DS21S07A
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 which is fed to a buffer amplifier. The buffer pro-
duces a 2.85V level and is capable of sourcing at least
24 mA 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 at least 200 mA, and
V
REF
will move less than 60 mV. When all lines settle in
the quiescent state, the regulator will consume about
2.5 mA. When the DS21S07A 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 iso-
late the DS21S07A from the signal lines and effectively
remove it from the circuit. The power–down pin (PD)
has an internal 50KΩ pull–up 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 re-
quires 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 in order 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 in order to terminate the 27
active signal lines.
DS21S07A BLOCK DIAGRAM
Figure 1
PD
TERMPWR1
TERMPWR2
50K Ohm
Power
Down
Circuitry
110 Ohms
R1
110 Ohms
R2
Bandgap
Reference
+
110 Ohms
R3
110 Ohms
R4
110 Ohms
R5
110 Ohms
R6
GND
110 Ohms
R7
110 Ohms
R8
110 Ohms
R9
VREF1
VREF2
–
022798 2/7
ETC [ ETC ]
