Analog Devices Inc./Maxim Integrated DS21S07AS

Overview

DS21S07AS from Maxim Integrated is a monolithic SCSI active terminator IC for single-ended Fast and Ultra SCSI bus systems, integrating nine laser-trimmed 110Ω termination resistors (±2% tolerance), a low-dropout 2.85V regulator, and power-down isolation logic. It operates from 4.0V–5.5V TERMPWR, consumes 2.5mA quiescent current, and supports hot-plug operation in disk drives and RAID controllers.

For engineers reviewing the DS21S07AS datasheet, DS21S07AS pinout, DS21S07AS application, or DS21S07AS equivalent, key selection considerations include its 16-pin SO package, dual TERMPWR/VREF inputs, active-low PD control with internal 50kΩ pullup, thermal shutdown protection, and compliance with SCSI-1/Fast/Ultra SCSI active termination requirements.

Technical Context

The DS21S07AS implements a bandgap-referenced 2.85V regulator that sources ≥24mA per termination line when pulled low and sinks ≥200mA to maintain VREF stability during actively negated SCSI signaling. Its nine switched 110Ω resistors are laser-trimmed to ±2% tolerance and isolated via an active-low power-down circuit with <150µA standby current.

Each DS21S07AS requires external 4.7µF decoupling on VREF pins and dual TERMPWR connections; two devices terminate an 8-bit SCSI bus (18 lines), while three handle a 16-bit bus (27 lines). The PD pin's internal 50kΩ pullup enables default activation when left open.

Key Specifications

Pinout & Package

DS21S07AS is housed in a 16-pin plastic SO (Small Outline) package with standard JEDEC MS-012AC dimensions (10.3mm × 7.5mm × 2.35mm), gull-wing leads, and exposed pad not internally connected.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar SCSI active termination applications.

Compared with DS2107A, DS21S07AS reduces component count by integrating regulator and precision resistors, while MAX3243ESE+ serves entirely different interface protocols and cannot substitute for SCSI termination functionality.

Availability

DS21S07AS is available at Aetrix Electronics and suitable for SCSI disk drive design, RAID controller manufacturing, and legacy server storage expansion requiring stable component supply and long-term industrial availability.

Supply support for DS21S07AS includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Maxim Integrated (now part of Analog Devices) is a U.S.-based semiconductor company specializing in high-performance analog, mixed-signal, and power management ICs for industrial, communications, and computing markets.

The DS21S07AS belongs to Maxim's legacy SCSI interface product line, designed specifically to replace discrete termination networks with monolithic, standards-compliant active termination for commercial storage subsystems.

FAQ

What is the primary function of the DS21S07AS in a SCSI system?

The DS21S07AS serves as a monolithic active terminator for single-ended SCSI buses, providing nine precisely matched 110Ω termination resistors, a 2.85V reference regulator, and power-down isolation. It replaces discrete resistor networks and external regulators in Fast and Ultra SCSI designs, ensuring compliance with SCSI-1, Fast SCSI, and Ultra SCSI standards while improving noise immunity and reducing quiescent current consumption compared to passive solutions.

How does the DS21S07AS handle hot-plug operations on a SCSI bus?

The DS21S07AS supports hot-plug compatibility through its active-low PD (power-down) pin and integrated isolation circuitry. When PD is asserted low, the device electrically disconnects all nine termination resistors from the SCSI signal lines without requiring physical removal. Its internal 50kΩ pullup resistor ensures default activation when PD is left open, enabling seamless insertion and removal of SCSI peripherals like disk drives or tape libraries without bus disruption.

What external components are required for proper DS21S07AS operation?

The DS21S07AS requires two external components: a 4.7µF tantalum capacitor between the VREF1 and VREF2 pins (which must be shorted externally), and decoupling capacitors (2.2µF or 4.7µF) on both TERMPWR1 and TERMPWR2 pins connected to the SCSI bus TERMPWR line. A 0.1µF high-frequency bypass capacitor may also be added in parallel with the 4.7µF VREF capacitor for fast-rise-time driver applications.

Is the DS21S07AS pin-compatible with earlier Maxim SCSI terminators?

Yes, the DS21S07AS is backward compatible with the DS2107 and DS2107A in the same 16-pin SO package, sharing identical pinout and terminal functions. However, unlike those predecessors, the DS21S07AS integrates a low-dropout regulator and laser-trimmed resistors, eliminating the need for external 2.85V supplies and discrete termination networks while adding power-down and thermal shutdown features.

What is the significance of the dual TERMPWR and VREF pins on the DS21S07AS?

The dual TERMPWR1/TERMPWR2 and VREF1/VREF2 pins on the DS21S07AS provide redundancy and improved noise rejection in high-density SCSI backplanes. TERMPWR1 and TERMPWR2 must both connect to the same SCSI bus TERMPWR line to ensure balanced current sourcing, while VREF1 and VREF2 must be shorted externally and decoupled with a shared 4.7µF capacitor to stabilize the 2.85V reference under dynamic sink loads from actively negated signals.