Texas Instruments LM81CIMTX-3

Overview

LM81CIMTX-3 from Texas Instruments is an SMBus 1.0–compatible hardware monitor IC for microprocessor-based systems, integrating a 9-/12-bit temperature sensor, six analog voltage inputs (+12V, +5V, +3.3V, +2.5V, Vccp1, Vccp2), two Schmitt-trigger fan tachometer inputs, and an 8-bit DAC (0–1.25 V) for fan speed control. It operates from 2.8 V to 3.8 V and supports chassis intrusion detection and critical temperature alarm (T_CRIT_A) in server and PC thermal management.

For engineers reviewing the LM81CIMTX-3 datasheet, LM81CIMTX-3 pinout, LM81CIMTX-3 application, or LM81CIMTX-3 equivalent, key selection considerations include its ±3 °C temperature error over −40 °C to +125 °C, 8-bit ADC with ≤±2% voltage monitoring error, SMBus timing compliance, 24-pin TSSOP package, and VID0–VID4 support for Pentium/PRO processor voltage identification.

Technical Context

The LM81CIMTX-3 implements a delta-sigma ADC architecture for simultaneous monitoring of six power rails and on-die temperature, with programmable watchdog limits per channel and dual interrupt outputs (INT and T_CRIT_A). Its internal register map includes dedicated configuration, interrupt status/mask, VID/fan divisor, and extended mode registers enabling 12-bit temperature resolution and critical threshold hysteresis.

It uses asynchronous internal clocking for ADC and fan counters independent of SMBus timing, supports serial bus timeout recovery (≥35 ms low time), and provides open-drain digital outputs (INT, T_CRIT_A, RESET, CI) with 0.4 V max logic-low at −3 mA. The DAC output drives fan PWM circuits with ±5% full-scale error and 100 ppm/°C temperature sensitivity.

Key Specifications

Pinout & Package

LM81CIMTX-3 is housed in a 24-pin TSSOP (PW24A) package with 0.65 mm pitch, requiring low-noise analog ground routing and separate bypassing at V+ (pin 9).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar hardware monitoring applications.

Compared with LM81BIMTX-3 and ADM1027ASTZ, the LM81CIMTX-3 provides SMBus 1.0 compatibility and native Pentium VID decoding at the cost of slightly relaxed voltage accuracy and no PWM fan output-making it optimal for cost-sensitive, legacy-compatible server platforms.

Availability

LM81CIMTX-3 is available at Aetrix Electronics and suitable for server thermal management, desktop PC power integrity validation, and industrial embedded controller monitoring requiring stable component supply across long-lifecycle programs.

Supply support for LM81CIMTX-3 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

Texas Instruments is a global semiconductor leader specializing in analog, embedded processing, and silicon design for industrial, automotive, and computing applications.

The LM81 product line was engineered specifically for ACPI-compliant hardware monitoring in x86-based servers and PCs, integrating voltage, temperature, fan, and chassis security functions into a single 24-pin IC.

FAQ

What is the SMBus compatibility of the LM81CIMTX-3?

The LM81CIMTX-3 implements SMBus 1.0 protocol compliance, supporting standard start/stop conditions, ACK/NACK handshaking, and 35 ms serial bus timeout recovery. It operates only as a slave device with a default 7-bit address of 01011xx (determined by A0/A1 pins), and does not support SMBus 1.1 or later features such as packet error checking or block read/write extensions. This ensures interoperability with legacy motherboard controllers while maintaining deterministic timing behavior.

How does the LM81CIMTX-3 handle negative voltage monitoring like −12V?

The LM81CIMTX-3 monitors negative voltages such as −12V using the Vccp2 analog input pin (14), which accepts an externally scaled signal. A precision resistor divider must be applied to translate −12V to a positive voltage within the LM81CIMTX-3's input range (−0.05 V to +3.6 V), typically targeting +2.5 V full scale. The device's internal ADC then converts this scaled value with 8-bit resolution and ≤±2% total unadjusted error, enabling accurate validation of negative supply rails in ATX-style power supplies.

What is the function of the VID0–VID4 inputs on the LM81CIMTX-3?

The VID0–VID4 inputs on the LM81CIMTX-3 directly interface with Pentium/PRO-class processor voltage identification pins to decode core supply voltage settings (e.g., 1.5 V to 2.9 V). These 5-bit digital signals are latched and stored in the VID/Fan Divisor Register (47h) and VID4 Register (49h), allowing host firmware to verify configured Vccp against expected values during boot and runtime-critical for detecting incorrect VID strap configurations that could cause processor instability or excessive power dissipation.

Can the LM81CIMTX-3 generate a hardware reset signal?

Yes, the LM81CIMTX-3 generates a hardware reset signal via its open-drain RESET pin (12), delivering an active-low pulse ≥20 ms wide when Bit 4 of the Configuration Register is set. This pulse is synchronized to the internal clock and independent of SMBus activity, enabling reliable power-on reset or recovery sequencing for attached microcontrollers or FPGAs. The RESET output can be disabled by clearing Bit 4, and its driver capability supports ≥5 mA sink current per TI specifications.

What is the temperature resolution mode supported by the LM81CIMTX-3?

The LM81CIMTX-3 supports two temperature resolution modes: 9-bit (0.5 °C LSB) and 12-bit (0.0625 °C LSB), selected via the Temperature Configuration Register (4Bh) and Extended Mode Register 1 (4Ch). In 9-bit mode, conversion completes in ~820 ms; in 12-bit mode, it takes ~1.2 s. Both modes maintain ±3 °C maximum error over −40 °C to +125 °C, with the higher resolution enabling finer thermal threshold granularity for critical temperature alarms in high-performance computing environments.