SNVSCQ8B November   2025  – July 2026 LM654A5-Q1 , LM654B0-Q1

PRODMIX  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  8. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Descriptions
      1. 7.3.1  Output Voltage Selection
      2. 7.3.2  EN Pin and Use as VIN UVLO
      3. 7.3.3  Device Configuration
      4. 7.3.4  Mode Selection
        1. 7.3.4.1 MODE/SYNC Pin Uses for Synchronization
        2. 7.3.4.2 Clock Locking
      5. 7.3.5  Adjustable Switching Frequency and Phase Shift
      6. 7.3.6  Dual Random Spread Spectrum (DRSS)
      7. 7.3.7  Internal LDO, VCC UVLO, and BIAS Input
      8. 7.3.8  Bootstrap Voltage (BST Pin)
      9. 7.3.9  Soft Start and Recovery From Dropout
      10. 7.3.10 Safety Features
        1. 7.3.10.1 Power-Good Monitor
        2. 7.3.10.2 Overcurrent and Short-Circuit Protection
        3. 7.3.10.3 Hiccup
        4. 7.3.10.4 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Active Mode
        1. 7.4.2.1 Peak Current Mode Operation
        2. 7.4.2.2 Auto Mode Operation
          1. 7.4.2.2.1 Diode Emulation
        3. 7.4.2.3 FPWM Mode Operation
  9. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Example Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Choosing the Switching Frequency
        2. 8.2.2.2 Inductor Selection
        3. 8.2.2.3 Output Capacitors
        4. 8.2.2.4 Input Capacitor Selection
        5. 8.2.2.5 Setting the Output Voltage
        6. 8.2.2.6 Compensation Components
        7. 8.2.2.7 Feed-forward Capacitor (CFF)
        8. 8.2.2.8 Maximum Ambient Temperature
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Third-Party Products Disclaimer
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

8.2.2.8 Maximum Ambient Temperature

As with any power conversion device, the LM654xx-Q1 family of regulators dissipates internal power while operating. The effect of this power dissipation is to raise the internal temperature of the converter above ambient. The internal die temperature (TJ) is a function of the ambient temperature, the power loss, and the effective thermal resistance, RθJA, of the device and PCB combination. The maximum junction temperature for the LM654xx-Q1 must be limited to 150°C. This limit establishes a limit on the maximum device power dissipation and, therefore, the load current. Equation 22 shows the relationships between the important parameters. Higher ambient temperatures (TA) and larger values of RθJA reduce the maximum available output current. The converter efficiency can be estimated by using the curves provided in this data sheet. If the desired operating conditions cannot be found in one of the curves, interpolation can be used to estimate the efficiency. Alternatively, the EVM can be adjusted to match the desired application requirements and the efficiency can be measured directly. The correct value of RθJA is more difficult to estimate. As stated in the Semiconductor and IC Package Thermal Metrics application note, many of the values given in the column labeled "JESD"of the Thermal Information table are not valid for design purposes and must not be used to estimate the thermal performance of the application. The values reported in that column of the table are measured under a specific set of conditions that are rarely obtained in an actual application. The column labeled "LM654B0EVM" represent measured data on the EVM, and can be helpful when estimating thermal performance. Also, the data given for RθJC(bott), RθJC(top), and ΨJT can be useful.

Equation 22. I O U T M A X   = T J   - T A   R θ J A   × ƞ 1 - ƞ   × 1 V O U T  

where

  • η = efficiency

The effective RθJA is a critical parameter and depends on many factors such as the following:

  • Power dissipation
  • Air temperature, flow
  • PCB area
  • Copper heat-sink area
  • External heat-sink
  • Number of thermal vias under the package
  • Adjacent component placement

The advanced package used for this regulator features a die attach paddle, or "thermal pad" (DAP), to provide a place to solder down to the PCB heat-sinking copper. This feature provides a good heat conduction path from the regulator junction to the heat sink and must be properly soldered to the PCB heat sink copper. A typical curve of RθJA versus copper board area can be found in Figure 8-8. The copper area given in the graph is for each of six layers. The top and bottom layers are 2oz copper each, while the inner layers are 1oz. Remember that the data given in this graph is for illustration purposes only, and the actual performance in any given application depends on all of the previously mentioned factors. As one data point, the EVM exhibits an approximate RθJA of about 16ºC/W for a copper area of about 100cm2, using a six layer design.

In addition, the package for this device family features an exposed top side. This allows the use of an external heat-sink to further reduce the effective RθJA when required.

The data in Figure 8-9 and Figure 8-10 provide an example of the allowable output current for a given ambient temperature. This data is valid only under the specific conditions given in the figures. The data is taken on the LM654B0EVM, with no external heat-sink and no air-flow.

The major advantage to the 'stackability" of the LM654xx family is that several regulators can be paralleled to share the total load current. Not only does advantage bring the total load current within the capability of a single regulator, but this advantage also reduces the individual power dissipation, thus reducing the regulator junction temperature.

The PCB Thermal Design Tips for Automotive DC/DC Converters application note is a good place to start when designing the thermal system for any DC/DC converter.

LM654A5-Q1 LM654B0-Q1 Thermal Resistance vs Copper Area Figure 8-8 Thermal Resistance vs Copper Area
LM654A5-Q1 LM654B0-Q1 Maximum Output Current vs. Ambient Temperature
VIN = 12V 400kHz TJ ⋍ 150ºC
1.2μH, 3.4mΩ
Figure 8-9 Maximum Output Current vs. Ambient Temperature
LM654A5-Q1 LM654B0-Q1 Maximum Output Current vs. Ambient Temperature
VIN = 12V 2100kHz TJ ⋍ 150ºC
0.35μH, 2.2mΩ
Figure 8-10 Maximum Output Current vs. Ambient Temperature

Use the following resources as guides to excellent thermal PCB design and estimating RθJA for a given application environment: