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GENERAL 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 DS21S07A 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
DS21S07A integrates a regulator and nine precise
switched 110Ω termination resistors into a
monolithic IC. The DS21S07A can be electrically
isolated from the SCSI bus without physical removal
from the SCSI device.
FEATURES
Fully Compliant with SCSI-1, Fast SCSI, and
Ultra SCSI
Backward Compatible to the DS2107 and
DS2107A
Provides Active Termination for Nine Signal
Lines
Laser-Trimmed 110Ω Termination Resistors
Have 2% Tolerance
Low Drop-Out Voltage Regulator
Power-Down Mode Isolates Termination
Resistors from the Bus
SCSI Bus Hot Plug Compatible
Fully Supports Actively Negated SCSI Signals
On-Board Thermal Shutdown Circuitry
16-Pin Plastic SO (DS21S07AS) and 20-Pin
Plastic TSSOP (Thin Shrink Small Outline
Package) (DS21S07AE)
PIN CONFIGURATIONS
ACTUAL FOOTPRINT SIZE
ORDERING INFORMATION
PART VOLTAGE
(V)
PIN-
PACKAGE
TOP
MARK
DS21S07AS 5 16 SO DS21T07S
DS21S07AS+ 5 16 SO DS21T07S
DS21S07AS/T&R 5 16 SO DS21T07S
DS21S07AS+T&R 5 16 SO DS21T07S
DS21S07AE 5 20 TSSOP DS21T07E
DS21S07AE/T&R 5 20 TSSOP DS21T07E
+ Denotes lead-free package. The top mark includes a “+" on
lead-free packages.
Note: All devices are specified over the 0°C to +70°C operating
temperature range.
DS21S07A
SCSI Terminato
r
www.maxim-ic.com
0.41"
0.41" 0.26"
0.27"
DS21S07AS DS21S07AE
DS21S087A
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FUNCTIONAL DESCRIPTION
The DS21S07A consists of a bandgap reference, buffer amplifier, and nine termination resistors
(Figure 1). The bandgap reference circuit produces a precise 2.55V level that is fed to a buffer amplifier.
The buffer produces a 2.85V level and can source at least 24mA 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 pulls
the signal line to 2.85V (quiescent state). To handle actively negated SCSI signals, the buffer can sink at
least 200mA, and VREF will move less than 60mV. When all lines settle in the quiescent state, the
regulator consumes about 2.5mA. When the DS21S07A is put into power-down mode by bringing PD
low, the power-down circuitry turns off the transistors on each signal line. This isolates the DS21S07A
from the signal lines and effectively removes it from the circuit. The power-down pin (PD) has an internal
50kΩ pullup resistor. To place the DS21S07A into an active state, the PD pin should be left open
circuited. When installed on disk drives or RAID system components, the DS21S07A 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
DS21S07A requires a 4.7µF capacitor connected between the VREF pins and ground. Figure 2 details a
typical SCSI bus configuration. In an 8-bit-wide SCSI bus arrangement (“A” Cable), two DS21S07A’s
would be needed at each end of the SCSI cable to terminate the 18 active signal lines. In a 16-bit-wide
SCSI bus arrangement (“P” Cable), three DS21S07A’s would be needed at each end of the SCSI cable to
terminate the 27 active signal lines.
Figure 1. DS21S087A Block Diagram
DS21S087A
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Figure 2. Typical Differential SCSI Bus Configuration
NOTE 1: C1 = 4.7µF TANTALUM CAPACITOR; C2 = 2.2
µ
F TANTALUM OR 4.7
µ
F ALUMINUM CAPACITOR.
NOTE 2: IF THE DS21S07A 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.
NOTE 3: A HIGH-FREQUENCY BYPASS CAPACITOR (0.1µF RECOMMENDED) CAN BE ADDED IN
PARALLEL TO C1 FOR APPLICATIONS USING FAST RISE/FALL TIME DRIVERS.
DS21S087A
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Table 1. Pin Description
PIN
TSSOP SO NAME FUNCTION
1 1 TERMPWR1
Termination Power 1. Should be connected to the SCSI
TERMPWR line. Must be decoupled with either a 2.2µF
capacitor or 4.7µF capacitor. See Figure 2.
2, 8, 12,
18 — HS-GND
Heat Sink Ground. Internally connected to the mounting pad.
Should be either grounded or electrically isolated from other
circuitry.
3–7,
13–16
2–6,
10–13 R1–R9 Signal Termination 1–9. 110Ω termination.
9 7 VREF1
Reference Voltage 1. Must be externally connected directly to
the VREF2 pin. Must be decoupled with a 4.7µF capacitor as
shown in Figure 2.
10 8 GND Ground. 0V, signal ground.
11 9 TERMPWR2
Termination Power 2. Should be connected to the SCSI
TERMPWR line. Must be decoupled with either a 2.2µF
capacitor or 4.7µF capacitor. See Figure 2.
17 14 NC No Connection. Do not connect any signal to this pin.
19 15 VREF2
Reference Voltage 2. Must be externally connected directly to
the VREF1 pin. Must be decoupled with a 4.7µF capacitor as
shown in Figure 2.
20 16 PD
Active-Low Power-Down. When tied low, the DS21S07A
enters a power-down mode. Contains an internal 50kΩ pullup
resistor. Strap low to deactivate the DS21S07A; leave open
circuited to activate the DS21S07A
DS21S087A
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ABSOLUTE MAXIMUM RATINGS
Voltage Range on Any Pin Relative to Ground……………………………………………..-1.0V to +7.0V
Operating Temperature Range………………………………………………………………..0°C to +70°C
Storage Temperature Range………………………………………………………………-55°C to +125°C
Soldering Temperature………………………………………….See IPC/JEDEC J-STD-020 Specification
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 device
reliability.
RECOMMENDED OPERATING CONDITIONS
(TA = 0°C to +70°C)
PARAMETER SYM MIN TYP MAX UNITS NOTES
TERMPWR Voltage VTP 4.00 5.50 V
PD Active VPDA -0.3 +0.8 V
PD Inactive VPDI 2.0 VTP + 0.3 V
DC CHARACTERISTICS
(TA = 0°C to +70°C)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
250 mA 1, 3
TERMPWR Current ITP 2.5 5 mA 1, 4
Power-Down Current IPD 100 150 µA 1, 2, 5
Termination Resistance RTERM 108 110 112
Ω 1, 2
Die Thermal Shutdown TSD 150 ºC 1, 6
Power-Down Termination
Capacitance CPD 3.0 5.0 pF 1, 2, 5, 6
Input Leakage High IIH -1.0 µA 1, 8
Input Leakage Low IIL 1.0 µA 1, 7
REGULATOR CHARACTERISTICS
(TA = 0°C to +70°C)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Output Voltage VREF 2.79 2.85 2.93 V 1, 2
Drop-Out Voltage VDROP 0.50 0.75 V 3, 6
Line Regulation LIREG 1.0 2.0 % 1, 4
Load Regulation LOREG 1.3 3.0 % 1, 3
Current Limit ILIM 350 mA 1
Sink Current ISINK 200 mA 1
DS21S087A
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NOTES:
1) 4.00V < TERMPWR < 5.50V.
2) 0.0V < signal lines < TERMPWR.
3) All signal lines = 0V.
4) All signal lines open.
5) PD = 0V.
6) Guaranteed by design; not production tested.
7) R1 to R9 only.
8) R1 to R9 and PD.
DS21S087A
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Maxim/Dallas Semiconductor cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim/Dallas Semiconductor product.
No circuit patent licenses are implied. Maxim/Dallas Semiconductor reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2006 Maxim Integrated Products • Printed USA
The Maxim logo is a registered trademark of Maxim Integrated Products, Inc. The Dallas logo is a registered trademark of Dallas Semiconductor Corporation.
PACKAGE INFORMATION (continued)
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline
information, go to www.maxim-ic.com/DallasPackInfo.)
