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

50k

Ω 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 4.7

µF 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