DS21T06

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FUNCTION DESCRIPTION

The DS21T06 is designed to be a single chip termination subsystem for use in Narrow SCSI systems. The

DS21T06 consists of a bandgap reference, buffer amplifier, and 18 termination resistors (Figure 1). The

voltage regulator circuitry (bandgap reference and class AB power amplifier) produces a precise laser

trimmed 2.85 volt level and is capable of sourcing 25 mA into each of the terminating resistors when the

signal line is low (active). 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 22 mA per line while keeping the voltage reference in regulation; the terminating

resistors maintain their 110 ohms value over the entire voltage range. When all lines settle in the

quiescent state, the regulator will consume about 10 mA.

turn off the transistors on each signal line. This will isolate the DS21T06 from the signal lines and

Ω=pullup resistor. To

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 bypassed locally. A 2.2

µF capacitor and a 0.01

µF high frequency capacitor is recommended between TPWR and ground and placed as close as possible

to the DS21T06. The DS21T06 should be placed as close as possible to the 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

µF capacitor is required between the VREF pin and ground of

each DS21T06. A high frequency cap (0.1

µF ceramic recommended) can also be placed on the VREF

pin in applications that use fast rise/fall time drivers. Figure 2 details a typical SCSI bus configuration. In

an 8-bit data SCSI bus (Narrow SCSI), one DS21T06 is needed for proper termination of the 18 active

signal lines. One DS21T06 and one DS21T07 can be used to terminate the 27 active signal lines in a 16-

bit data SCSI bus (Wide SCSI).

DS21T06 BLOCK DIAGRAM Figure 1