DS21T05
SCSI Terminator
www.dalsemi.com
FEATURES
PIN ASSIGNMENT
Fully compliant with SCSI-1, Fast SCSI and
Ultra SCSI
16
1
PD
TERMPWR1
Functionally compatible to the DS21T07,
15
2
R1
VREF2
targeted for high volume applications
14
3
R2
R9
R8
4
13
Provides active termination for nine signal
R3
12
5
R4
R7
lines
11
6
R5
R6
Laser-trimmed 110-ohm termination resistors
10
7
VREF1
NC
have 5% tolerance
9
8
GND
TERMPWR2
Low dropout voltage
DS21T05Z 16-Pin SOIC (150-mil)
Power-down mode isolates termination
resistors from the bus
PD
TERMPWR1
1
16
SCSI bus hot-plug compatible
R1
VREF2
2
15
Fully supports actively negated SCSI signals
R2
NC
3
14
Onboard thermal shutdown circuitry
R9
R3
4
16-pin plastic narrow SOIC
13
R4
R5
VREF1
GND
5
6
7
8
12
11
10
9
R8
R7
R6
TERMPWR2
DS21T05S 16-Pin SOIC (300-mil)
DESCRIPTION
Fast SCSI and Ultra SCSI require the use of active terminations at both ends of every cable segment in a
SCSI system with single-ended drivers and receivers. The DS21T05 SCSI Terminator, which is fully
compliant with these standards, enables the designer to gain the benefits of active termination: greater
immunity to voltage drops on the TERMPWR (TERMination PoWeR) line, enhanced high-level noise
immunity, intrinsic TERMPWR decoupling, and very low quiescent current consumption. The DS21T05,
which integrates a regulator and nine precise-switched, 110-ohm termination resistors into a monolithic
IC, is a functionally compatible version of the DS21T07. With relaxed output current and termination
tolerances, the DS21T05 is intended for high volume applications which require active termination but
not the high performance of the DS21T07.
FUNCTIONAL DESCRIPTION
The DS21T05 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 produces 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,
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112099
DALLAS
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