TI UCC5614Z

The UCC5614 provides 9 active termination lines for the Small Computer System Interface (SCSI) parallel bus. The SCSI standard recommends active termination at both ends of a cable segment.

The UCC5614 is well-suited for high-performance 3.3-V SCSI systems. The key features enabling operation at such low supply voltages are a 0.1-V dropout regulator and a 2.7-V reference voltage. Supply current in the disconnected state is typically only 0.5 μA, making the device suitable for battery-powered systems.

The UCC5614 is designed with an ultra-low per-channel capacitance of 1.8 pF, eliminating the signal integrity impact that unterminated terminators have on midpoint signals on the bus.

The UCC5614 can be programmed for either 110-Ω or 2.5-kΩ termination. The 110-Ω termination is used for standard SCSI bus lengths, while the 2.5-kΩ termination is typically used for short-bus applications. When the TTL-compatible DISCNCT pin is driven directly, pulling DISCNCT low connects the 110-Ω termination; pulling it high disconnects it. When the DISCNCT pin is driven through an impedance between 80 kΩ and 150 kΩ, the terminator enters short-bus mode. The 2.5-kΩ termination is connected when DISCNCT is pulled low and disconnected when pulled high.

The power amplifier output stage allows the UCC5614 to source full termination current and sink active negation current when all termination lines are actively negated.

The UCC5614 is pin-for-pin compatible with Unitrode's other 9-line SCSI terminators, allowing existing systems to be upgraded to lower capacitance and lower voltage. Like all other Unitrode terminators, the UCC5614 fully supports hot-plugging, and the termination channels present high impedance when VTRMPWR = 0 V or open.

Internal circuit trimming first adjusts the 110-Ω termination impedance to a 7% tolerance, and more importantly, trims the output current to a 4% tolerance — as close as possible to the maximum SCSI specification — thereby maximizing noise margin in Fast SCSI operation.

Additional features include thermal shutdown and current limiting.

The device is offered in the industry-standard 16-pin narrow-body SOIC, 16-pin N, and a low-thermal-resistance 24-pin TSSOP package.

• Compliant with SCSI, SCSI-2, and SPI-2 single-ended standards
• 2.7V to 5.25V operating voltage
• 1.8 pF channel capacitance when disconnected
• 0.5 μA supply current in disconnect mode
• Programmable termination resistance: 110 Ω/2.5 kΩ
• Fully meets SCSI hot-plug requirements
• -400 mA current sourcing during termination
• +400 mA current sinking during active negation driving
• Termination current trimmed to 4%
• Impedance trimmed to 7%
• Current limiting and thermal shutdown protection