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DS2121A Ultra3 LVD/SE SCSI Terminator
www.dalsemi.com
FEATURES
Fully compliant with Ultra2, Ultra3, and Ultra 160 LVD-only SCSI Provides Low Voltage Differential (LVD) termination for 9 signal line pairs 5% tolerance on SE and LVD termination resistance Low power-down capacitance of 3 pF Onboard thermal shutdown circuitry SCSI bus hot plug compatible
PIN ASSIGNMENT
VREF RIP RIN R2P R2N HS GND R3P R3N R4P R4N R5P R5N ISO GND 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 TPWR TPWR R9N R9P R8N R8P HS GND R7N R7P R6N R6P DIFF_CAP DIFFSENSE MSTR/SLV
DS2121AE 28-Pin TSSOP
DESCRIPTION
The DS2121A LVD SCSI Terminator is a Low Voltage Differential (LVD) terminator. If the device is connected in an LVD-only bus, the DS2121A will use LVD termination. If any SE devices are connected to the bus, the DS2121A will disconnect from the bus. This is accomplished automatically inside the part by sensing the voltage on the SCSI bus DIFFSENS line. For the LVD termination, the DS2121A integrates two current sources with nine precision resistor strings. Three DS2121A terminators are needed for a wide SCSI bus.
REFERENCE DOCUMENTS
SCSI Parallel Interface 2 (SPI-2) {X310/1142D} SCSI-3 Parallel Interface (SPI) {X3T10/855D} SCSI-3 Fast-20 {X3T10/1071D} SCSI-2 {X3.131-1994} 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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031400
DS2121A
FUNCTIONAL DESCRIPTION
The DS2119 combines LVD and SE termination with DIFFSENS sourcing and detection. A bandgap reference is fed into two amplifiers, which creates a 1.25V If the SCSI bus is in LVD mode, then the 1.25V reference will be used. The DIFFSENSE circuitry decodes trinary logic. There will be one of three voltages on the SCSI control line called DIFFSENS. Two comparators and a NAND gate determine if the voltage is below 0.6V, above 2.15V, or in between. That indicates the mode of the bus to be SE, HVD, or LVD, respectively. The DS2121A's DIFF_CAP pin monitors the DIFFSENS line to determine the proper operating mode of the device. The DIFFSENSE pin can also drive the SCSI DIFFSENS line (when MSTR/SLV = 1) to determine the SCSI bus operating mode. The DS2121A switches to the termination mode that is appropriate for the bus based on the value of the DIFFSENS voltage. These modes are: LVD mode LVD termination is provided by a precision laser trimmed resistor string with two current sources. This configuration yields a 105=differential and 150=common mode impedance. A fail-safe bias of 112 mV is maintained when no drivers are connected to the SCSI bus. SE mode The DS2121A identifies that there is a SE (single-ended) device on the SCSI bus and isolates the termination pins from the bus. HVD Isolation Mode The DS2121A identifies that there is an HVD (high voltage differential) device on the SCSI bus and isolates the termination pins from the bus. When ISO = 1, the termination pins are isolated from the SCSI bus, Vref remains active. During thermal shutdown, the termination pins are isolated from the SCSI bus and Vref becomes high impedance. The DIFFSENSE driver is shut down during either of these two events. To ensure proper operation, the TPWR pin should be connected to the SCSI bus TERMPWR line. As with all analog circuitry, the TERMPWR and VDD lines should be bypassed locally. A 2.2 F capacitor and a 0.01 F high frequency capacitor are recommended between TPWR and ground and placed as close as possible to the DS2121A. The DS2121A should be placed as close as possible to the SCSI 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 F capacitor is required between the Vref pin (VREF) and ground of each DS2121A. A high frequency cap (0.1 F ceramic recommended) can also be placed on the Vref pin in applications that use fast rise/fall time drivers. A typical SCSI bus configuration is shown in Figure 2.
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DS2121A
DIFFSENS noise filtering The DS2121A incorporates a digital filter to remove high frequency transients on the DIFFSENS control line, thereby eliminating erroneous switching between modes. This filter eliminates the need for the external capacitor and resistor, which heretofore performed this function.
NOTE:
DIFFSENS - Refers to the SCSI bus signal. DIFFSENSE - Refers to the Dallas Semiconductor pin name and internal circuitry relating the DIFFSENS driver.
DS2121A BLOCK DIAGRAM Figure 1
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DS2121A
SCSI BUS CONFIGURATION Figure 2
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DS2121A
PIN DESCRIPTION Table 1
PIN 1 2-5, 7-12, 18-21, 23-26 6, 22 13 14 15 16 17 SYMBOL VREF RxP, RxN HS GND DESCRIPTION Reference Voltage. 1.25-volt reference in LVD mode; must be decoupled with a 4.7 F cap. Signal Termination. Connect to SCSI bus signal lines.
27, 28
Heat Sink Ground. Internally connected to the mounting pad. Should be grounded. ISO Isolation. When pulled high, the DS2121A isolates its bus pins (RxP, RxN) from the SCSI bus. GND Ground. Signal ground; 0.0 volts. MSTR/SLV Master/slave. Mode select for the non-controlling terminator. When pulled high, MSTR enables the DIFFSENSE driver. DIFFSENSE DIFFSENSE. Output to drive the SCSI bus DIFFSENS line. DIFF_CAP DIFFSENSE CAPACITOR. Connect a 0.1 F capacitor for DIFFSENSE filter. Input to detect the type of device (differential or single-ended) on the SCSI bus. TPWR Termination Power. Connect to the SCSI TERMPWR line and decouple with a 2.2 F capacitor.
RECOMMENDED OPERATING CONDITIONS
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Termpower Voltage LVD mode Logic 0 Logic 1 Operating Temperature
Vtpwr(LVD) Vil Vih Vamb
2.7 -0.3 2.0 0
5.5 +0.8 Vtpwr + 0.3 70
V V V C
LOW VOLTAGE DIFFERENTIAL CHARACTERISTICS
PARAMETER
Differential Mode Termination Resistance Common Mode Termination Resistance Differential Mode Bias Common Mode Bias
SYMBOL Rdm Rcm Vdm Vcm
MIN 100 110 100 1.125
TYP
MAX 110 190 125 1.375
UNITS Ohms Ohms mV V
NOTES
4
DC CHARACTERISTICS
PARAMETER
Termpower Current Input Leakage High Input Leakage Low Output Current High Output Current Low DIFFSENS SE Operating Range DIFFSENS LVD Operating Range DIFFSENS HVD Operating Range
SYMBOL Itpmr Iih Iil Ioh Iol Vseor Vlvdor Vhvdor
MIN -1.0
TYP 12
MAX
1.0 -1.0 4.0 -0.3 0.7 2.4
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UNITS mA A A mA mA V V V
NOTES 4
5, 7 6, 7
0.5 1.9 Vtpwr + 0.3
DS2121A
PARAMETER
DIFFSENSE Driver Output Voltage DIFFSENSE Driver Source Current DIFFSENSE Driver Sink Current Thermal Shutdown
SYMBOL Vdso Idsh Idsl
MIN 1.2 5 20
TYP
MAX 1.4 15 200
UNITS V mA A C
NOTES 8, 9 8, 10, 12 8, 11 3
150
REGULATOR CHARACTERISTICS
PARAMETER
Line Regulation Load Regulation Current Limit Sink Current
(0C to 70C)
MIN TYP 1.0 1.3 550 MAX 2.5 3.5 UNITS % % mA mA NOTES
SYMBOL LIREG LOREG ILIM ISINK
200
NOTES:
1. Vline = 0-3.0 volts. 2. Vline = 0.2 volts. 3. Guaranteed by design. 4. All lines open. 5. VOUT = 2.4 volts. 6. VOUT = 0.4 volts. 7. SE/LVD/HVD pins only. 8. MSTR/SLV = 1. 9. Ids = 0-5 mA. 10. Vdso = 0.0 volts. 11. Vdso = 2.75 volts. 12. TPWR = 5.5V
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DS2121A
DS2121A 28-PIN SSOP PACKAGE NOTES:
1. DIMENSION "D" DOES NOT INCLUDE MOLD MISSMATCH, FLASH OR PROTRUSIONS. MOLD MISSMATCH, FLASH AND PROTRUSIONS SHALL NOT EXCEED 0.15 PER SIDE. 2. DIMENSION "B" DOES NOT INCLUDE DAMBAR PROTRUSION. DAMBAR PROTRUSION SHALL NOT BE LOCATED ON THE LOWER RADIUS OF THE FOOT.
DIM A A1 A2 c L e1 b D E1 E
MIN MAX 1.10 0.05 0.75 1.05 0.09 0.20 0.50 0.75 0.65 BSC 0.18 0.30 9.60 9.80 4.40 BSC 6.20 6.60
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