National Semiconductor LM64 Data Handbook/Specifications

Summary

The LM64 is a high-precision remote diode temperature sensor featuring integrated PWM fan control and five programmable GPIOs, ideal for advanced thermal management solutions. This manual provides comprehensive technical documentation, covering detailed specifications, operational requirements (DC 3.0V–3.6V), pin connections, and integration guidelines. It is designed for embedded systems engineers developing computer processor monitoring, graphics cooling, power supply systems, and general electronic instrumentation that requires accurate temperature measurement and controlled fan output.

LM64
- General Description
- Features
- Key Specifications
- Applications
- Connection Diagram
- Pin Descriptions
- Simplified Block Diagram
- Ordering Information
- Typical Application
- Absolute Maximum Ratings
- Operating Ratings (Notes , )
- DC Electrical Characteristics
- Operating Electrical Characteristics
- AC Electrical Characteristics
- Digital Electrical Characteristics
- SMBus Logical Electrical Characteristics
- SMBus Digital Switching Characteristics
- FIGURE 1. SMBus Timing Diagram for SMBCLK and SMBDAT Signals
- FIGURE 2. ESD Protection Input Structure
- 1.0 Functional Description
  - 1.1 CONVERSION SEQUENCE
    
    FIGURE 3. Supply Current vs Conversion Rate
  - 1.2 THE ALERT OUTPUT
    
    1.2.1 ALERT Output as a Temperature Comparator
    
    1.2.2 ALERT Output as an Interrupt
    
    1.2.3 ALERT Output as an SMBus ALERT
  - 1.3 SMBus INTERFACE
  - 1.4 POWER-ON RESET (POR) DEFAULT STATES
  - 1.5 TEMPERATURE DATA FORMAT
    
    Actual vs. LM64 Remote Temperature Conversion
    
    Actual vs. Remote T_Crit Setpoint Example
  - 1.6 OPEN-DRAIN OUTPUTS, INPUTS, AND PULL-UP RESISTORS
  - 1.7 DIODE FAULT DETECTION
  - 1.8 COMMUNICATING WITH THE LM64
  - 1.9 DIGITAL FILTER
    
    Digital Filter Selection Table
    
    FIGURE 7. Step Response of the Digital Filter
    
    FIGURE 8. Impulse Response of the Digital Filter
    
    FIGURE 9. Digital Filter Response in an Intel Pentium 4 processor System. The Filter on and off curv
  - 1.10 FAULT QUEUE
    
    FIGURE 10. Fault Queue Temperature Response Diagram
  - 1.11 ONE-SHOT REGISTER
  - 1.12 SERIAL INTERFACE RESET
- 2.0 LM64 Registers
  - 2.1 LM64 REGISTER MAP IN HEXADECIMAL ORDER
  - 2.2 LM64 REGISTER MAP IN FUNCTIONAL ORDER
  - 2.3 LM64 INITIAL REGISTER SEQUENCE AND REGISTER DESCRIPTIONS IN FUNCTIONAL ORDER
    
    2.3.1 LM64 Required Initial Fan Control Register Sequence
  - 2.4 LM64 REGISTER DESCRIPTIONS IN FUNCTIONAL ORDER
    
    Fan Control Registers
    
    Configuration Register
    
    Tachometer Count And Limit Registers
    
    Local Temperature And Local High Setpoint Registers
    
    Remote Diode Temperature, Offset And Setpoint Registers
    
    ALERT Status And Mask Registers
    
    Conversion Rate And One-Shot Registers
    
    ID Registers
- General Purpose Registers
- 3.0 Application Notes
  - 3.1 FAN CONTROL DUTY CYCLE VS. REGISTER SETTINGS AND FREQUENCY
    
    3.1.1 Computing Duty Cycles for a Given Frequency
  - 3.2 USE OF THE LOOKUP TABLE FOR NON-LINEAR PWM VALUES VS TEMPERATURE
  - 3.3 NON-IDEALITY FACTOR AND TEMPERATURE ACCURACY
    
    3.3.1 Diode Non_Ideality
    
    3.3.2 Compensating for Diode Non-Ideality
  - 3.4 COMPUTING RPM OF THE FAN FROM THE TACH COUNT
  - 3.5 PCB LAYOUT FOR MINIMIZING NOISE
    
    FIGURE 11. Ideal Diode Trace Layout
- Physical Dimensions

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Manual Details

Brand	National
Pages	29
File Size	376.58 KB
Published	June 18, 2026

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Frequently Asked Questions

What is the required DC voltage range for this sensor?

The supply DC Voltage must be between 3.0 V and 3.6 V.

How does the LM64 fan control function?

It has an integrated PWM output and a programmable 8-step Lookup Table to quiet acoustic fan noise, showing non-linear speed vs. temperature.

Are there dedicated pins for General Purpose I/O (GPIO)?

Yes, it features five GPIO pins (GPIO1 through GPIO5), which can act as open-drain digital outputs or inputs.

What is the primary function of the T_Crit and ALERT outputs?

These are both open-drain digital outputs used to signal critical temperature thresholds or alert conditions.

National Semiconductor LM64 Data Handbook/Specifications

Summary

1.1 CONVERSION SEQUENCE

FIGURE 3. Supply Current vs Conversion Rate

1.2 THE ALERT OUTPUT

1.2.1 ALERT Output as a Temperature Comparator

1.2.2 ALERT Output as an Interrupt

1.2.3 ALERT Output as an SMBus ALERT

1.3 SMBus INTERFACE

1.4 POWER-ON RESET (POR) DEFAULT STATES

1.5 TEMPERATURE DATA FORMAT

Actual vs. LM64 Remote Temperature Conversion

Actual vs. Remote T_Crit Setpoint Example

1.6 OPEN-DRAIN OUTPUTS, INPUTS, AND PULL-UP RESISTORS

1.7 DIODE FAULT DETECTION

1.8 COMMUNICATING WITH THE LM64

1.9 DIGITAL FILTER

Digital Filter Selection Table

FIGURE 7. Step Response of the Digital Filter

FIGURE 8. Impulse Response of the Digital Filter

FIGURE 9. Digital Filter Response in an Intel Pentium 4 processor System. The Filter on and off curv

1.10 FAULT QUEUE

FIGURE 10. Fault Queue Temperature Response Diagram

1.11 ONE-SHOT REGISTER

1.12 SERIAL INTERFACE RESET

2.1 LM64 REGISTER MAP IN HEXADECIMAL ORDER

2.2 LM64 REGISTER MAP IN FUNCTIONAL ORDER

2.3 LM64 INITIAL REGISTER SEQUENCE AND REGISTER DESCRIPTIONS IN FUNCTIONAL ORDER

2.3.1 LM64 Required Initial Fan Control Register Sequence

2.4 LM64 REGISTER DESCRIPTIONS IN FUNCTIONAL ORDER

Fan Control Registers

Configuration Register

Tachometer Count And Limit Registers

Local Temperature And Local High Setpoint Registers

Remote Diode Temperature, Offset And Setpoint Registers

ALERT Status And Mask Registers

Conversion Rate And One-Shot Registers

ID Registers

3.1 FAN CONTROL DUTY CYCLE VS. REGISTER SETTINGS AND FREQUENCY

3.1.1 Computing Duty Cycles for a Given Frequency

3.2 USE OF THE LOOKUP TABLE FOR NON-LINEAR PWM VALUES VS TEMPERATURE

3.3 NON-IDEALITY FACTOR AND TEMPERATURE ACCURACY

3.3.1 Diode Non_Ideality

3.3.2 Compensating for Diode Non-Ideality

3.4 COMPUTING RPM OF THE FAN FROM THE TACH COUNT

3.5 PCB LAYOUT FOR MINIMIZING NOISE

FIGURE 11. Ideal Diode Trace Layout

LM64

General Description

Features

Key Specifications

Applications

Connection Diagram

Pin Descriptions

Simplified Block Diagram

Ordering Information

Typical Application

Absolute Maximum Ratings

Operating Ratings (Notes , )

DC Electrical Characteristics

Operating Electrical Characteristics

AC Electrical Characteristics

Digital Electrical Characteristics

SMBus Logical Electrical Characteristics

SMBus Digital Switching Characteristics

FIGURE 1. SMBus Timing Diagram for SMBCLK and SMBDAT Signals

FIGURE 2. ESD Protection Input Structure

1.0 Functional Description