JAJSBH3D March   2011  – May 2019 LM21215A

PRODUCTION DATA.  

  1. 1 特長
  2. 2 アプリケーション
    1.     2.5V、500kHz での効率
  3. 3 概要
    1.     代表的なアプリケーション回路
      1.      Device Images
  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 Precision Enable
      2. 7.3.2 Input Voltage UVLO
      3. 7.3.3 Soft-Start Capability
      4. 7.3.4 PGOOD Indicator
      5. 7.3.5 Frequency Synchronization
      6. 7.3.6 Current Limit
      7. 7.3.7 Short Circuit Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Light-Load Operation
      2. 7.4.2 Overvoltage and Undervoltage Handling
      3. 7.4.3 Thermal Shutdown
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Applications
      1. 8.2.1 Typical Application 1
        1. 8.2.1.1 Design Requirements
        2. 8.2.1.2 Detailed Design Procedure
          1. 8.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 8.2.1.2.2 Output Voltage Setpoint
          3. 8.2.1.2.3 Precision Enable
          4. 8.2.1.2.4 Filter Inductor Selection
          5. 8.2.1.2.5 Output Capacitor Selection
          6. 8.2.1.2.6 Input Capacitor Selection
          7. 8.2.1.2.7 Control Loop Compensation
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Typical Application 2
        1. 8.2.2.1 Design Requirements
        2. 8.2.2.2 Detailed Design Procedure
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
      1. 10.1.1 Compact PCB Layout for EMI Reduction
      2. 10.1.2 Thermal Design
      3. 10.1.3 Ground Plane Design
    2. 10.2 Layout Example
  11. 11概要（続き）
  12. 12デバイスおよびドキュメントのサポート
    1. 12.1 デバイス・サポート
      1. 12.1.1 デベロッパー・ネットワークの製品に関する免責事項
      2. 12.1.2 開発サポート
        1. 12.1.2.1 WEBENCH® ツールによるカスタム設計
    2. 12.2 ドキュメントのサポート
      1. 12.2.1 関連資料
    3. 12.3 コミュニティ・リソース
    4. 12.4 商標
    5. 12.5 静電気放電に関する注意事項
    6. 12.6 Glossary
  13. 13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

8.2.1.2.5 Output Capacitor Selection

The output capacitor, designated COUT, filters the inductor ripple current and provides a source of charge for transient load conditions. A wide range of output capacitors may be used with the LM21215A that provide various advantages. The best performance is typically obtained using ceramic, SP or OSCON type chemistries. Typical trade-offs are that the ceramic capacitor provides extremely low ESR to reduce the output ripple voltage and noise spikes, while the SP and OSCON capacitors provide a large bulk capacitance in a small volume for transient loading conditions.

When selecting an output capacitor, the two performance characteristics to consider are the output voltage ripple and load transient response. Approximate the output voltage ripple by using Equation 7.

Equation 7. LM21215A q_delta_Vout_nosb87.gif

where

  • ΔVOUT is the peak-to-peak output voltage ripple
  • RESR is the effective series resistance (ESR) of the output capacitor
  • FSW is the switching frequency
  • COUT is the effective output capacitance

The amount of output voltage ripple is application specific. A general recommendation is to keep the output ripple less than 1% of the rated output voltage. Keep in mind that ceramic capacitors are sometimes preferred because they have very low ESR. However, depending on package and voltage rating of the capacitor, the effective in-circuit capacitance can drop significantly with applied voltage. The output capacitor selection also affects the output voltage droop during a load transient. The peak deviation of the output voltage during a load transient is dependent on many factors. An approximation of the transient dip ignoring loop bandwidth is obtained using Equation 8:

Equation 8. LM21215A q_Vdroop_nosb87.gif

where

  • COUT is the minimum required output capacitance
  • LF is the filter inductance
  • VDROOP is the output voltage deviation ignoring loop bandwidth considerations
  • ΔIOUT-STEP is the load step change
  • RESR is the output capacitor ESR
  • VIN is the input voltage
  • VOUT is the output voltage setpoint

Three 100-µF, 6.3-V ceramic capacitors with X5R dielectric and 1210 footprint are selected here based on a review of the capacitor's tolerance and voltage coefficient to meet output ripple specification.