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112099
FEATURES
Fully compliant with SCSI-1, Fast SCSI and
Ultra SCSI
Functionally compatible to the DS21T07,
targeted for high volume applications
Provides active termination for nine signal
lines
Laser-trimmed 110-ohm termination resistors
have 5% tolerance
Low dropout voltage
Power-down mode isolates termination
resistors from the bus
SCSI bus hot-plug compatible
Fully supports actively negated SCSI signals
Onboard thermal shutdown circuitry
16-pin plastic narrow SOIC
PIN ASSIGNMENT
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. The DS21T05 SCSI Terminator, which is fully
compliant with these standards, enables the designer to gain the benefits of active termination: 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. The DS21T05,
which integrates a regulator and nine precise-switched, 110-ohm termination resistors into a monolithic
IC, is a functionally compatible version of the DS21T07. With relaxed output current and termination
tolerances, the DS21T05 is intended for high volume applications which require active termination but
not the high performance of the DS21T07.
FUNCTIONAL DESCRIPTION
The DS21T05 consists of a bandgap reference, buffer amplifier, and nine termination resistors (Figure 1).
The bandgap reference circuit produces a precise 2.55V level which is fed to a buffer amplifier. The
buffer produces a 2.85V level and is capable of sourcing at least 24 mA into each of the termination
resistors when the signal line is low (active). When the driver for a given signal line turns off, the
terminator will pull the signal line to 2.85V (quiescent state). To handle actively negated SCSI signals,
DS21T05
SCSI Terminator
www.dalsemi.com
1
2
3
4
5
6
7
8
TERMPWR1
R1
R3
R4
R5
VREF1
GND
R2
DS21T05Z 16-Pin SOIC (150-mil)
13
16
15
14
12
11
10
9
PD
VREF2
R9
R7
R6
NC
TERMPWR2
R8
1
2
3
4
5
6
7
8
13
16
15
14
12
11
10
9
PD
VREF2
NC
R8
R7
R6
TERMPWR2
R9
TERMPWR1
R1
R3
R4
R5
VREF1
GND
R2
DS21T05S 16-Pin SOIC (300-mil)
DS21T05
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the buffer can sink 200 mA, and V
REF
will move less than 60 mV. When all lines settle in the quiescent
state, the regulator will consume about 5 mA. When the DS21T05 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 DS21T05 from the signal lines and effectively remove it from the circuit. The power-down pin
(
PD
) has an internal 50 k
=pullup resistor. To place the DS21T05 into an active state, the
PD
pin should
be left open circuited. When installed on disk drives or RAID system components, the DS21T05 will not
affect the SCSI bus during a hot plug operation.
To ensure proper operation, both the TERMPWR1 and TERMPWR2 pins must be connected to the SCSI
bus TERMPWR line and both the VREF1 and VREF2 pins must be tied together externally. Each
DS21T05 requires a 4.7
µ
F capacitor connected between the VREF pins and ground. Figure 2 details a
typical SCSI bus configuration. In an 8-bit wide SCSI bus arrangement, two DS21T05s would be needed
at each end of the SCSI cable in order to terminate the 18 active signal lines. In a 16-bit wide SCSI bus
arrangement, three DS21T05's would be needed at each end of the SCSI cable in order to terminate the
27 active signal lines.
DS21T05 BLOCK DIAGRAM Figure 1
DS21T05
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TYPICAL SCSI BUS CONFIGURATION Figure 2
NOTES:
1.
C1 = 4.7
µ
F tantalum
C2 = 2.2
µ
F tantalum or 4.7
µ
F aluminum
2.
If the DS21T05 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
µ
F recommended) can be added in parallel to C1 for
applications using fast rise/fall time drivers.
DS21T05
DS21T05
DS21T05
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PIN DESCRIPTION Table 1
DS21T05Z
PIN
SYMBOL
DESCRIPTION
1
TERMPWR1
Termination Power 1. Should be connected to the SCSI TERMPWR
line. Must be decoupled with either a 2.2 µF or 4.7 µF. See Figure 2.
2
R1
Signal Termination 1. 110-ohm termination.
3
R2
Signal Termination 2. 110-ohm termination.
4
R3
Signal Termination 3. 110-ohm termination.
5
R4
Signal Termination 4. 110-ohm termination.
6
R5
Signal Termination 5. 110-ohm termination.
7
VREF1
Reference Voltage 1. Must be externally connected directly to the
VREF2 pin. Must be decoupled with a 4.7 µF capacitor as shown in
Figure 2.
8
GND
Ground. Signal ground; 0.0V.
9
TERMPWR2
Termination Power 2. Should be connected to the SCSI TERMPWR
line. Must be decoupled with either a 2.2 µF or 4.7 µF. See Figure 2.
10
NC
No Connect. Do not connect any signal to this pin.
11
R6
Signal Termination 6. 110-ohm termination.
12
R7
Signal Termination 7. 110-ohm termination.
13
R8
Signal Termination 8. 110-ohm termination.
14
R9
Signal Termination 9. 110-ohm termination.
15
VREF2
Reference Voltage 2. Must be externally connected directly to the
VREF1 pin. Must be decoupled with a 4.7 µF capacitor as shown in
Figure 2.
16
PD
Power Down. When tied low, the DS21T05 enters a power-down
mode. Contains an internal 50K pull-up. Strap low to deactivate the
DS21T05, leave open circuited to activate the DS21T05.
DS21T05
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ABSOLUTE MAXIMUM RATINGS*
Voltage on Any Pin Relative to Ground
-1.0V to +7.0V
Operating Temperature
0
°
C to 70
°
C
Storage Temperature
-55
°
C to +125
°
C
Soldering Temperature
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 DS21T05 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 DS21T05 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
(0°C to 70°C)
PARAMETER
SYM
MIN
TYP
MAX
UNITS
NOTES
TERMPWR Voltage
V
TP
4.00
5.50
V
PD
Active
V
PDA
-0.3
+0.8
V
PD
Inactive
V
PDI
2.0
V
TP
+ 0.3
V
DC CHARACTERISTICS
(0°C to 70°C)
PARAMETER
SYMBOL
MIN
TYP
MAX
UNITS
NOTES
TERMPWR Current
I
TP
I
TP
5
250
8
mA
mA
1, 3
1, 4
Power-Down Current
I
PD
500
µA
1, 2, 5
Termination Resistance
R
TERM
107.5
110
119.5
ohm
1, 2
Die Thermal Shutdown
T
SD
150
°C
1, 6
Power Down
Termination Capacitance
C
PD
3.0
pF
1, 2, 5, 6
Input Leakage High
I
IH
-1.0
µA
1, 8
Input Leakage Low
I
IL
1.0
µA
1, 7
REGULATOR CHARACTERISTICS
(0°C to 70°C)
PARAMETER
SYMBOL
MIN
TYP
MAX
UNITS
NOTES
Output Voltage
V
REF
2.7
2.85
3.1
V
1, 2
Drop Out Voltage
V
DROP
0.75
1.0
V
3, 6
Output Current
I
OUT
25.2
mA
9
Line Regulation
LI
REG
1.0
2.0
%
1, 4
Load Regulation
LO
REG
1.3
3.0
%
1, 2
Current Limit
I
LIM
350
mA
1
Sink Current
I
SINK
200
mA
1
DS21T05
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NOTES:
1.
4.00V < TERMPWR < 5.50V.
2.
0.0V < signal lines < 3.0V.
3.
All signal lines = 0.0V.
4.
All signal lines open.
5.
PD
= 0.0V.
6.
Guaranteed by design; not production tested.
7.
R
1
through R
9
only.
8.
R
1
through R
9
and
PD
.
9.
V
SIGNAL
= 0.2V.
DS21T05Z SCSI TERMINATOR 16-PIN SOIC (150-MIL)
PKG
16-PIN
DIM
MIN
MAX
A IN.
MM
0.053
1.35
0.069
1.75
A1 IN.
MM
0.004
0.10
0.010
0.25
A2 IN.
MM
0.048
1.24
0.062
1.57
b IN.
MM
0.012
0.30
0.020
0.50
C IN.
MM
0.007
0.17
0.011
0.28
D IN.
MM
0.386
9.80
0.393
9.98
e IN.
MM
0.050 BSC
1.27 BSC
E1 IN.
MM
0.150
3.81
0.158
4.01
H IN.
MM
0.230
5.84
0.244
6.20
L IN.
MM
0.016
0.40
0.050
0.89
0°
8°
DS21T05
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DS21T05S SCSI TERMINATOR 16-PIN SOIC (300-MIL)
The chamfer on the body is optional. If it is not present, a terminal 1 identifier must be positioned so that
½ or more of its area is contained in the hatched zone.
PKG
16-PIN
DIM
MIN
MAX
A IN.
MM
0.094
2.38
0.105
2.68
A1 IN.
MM
0.004
0.102
0.012
0.30
A2 IN.
MM
0.089
2.26
0.095
2.41
b IN.
MM
0.013
0.33
0.020
0.51
C IN.
MM
0.009
0.229
0.013
0.33
D IN.
MM
0.398
10.11
0.412
10.46
e IN.
MM
0.050 BSC
1.27 BSC
E1 IN.
MM
0.290
7.37
0.300
7.62
H IN.
MM
0.398
10.11
0.416
10.57
L IN.
MM
0.016
0.40
0.040
1.02
0°
8°
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