JAJSAY1E March   2008  – October 2017 LM2854

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
  3. 3 概要
    1.     Device Images
      1.      代表的なアプリケーション回路
  4. 4 改訂履歴
  5. 5 Pin Configuration and Functions
    1.     Pin Functions
  6. 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
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1  Switching Frequency
      2. 7.3.2  Enable
      3. 7.3.3  Soft-Start
      4. 7.3.4  Tracking
      5. 7.3.5  Pre-Biased Start-up Capability
      6. 7.3.6  Feedback Voltage Accuracy
      7. 7.3.7  Positive Current Limit
      8. 7.3.8  Negative Current Limit
      9. 7.3.9  Overtemperature Protection
      10. 7.3.10 Loop Compensation
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Soft-Start and Track Mode
      3. 7.4.3 Normal Operating Mode
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Input Filter Capacitor
        2. 8.2.2.2 AVIN Filtering Components
        3. 8.2.2.3 Soft-Start Capacitor
        4. 8.2.2.4 Tracking - Equal Soft-Start Time
        5. 8.2.2.5 Tracking - Equal Slew Rates
        6. 8.2.2.6 Enable and UVLO
        7. 8.2.2.7 Output Voltage Setting
        8. 8.2.2.8 Compensation Component Selection
        9. 8.2.2.9 Filter Inductor and Output Capacitor Selection
      3. 8.2.3 Application Curves
      4. 8.2.4 System Examples
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11デバイスおよびドキュメントのサポート
    1. 11.1 ドキュメントのサポート
      1. 11.1.1 関連資料
    2. 11.2 商標
    3. 11.3 静電気放電に関する注意事項
    4. 11.4 Glossary
  12. 12メカニカル、パッケージ、および注文情報

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）
サーマルパッド・メカニカル・データ
発注情報

8.2.2.4 Tracking - Equal Soft-Start Time

One way to use the tracking feature is to design the tracking resistor divider so that the master supply output voltage, VOUT1, and the LM2854 output voltage, VOUT2, both rise together and reach their target values at the same time. This is termed ratiometric start-up. For this case, the equation governing the values of tracking divider resistors RT1 and RT2 is given in Equation 7.

Equation 7. LM2854 30052858.gif

The above equation includes an offset voltage to ensure that the final value of the SS pin voltage exceeds the reference voltage of the LM2854. This offset will cause the LM2854 output voltage to reach regulation slightly before the master supply. A value of 33 kΩ 1% is recommended for RT2 as a compromise between high precision and low quiescent current through the divider while minimizing the effect of the 2 µA soft-start current source.

For example, If the master supply voltage VOUT1 is 3.3V and the LM2854 output voltage was 1.8V, then the value of RT1 needed to give the two supplies identical soft-start times would be 14.3 kΩ. A timing diagram for this example, the equal soft-start time case, is shown in Figure 17.

LM2854 30052859.gifFigure 17. Simplified Start-up Waveforms When Using Proportional Tracking