ST21S07A

4/10

FUNCTIONAL DESCRIPTION

The ST21S07A consists of a bandgap reference,

buffer amplifier, and nine termination resistors

Figure 1). The bandgap reference circuit produces

a precise 1.25V level which is fed to a buffer

amplifier. The buffer produce a 2.85V level and is

capable of sourcing 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 will pull the signal line to

2.85V (quiescent state). To

handle actively

negated SCSI signals, the buffer can sink 200mA.

When all lines settle in the quiescent state, the

regulator will consume about 1.5mA. When the

ST21S07A is put into power-down mode by

bringing PD low, the power-down circuitry will turn

off the transistors on each signal line. This will

isolated the ST21S07A from the signal lines and

effectively

remove

it

from

the

circuit.

The

power-down pin (PD) has an internal pull-up

resistor.To place the ST21S07A into an active

state, the PD pin should be left open circuited.

To ensure proper operation, both the TERMPWR1

and TERMPWR2 pins must be connected to the

SCSI bus TREMPWR line and both the VREF1

and VREF2 pins must be tied together externally.

Each ST21S07A 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 ST21S07A’s would be needed at each

end of the SCSI cable in order to terminate the 18

active

signal

lines.

In

a

16-bit

SCSI

bus

arrangement ("P" Cable), three ST21S07A’s

would be need at each end of the SCSI cable in

order to terminate the 27 active signal lines.

Figure 1 : Block Diagram