SNIS152E May   2009  – July 2015 LM57

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics - Accuracy Characteristics - Trip Point Accuracy
    6. 7.6 Electrical Characteristics - Accuracy Characteristics - VTEMP Analog Temperature Sensor Output Accuracy
    7. 7.7 Electrical Characteristics
    8. 7.8 Switching Characteristics
    9. 7.9 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 LM57 VTEMP Temperature-to-Voltage Transfer Function
        1. 8.3.1.1 LM57 VTEMP Voltage-to-Temperature Equations
      2. 8.3.2 RSENSE
      3. 8.3.3 Resistor Selection
      4. 8.3.4 TOVER and TOVER Digital Outputs
        1. 8.3.4.1 TOVER and TOVER Noise Immunity
      5. 8.3.5 Trip Test Digital Input
      6. 8.3.6 VTEMP Analog Temperature Sensor Output
        1. 8.3.6.1 VTEMP Noise Considerations
        2. 8.3.6.2 VTEMP Capacitive Loads
        3. 8.3.6.3 VTEMP Voltage Shift
    4. 8.4 Device Functional Modes
  9. 9 Application and Implementation
    1. 9.1 Application Information
      1. 9.1.1 ADC Input Considerations
    2. 9.2 Typical Application
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1 Selection of RSENSE Resistors
      3. 9.2.3 Application Curves
      4. 9.2.4 Grounding of the TRIP TEST Pin
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
    3. 11.3 Temperature Considerations
  12. 12Device and Documentation Support
    1. 12.1 Documentation Support
      1. 12.1.1 Related Documentation
    2. 12.2 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

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9 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.

9.1 Application Information

The LM57 has several outputs allowing for varying system implementations.

9.1.1 ADC Input Considerations

The LM57 has an analog temperature sensor output (VTEMP) that can be directly connected to an ADC (Analog to Digital Converter) input. Most CMOS ADCs found in microcontrollers and ASICs have a sampled data comparator input structure. When the ADC charges the sampling cap, it requires instantaneous charge from the output of the analog source such as the LM57 temperature sensor and many op amps. This requirement is easily accommodated by the addition of a capacitor (CFILTER). The size of CFILTER depends on the size of the sampling capacitor and the sampling frequency. Because not all ADCs have identical input stages, the charge requirements will vary. The general ADC application shown in Figure 20 is an example only.

LM57 30080528_source.gifFigure 20. Suggested Connection to a Sampling Analog-to-Digital Converter Input Stage

9.2 Typical Application

LM57 30080576.gifFigure 21. Typical Application Schematic with Microcontroller TRIP TEST Control

9.2.1 Design Requirements

By simply selecting the value of two resistors the trip point of the LM57 can easily be programmed as described in the following section. If standard 1% values are used the actual trip point threshold is not degraded and stands as described in the Electrical Characteristics section ().

9.2.2 Detailed Design Procedure

9.2.2.1 Selection of RSENSE Resistors

To set the trip point:

  1. Locate the desired trip temperature in Table 4.
  2. Identify the corresponding RSENSE2 value by following the column up to the resistor value.
  3. Identify the corresponding RSENSE1 value by following the row leftwards to the resistor value.
  4. Use only the EIA E96 standard resistor values from the list.
  5. Use only a resistor with 1% tolerance and a temperature coefficient of 100 ppm (or better). These restrictions are necessary to stay at the selected setting, and not to slip into an adjacent setting.
  6. This is consistent with using resistors from the thick film chip resistors CRCW0402 family. These are available with very small dimensions of L = 1 mm, W = 0.5 mm, H = 0.35 mm.
  7. Note that the resistor tolerance does not diminish the accuracy of the trip point. As can be seen in the block diagram these inputs drive the logic inputs of a DAC thus their tolerance does affect the trip point accuracy unless the DAC setting slips into an adjacent level. See patent number 6924758.

9.2.3 Application Curves

The typical performance of the LM57 temperature sensor output can be seen in Figure 22. Figure 23 shows the output behavior of the LM57 TOVER output.

LM57 C102_SNIS152.pngFigure 22. J2 VTEMP Accuracy Characteristics
LM57 TransferChar_SNIS152.gif
Figure 23. Output Transfer Characteristic

9.2.4 Grounding of the TRIP TEST Pin

The circuit in Figure 24 shows the TRIP TEST pin grounded. This allows the LM57 to function autonomously without microcontroller intervention. In all other respects this circuit functions similarly to the circuit shown in Figure 21.

LM57 LM57_TypApp_SimpleMicroT_OVER.gifFigure 24. Typical Application Schematic without Microcontroller TRIP TEST Control