JAJSVQ2 November   2024 LM644A2-Q1

PRODUCTION DATA  

  1.   1
  2. 1 特長
  3. 2 アプリケーション
  4. 3 概要
  5. 4 Device Comparison Table
  6. 5 Pin Configuration and Functions
    1. 5.1 Wettable Flanks
  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 Description
      1. 7.3.1  Input Voltage Range (VIN)
      2. 7.3.2  Enable EN Pin and Use as VIN UVLO
      3. 7.3.3  Output Voltage Selection and Soft Start
      4. 7.3.4  SYNC Allows Clock Synchronization and Mode Selection
      5. 7.3.5  Clock Locking
      6. 7.3.6  Adjustable Switching Frequency
      7. 7.3.7  Power-Good Output Voltage Monitoring
      8. 7.3.8  Internal LDO, VCC UVLO, and BIAS Input
      9. 7.3.9  Bootstrap Voltage and VCBOOT-UVLO (CB1 and CB2 Pin)
      10. 7.3.10 CONFIG Device Configuration Pin
      11. 7.3.11 Spread Spectrum
      12. 7.3.12 Soft Start and Recovery From Dropout
      13. 7.3.13 Overcurrent and Short-Circuit Protection
      14. 7.3.14 Hiccup
      15. 7.3.15 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Mode
      2. 7.4.2 Standby Mode
      3. 7.4.3 Active Mode
        1. 7.4.3.1 Peak Current Mode Operation
        2. 7.4.3.2 Auto Mode Operation
          1. 7.4.3.2.1 Diode Emulation
        3. 7.4.3.3 FPWM Mode Operation
        4. 7.4.3.4 Minimum On-time (High Input Voltage) Operation
        5. 7.4.3.5 Dropout
        6. 7.4.3.6 Recovery from Dropout
        7. 7.4.3.7 Other Fault Modes
  9. 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  Choosing the Switching Frequency
        2. 8.2.2.2  Setting the Output Voltage
        3. 8.2.2.3  Inductor Selection
        4. 8.2.2.4  Output Capacitor Selection
        5. 8.2.2.5  Input Capacitor Selection
        6. 8.2.2.6  BOOT Capacitor
        7. 8.2.2.7  VCC
        8. 8.2.2.8  CFF and RFF Selection
        9. 8.2.2.9  SYNCHRONIZATION AND MODE
        10. 8.2.2.10 External UVLO
        11. 8.2.2.11 Typical Thermal Performance
      3. 8.2.3 Application Curves
    3. 8.3 Power Supply Recommendations
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Ground and Thermal Considerations
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 サード・パーティ製品に関する免責事項
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 ドキュメントの更新通知を受け取る方法
    4. 9.4 サポート・リソース
    5. 9.5 Trademarks
    6. 9.6 静電気放電に関する注意事項
    7. 9.7 用語集
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

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

7.3.13 Overcurrent and Short-Circuit Protection

The LM644A2-Q1 is protected from overcurrent conditions by cycle-by-cycle current limiting on both the high-side and the low-side MOSFETs.

High-side MOSFET overcurrent protection is implemented by the nature of the peak current mode control. The HS switch current is sensed when the HS is turned on after a short blanking time. The HS switch current is compared to the output of the voltage regulation loop minus slope compensation, every switching cycle. Because the voltage loop has a maximum value and slope compensation increases with duty cycle, the HS current limit decreases with increased duty cycle such that the HS current limit is reduced by 35% for high output to input voltage duty ratios. See Figure 7-14.

LM644A2-Q1 Maximum Current Allowed Through the HS FET - Function of Duty Cycle for LM644A2-Q1 Figure 7-14 Maximum Current Allowed Through the HS FET - Function of Duty Cycle for LM644A2-Q1

When the LS switch is turned on, the current going through the LS switch is also sensed and monitored. Like the high-side device, the low-side device turn-off is commanded by the voltage control loop. For a low-side device, turn-off is prevented if current exceeds this value, even if the oscillator normally starts a new switching cycle. See Section 7.4.3.4. Also like the high-side device, there is a limit on how high the turn-off current is allowed to be. This is called the low-side current limit; see the Electrical Characteristics for values. If the LS current limit is exceeded, the LS MOSFET stays on and the HS switch is not turned on. The LS switch is turned off after the LS current falls below the limit. The HS switch is turned on again as long as at least one clock period has passed since the last time the HS device has turned on.

LM644A2-Q1 Current Limit WaveformsFigure 7-15 Current Limit Waveforms

The net effect of the operation of high-side and low-side current limit is that the IC operates in hysteretic control. Because the current waveform assumes values between IL-HS and IL-LS, output current is close to the average of these two values unless duty cycle is very high. After operating in current limit, hysteretic control is used and current does not increase as output voltage approaches zero.

If duty cycle is very high, current ripple must be very low to prevent instability; see Section 8.2.2.3. Because current ripple is low, the part is able to deliver full current unless dropout across device is less than 0.5V. The current delivered is very close to IL-LS.

If overloaded, after the overload is removed, the device recovers as though in soft start; see Section 7.3.12. Note that hiccup can be triggered if output voltage drops below approximately 0.4 times the intended output voltage.