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SS8218-S4TR датащи(PDF) 2 Page - Silicon Standard Corp. |
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SS8218-S4TR датащи(HTML) 2 Page - Silicon Standard Corp. |
2 / 11 page www.SiliconStandard.com 2 of 11 SS8218 Rev.2.01 6/06/2003 Ordering Information PART NUMBER TEMP. RANGE PIN-PACKAGE FEATURE SS8218-S4TB 0ºC-70ºC SSOP-28 pin / Tube DISCNCT does not disable VREF SS8218-S4TR 0ºC-70ºC SSOP-28 pin / Tape & Reel DISCNCT does not disable VREF SS8218-S5TB 0ºC-70ºC SSOP-36 pin / Tube DISCNCT does not disable VREF SS8218-S5TR 0ºC-70ºC SSOP-36 pin / Tape & Reel DISCNCT does not disable VREF SS8218-D3TB 0ºC-70ºC TSSOP-28 pin / Tube DISCNCT does not disable VREF SS8218-D3TR 0ºC-70ºC TSSOP-28 pin / Tape & Reel DISCNCT does not disable VREF Note: DIFFSENS Refers to the SCSI bus signal. DIFFSENSE Refers to the SS8218 pin name and internal circuitry relating to differential sensing. DIFFB Refers to the SS8218 pin name and internal circuitry relating to monitoring the DIFFSENS line. Functional Description The SS8218 is used in multi-mode active termination applications, where single ended (SE) and low voltage differential (LVD) devices might coexist. The LVD ter- mination section consists of 18 source/sink amplifiers (VTOP, VBOT), biasing circuitry and nine precision resistor strings (RTOP, RMID, RBOT). The SE termi- nation section consists of a 2.85V source/sink regula- tor with 9 precision 110ohm resistor. The DIFFSENSE section consists of a 1.3V, 5mA driver and a sensing circuit (Figure 1) DIFFSENS is used to identify which types of SCSI devices are present on the bus. If the voltage on DIFFSENS is between 0-0.5V the bus is single-ended; if it is between 0.7-1.9V the bus is LVD; and if it is greater than 2.4V, the bus is HVD. The SS8218 DIFFB pin monitors the DIFFSENS line to determine the proper operating mode of the device; this mode is indicated by the SE/LVD/HVD outputs. HVD Isolation Mode The SS8218 identifies that there is an HVD (high voltage differential) device on the SCSI bus and isolates the termination pins from the bus. When DISCNCT is pulled high, the termination pins are isolated from the SCSI bus, and the bus mode indicators (SE/LVD/HVD) as well as 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 lines should be by- passed locally. A 2.2µF capacitor and a 0.01µF high frequency capacitor is recommended between TPWR and ground and placed as close as possible to the SS8218. The 8218 should be placed as close as possible to the SCSI connector to minimize signal and power trace length, thereby resulting in less input ca- pacitance and reflections which can degrade the bus signals. The DIFFSENSE pin can drive the SCSI DIFFSENS line (when M/S is pulled high) to determine the SCSI bus operating mode. The SS8218 switches to the ter- mination 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 resistor string with two amplifiers. This configuration yields a 105 Ω differential and 150Ω common mode impedance. A fail safe bias of 112mV is maintained when no drivers are connected to the SCSI bus. SE mode 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 22mA per line while keeping the voltage reference in regula- tion. The terminating resistors maintain their 110 Ω value. To maintain the specified regulation, a 4.7µF capacitor is required between the VREF pin and ground of each SS8218. A high frequency cap (0.1µF ceramic recom- mended) can also be placed on the VREF pin in ap- plications that use fast rise/fall time drivers. A typical SCSI bus configuration is shown in Figure2. An internal pull down resistor assures that the SS8218 will be terminating the bus if the DISCNCT pin is left floating. |
Аналогичный номер детали - SS8218-S4TR |
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Аналогичное описание - SS8218-S4TR |
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