DS2120B Ver la hoja de datos (PDF) - Maxim Integrated

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DS2120B

Ultra3 LVD SCSI Terminator

Maxim Integrated 

DS2120

During thermal shutdown, the termination pins are isolated from the SCSI bus and VREF becomes high

impedance. The DIFFSENS driver is shut down during either of these two events. The DIFF_CAP

receiver is disabled and the LVD goes low, indicating a non-LVD condition.

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 2.2µF capacitor

and a 0.01µF high-frequency capacitor are recommended between TPWR and ground and placed as close

as possible to the DS2120. The DS2120 should be placed as close as possible to the SCSI connector to

minimize signal and power trace length, thereby lessening input capacitance and reflections that can

degrade the bus signals.

To maintain the specified regulation, a 4.7µF capacitor is required between the VREF pin and ground of

each DS2120. A high-frequency cap (0.1µF 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.

DIFFSENS Noise Filtering: The DS2120 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 previously performed this function.

The external filter can be used in addition to the digital filter if the DS2120 and DS2118M or DS2119M

are to be used interchangeably.

NOTES:

1) DIFFSENS: Refers to the SCSI bus signal.

2) DIFFSENSE: Refers to the Dallas Semiconductor pin name and internal circuitry relating to

differential sensing.

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