Analog Devices Inc./Maxim Integrated DS1231S-20

Overview

DS1231S-20 from Maxim Integrated (now Analog Devices) is a precision power monitor IC that detects impending AC/DC power failure and initiates orderly microprocessor shutdown before VCC drops below 4.75 V (–5% threshold), provides NMI warning via high-impedance input (VTP– = 2.3 V), delivers dual active-high/active-low reset outputs, and supports automatic restart after power recovery - used in industrial controllers, embedded systems with nonvolatile memory, and power-supply supervision circuits.

For engineers reviewing the DS1231S-20 datasheet, DS1231S-20 pinout, DS1231S-20 application, or DS1231S-20 equivalent, key selection criteria include VCC trip accuracy (±5% or ±10%), IN pin hysteresis (+60 mV), dual RST output types (push-pull and open-drain), MODE-selectable input interface (high-Z or 30 µA current source), and SOIC-16 package compatibility with legacy DIP-8 footprint layouts.

Technical Context

The DS1231S-20 integrates two independent comparators: one monitors VCC against a temperature-compensated reference to generate RST/RST at programmable thresholds (4.75 V or 4.50 V), while the second monitors the IN pin with MODE-selectable input characteristics - high-impedance (Mode = VCC, VTP– = 2.3 V) or 30 µA current-source (Mode = GND, VTP+ = 2.5 V). Both paths include built-in hysteresis (±60 mV) for noise immunity.

On power-up, RST and RST remain asserted for ≥150 ms to ensure supply stabilization; on power-down, NMI triggers first (tIPU = 200 µs), followed by RST activation (tRPD = 100 ns) just before hold-up time exhaustion. The device requires no external capacitors and interfaces directly with opto-isolators or resistor-divider sense networks.

Key Specifications

Pinout & Package

DS1231S-20 is housed in a 16-pin SOIC (300-mil width) package with 0.050" pitch, pin-compatible with DS1231 DIP-8 layout when using only pins 1, 2, 3, 5, 6, 7, 8, and 16 as functional equivalents.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar power monitor applications.

Compared with DS1231S-20, MAX690TSA+ sacrifices NMI warning and programmable threshold for lower cost and smaller SOT-23 package, while TPS3808G33DBVR trades IN pin flexibility for fine-grained VCC adjustment and watchdog functionality - neither offers the DS1231S-20's dual-sensing architecture for AC/DC hybrid monitoring.

Availability

DS1231S-20 is available at Aetrix Electronics and suitable for industrial PLCs, medical infusion pumps, telecom line cards, and automated test equipment requiring stable component supply across extended product lifecycles.

Supply support for DS1231S-20 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, designs precision analog and mixed-signal ICs for power management, sensing, and interface applications in industrial, medical, and communications systems.

The DS1231S-20 belongs to Maxim's legacy power-monitoring product line, engineered specifically for fail-safe microprocessor supervision in systems where nonvolatile memory integrity depends on deterministic power-fail response.

FAQ

What is the function of the MODE pin on the DS1231S-20?

The MODE pin on the DS1231S-20 selects the electrical behavior of the IN pin: when tied to VCC, it configures Pin 1 as a high-impedance input (VTP– = 2.3 V) for resistor-divider sensing; when grounded, it enables a 30 µA current source (VTP+ = 2.5 V) for direct opto-isolator interfacing. This dual-mode capability allows the DS1231S-20 to adapt to either AC-line or isolated DC sense topologies without external components.

How does the DS1231S-20 provide early warning of power failure?

The DS1231S-20 provides early warning via its NMI output, which activates when the IN pin voltage crosses its trip threshold (2.3 V or 2.5 V depending on MODE) - typically derived from an unregulated AC/DC sense point upstream of the main 5 V regulator. Because this occurs well before VCC drops out of spec, the DS1231S-20 gives the processor time to execute shutdown routines before RST is asserted, ensuring DS1231S-20 remains a critical element in fail-safe system design.

Can the DS1231S-20 replace a standard power-on reset IC?

Yes, the DS1231S-20 replaces basic power-on reset ICs by providing guaranteed ≥150 ms RST assertion on power-up, eliminating the need for external RC networks. Unlike simple reset generators, DS1231S-20 adds power-fail warning (NMI), dual reset outputs (RST and RST), and programmable VCC thresholds - making DS1231S-20 suitable for systems requiring both robust startup sequencing and graceful shutdown under brownout conditions.

What is the purpose of the TOL pin on the DS1231S-20?

The TOL pin on the DS1231S-20 selects the VCC undervoltage detection threshold: grounding TOL sets the trip point to 4.75 V (–5% of nominal 5 V), while connecting TOL to VCC lowers it to 4.50 V (–10%). This allows the DS1231S-20 to match system-level tolerance requirements - tighter thresholds suit high-reliability applications, while looser thresholds accommodate noisy or aging power supplies - all without changing the DS1231S-20 hardware.

Does the DS1231S-20 require external pull-up resistors?

The DS1231S-20 RST output (Pin 5) is push-pull and requires no external pull-up; however, the RST output (Pin 6) is open-drain and must be pulled up to VCC with a resistor (typically 4.7 kΩ). Additionally, the NMI output is push-pull and self-contained. Therefore, only one external resistor is mandatory for DS1231S-20 operation - simplifying layout and reducing component count compared to alternatives requiring multiple pull-ups.