ZHCSJW8D October   2011  – June 2019 LMR10515

PRODUCTION DATA.  

  1. 特性
  2. 应用
  3. 说明
    1.     Device Images
      1.      简化应用
  4. 修订历史记录
  5. 说明(续)
  6. Pin Configuration and Functions
    1.     Pin Description: 5-Pin SOT-23
    2.     Pin Descriptions 6-Pin WSON
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 Recommended Operating Ratings
    3. 7.3 Electrical Characteristics
    4. 7.4 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Soft-Start
      2. 8.3.2 Output Overvoltage Protection
      3. 8.3.3 Undervoltage Lockout
      4. 8.3.4 Current Limit
      5. 8.3.5 Thermal Shutdown
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Application
      1. 9.2.1 Detailed Design Procedure
        1. 9.2.1.1 Custom Design With WEBENCH® Tools
        2. 9.2.1.2 Inductor Selection
        3. 9.2.1.3 Input Capacitor
        4. 9.2.1.4 Output Capacitor
        5. 9.2.1.5 Catch Diode
        6. 9.2.1.6 Output Voltage
        7. 9.2.1.7 Calculating Efficiency, And Junction Temperature
      2. 9.2.2 Application Curves
      3. 9.2.3 Other System Examples
        1. 9.2.3.1 LMR10510x Design Example 1
        2. 9.2.3.2 Lmr10510X Design Example 2
        3. 9.2.3.3 LMR10510Y Design Example 3
        4. 9.2.3.4 LMR10510Y Design Example 4
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Thermal Definitions
    4. 10.4 WSON Package
  11. 11器件和文档支持
    1. 11.1 器件支持
      1. 11.1.1 开发支持
        1. 11.1.1.1 使用 WEBENCH® 工具创建定制设计
    2. 11.2 接收文档更新通知
    3. 11.3 社区资源
    4. 11.4 商标
    5. 11.5 静电放电警告
    6. 11.6 Glossary
  12. 12机械、封装和可订购信息

封装选项

机械数据 (封装 | 引脚)
散热焊盘机械数据 (封装 | 引脚)
订购信息

Thermal Definitions

TJ = Chip junction temperature

TA = Ambient temperature

RθJC = Thermal resistance from chip junction to device case

RθJA = Thermal resistance from chip junction to ambient air

Heat in the LMR10515 due to internal power dissipation is removed through conduction and/or convection.

Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the transfer of heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor).

Heat Transfer goes as:

Silicon → package → lead frame → PCB

Convection: Heat transfer is by means of airflow. This could be from a fan or natural convection. Natural convection occurs when air currents rise from the hot device to cooler air.

Thermal impedance is defined as:

LMR10515 30166173.gif

Thermal impedance from the silicon junction to the ambient air is defined as:

LMR10515 30166174.gif

The PCB size, weight of copper used to route traces and ground plane, and number of layers within the PCB can greatly effect RθJA. The type and number of thermal vias can also make a large difference in the thermal impedance. Thermal vias are necessary in most applications. They conduct heat from the surface of the PCB to the ground plane. Four to six thermal vias should be placed under the exposed pad to the ground plane if the WSON package is used.

Thermal impedance also depends on the thermal properties of the application operating conditions (Vin, Vo, Io etc), and the surrounding circuitry.

Silicon Junction Temperature Determination Method 1:

To accurately measure the silicon temperature for a given application, two methods can be used. The first method requires the user to know the thermal impedance of the silicon junction to case temperature.

RθJC is approximately 18°C/Watt for the 6-pin WSON package with the exposed pad. Knowing the internal dissipation from the efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench we have:

LMR10515 30166175.gif

where TC is the temperature of the exposed pad and can be measured on the bottom side of the PCB.

Therefore:

Tj = (RθJC × PLOSS) + TC

From the previous example:

Tj = (RθJC × PINTERNAL) + TC
Tj = 18°C/W × 0.213W + TC

The second method can give a very accurate silicon junction temperature.

The first step is to determine RθJA of the application. The LMR10515 has over-temperature protection circuitry. When the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a hysteresis of about 15°C. Once the silicon temperature has decreased to approximately 150°C, the device will start to switch again. Knowing this, the RθJA for any application can be characterized during the early stages of the design one may calculate the RθJA by placing the PCB circuit into a thermal chamber. Raise the ambient temperature in the given working application until the circuit enters thermal shutdown. If the SW-pin is monitored, it will be obvious when the internal PFET stops switching, indicating a junction temperature of 165°C. Knowing the internal power dissipation from the above methods, the junction temperature, and the ambient temperature RθJA can be determined.

LMR10515 30166176.gif

Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be found.

An example of calculating RθJA for an application using the LMR10515 is shown below.

A sample PCB is placed in an oven with no forced airflow. The ambient temperature was raised to 140°C, and at that temperature, the device went into thermal shutdown.

From the previous example:

PINTERNAL = 213 mW

LMR10515 30166177.gif

Since the junction temperature must be kept below 125°C, then the maximum ambient temperature can be calculated as:

Tj - (RθJA x PLOSS) = TA
125°C – (117°C/W x 213 mW) = 100°C