power-down circuitry will
turn off the transistors on each signal line. This will isolate the DS21T06 from the signal lines and
effectively remove it from the circuit. The power-down pin (
PD
) has an internal 50KΩ=pullup resistor. To
place the DS21T06 into an active state, the
PD
pin should be tied higher left open circuited.
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 bypassed locally. A 2.2
µF
capacitor and a 0.01
µF
high frequency capacitor is recommended between TPWR and ground and placed as close as possible
to the DS21T06. The DS21T06 should be placed as close as possible to the connector to minimize signal
and power trace length, thereby resulting in less input capacitance and reflections which can degrade the
bus signals.
To maintain the specified regulation, a 4.7
µF
capacitor is required between the VREF pin and ground of
each DS21T06. A high frequency cap (0.1
µF
ceramic recommended) can also be placed on the VREF
pin in applications that use fast rise/fall time drivers. Figure 2 details a typical SCSI bus configuration. In
an 8-bit data SCSI bus (Narrow SCSI), one DS21T06 is needed for proper termination of the 18 active
signal lines. One DS21T06 and one DS21T07 can be used to terminate the 27 active signal lines in a 16-
bit data SCSI bus (Wide SCSI).
DS21T06 BLOCK DIAGRAM
Figure 1
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DS21T06
TYPICAL SCSI BUS CONFIGURATION
Figure 2
NOTES:
1. C1 = 4.7
µF
tantalum
C2 = 2.2
µF
tantalum or 4.7
µF
aluminum
C3 = 0.01
µF
ceramic
2. If the DS21T06 is to be embedded into a peripheral that will act as a target on a SCSI bus, it is
recommended that TERMPWR be derived from the SCSI cable, not generated locally. In this
configuration, if a power failure occurs in the peripheral, it will not affect the bus.
3. A high frequency bypass capacitor (0.1
µF
recommended) can be added in parallel to C1 for
applications using fast rise/fall time drivers.
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DS21T06
PIN DESCRIPTION
Table 1
PIN
1
2-6, 10-13,
16-19, 23-27
7-9, 20-22
14
15
28
SYMBOL
PD
R1…R13
R14…R18
HS-GND
TPWR
VREF
GND
DESCRIPTION
Power Down.
When tied low, the DS21T06 enters power-down mode.
Contains an internal 50 kΩ pull-up. Tie low to deactivate the DS21T06.
Tie high or leave open circuited to activate the DS21T06.
Signal Termination 1.
110 ohms termination. Connect to SCSI bus
signal lines.
Heat Sink Ground.
Internally connected to the mounting pad. Should
be either grounded or electrically isolated from other circuitry.
Termination Power.
Connect to the SCSI TERMPWR line. Bypass
with a 2.2 µF cap.
Reference Voltage.
2.85 volt reference; must be decoupled with a 4.7
µF capacitor.
Ground.
Signal ground; 0.0 volts.
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DS21T06
ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground
Operating Temperature
Storage Temperature
Soldering Temperature
*
-1.0V to +7.0V
0°C to 70°C
-55°C to +125°C
260°C for 10 seconds
This is a stress rating only and functional operation of the device at these or any other conditions
above those indicated in the operation sections of this specification is not implied. Exposure to
absolute maximum rating conditions for extended periods of time may affect reliability.
The Dallas Semiconductor DS21T06 is built to the highest quality standards and manufactured for long
term reliability. All Dallas Semiconductor devices are made using the same quality materials and
manufacturing methods. However, the DS21T06 is not exposed to environmental stresses, such as burn-
in, that some industrial applications require. For specific reliability information on this product, please
contact the factory in Dallas at (972) 371-4448.
RECOMMENDED OPERATING CONDITIONS
PARAMETER
TERMPWR Voltage
PD
Active
PD
Inactive
SYM
V
TP
V
PDA
V
PDI
MIN
4.00
-0.3
2.00
TYP
MAX
5.25
0.8
V
TP
+ 0.3
(0°C to 70°C)
UNITS
V
V
V
NOTES
DC CHARACTERISTICS
PARAMETER
TERMPWR Current
Power-Down Current
Termination Resistance
Die Thermal Shutdown
Power-Down
Termination Capacitance
Input Leakage High
Input Leakage Low
SYMBOL
I
TP
I
PD
R
TERM
TS
D
C
PD
I
IH
I
IL
107.5
MIN
TYP
10
500
110
150
MAX
500
14
119.5
3
-1.0
1.0
(0°C to 70°C)
UNITS
mA
µA
Ω
°C
pF
µA
µA
NOTES
1, 3
1, 4
1, 2, 5
1, 2
1
1, 2, 5, 6
1
1, 7
REGULATOR CHARACTERISTICS
PARAMETER
Output Voltage
Drop Out Voltage
Output Current
Line Regulation
Load Regulation
Current Limit
Sink Current
SYMBOL
V
REF
V
DROP
I
OUT
LI
REG
LO
REG
I
L
I
SINK
MIN
2.7
TYP
2.85
0.75
1.0
1.3
700
400
MAX
3.0
1.0
25.2
2.0
3.0
(0°C to 70°C)
UNITS
V
V
mA
%
%
mA
mA
NOTES
1, 2
3, 6
9
1, 4
1, 2
1
1
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