DS21S07AS+

Overview

The DS21S07AS+ is a monolithic SCSI terminator compliant with SCSI-1, Fast SCSI, and Ultra SCSI standards. It integrates a low-dropout voltage regulator and nine laser-trimmed 110Ω termination resistors with 2% tolerance into a 16-pin SOIC package. The device operates on a TERMPWR supply ranging from 4.0V to 5.5V, consuming approximately 2.5mA in active mode and 150µA in power-down mode. It provides active termination for nine signal lines, featuring intrinsic decoupling and thermal shutdown circuitry at 150°C.

Feature Summary

• Integrates a bandgap reference and buffer amplifier to source at least 24mA per line when active and sink at least 200mA for actively negated signals.
• Includes an active-low power-down pin that isolates the termination resistors from the bus, enabling hot-plug compatibility.
• Features internal 50kΩ pullup resistor on the power-down pin and requires external 4.7µF decoupling capacitors on VREF pins.

Applications

• Terminating active signal lines in single-ended SCSI systems
• Disk drive and RAID system components requiring hot-plug support
• Ultra SCSI cable segment termination

Procurement Notes

Verify component authenticity through authorized Analog Devices or Maxim Integrated distributors. Confirm the lead-free status indicated by the '+' suffix matches project requirements. Check datasheet revision history for any updates to electrical characteristics or pin configurations before finalizing procurement specifications.

Selection Notes

Select this component for designs requiring compliance with legacy SCSI standards and active termination capabilities. Ensure the PCB layout accommodates the 16-pin SOIC package dimensions. Verify that the system TERMPWR line remains stable within the 4.0V to 5.5V range to maintain proper regulator operation and termination accuracy.

Part Context

The DS21S07AS+ belongs to the DS21S07A family of SCSI terminators manufactured by Analog Devices Inc./Maxim Integrated. It serves as a backward-compatible replacement for earlier models like the DS2107 and DS2107A. The 'AS' designation specifically identifies the 16-pin Plastic SO package variant within this product line.

Replacement Considerations

The DS21S07AE is a compatible alternative offering identical functionality in a 20-pin TSSOP package. Designers must verify pin compatibility and footprint changes when substituting between the SO and TSSOP variants.

FAQ

How does the device handle actively negated SCSI signals?

The internal buffer can sink at least 200mA, ensuring that the reference voltage (VREF) moves less than 60mV during actively negated signal transitions.

What is the function of the PD pin?

The Power-Down (PD) pin is active-low. Tying it low activates power-down mode, turning off transistors to isolate the device from the bus. Leaving it open-circuited activates the device due to an internal 50kΩ pullup resistor.

What are the external capacitor requirements?

A 4.7µF capacitor is required between the VREF pins and ground. Additionally, either a 2.2µF or 4.7µF capacitor is needed to decouple the TERMPWR pins.