ZHCSPC5 October   2022 LM51231-Q1

PRODUCTION DATA  

  1. 1 特性
  2. 2 应用
  3. 3 说明
  4. 4 Revision History
  5. 5 Pin Configuration and 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  Device Enable/Disable (EN, VH Pin)
      2. 7.3.2  High Voltage VCC Regulator (BIAS, VCC Pin)
      3. 7.3.3  Light Load Switching Mode Selection (MODE Pin)
      4. 7.3.4  VOUT Range Selection (RANGE Pin)
      5. 7.3.5  Line Undervoltage Lockout (UVLO Pin)
      6. 7.3.6  Fast Restart using VCC HOLD (VH Pin)
      7. 7.3.7  Adjustable Output Regulation Target (VOUT, TRK, VREF Pin)
      8. 7.3.8  Overvoltage Protection (VOUT Pin)
      9. 7.3.9  Power Good Indicator (PGOOD Pin)
      10. 7.3.10 Dynamically Programmable Switching Frequency (RT)
      11. 7.3.11 External Clock Synchronization (SYNC Pin)
      12. 7.3.12 Programmable Spread Spectrum (DITHER Pin)
      13. 7.3.13 Programmable Soft-start (SS Pin)
      14. 7.3.14 Wide Bandwidth Transconductance Error Amplifier and PWM (TRK, COMP Pin)
      15. 7.3.15 Current Sensing and Slope Compensation (CSP, CSN Pin)
      16. 7.3.16 Constant Peak Current Limit (CSP, CSN Pin)
      17. 7.3.17 Maximum Duty Cycle and Minimum Controllable On-time Limits
      18. 7.3.18 MOSFET Drivers, Integrated Boot Diode, and Hiccup Mode Fault Protection (LO, HO, HB Pin)
      19. 7.3.19 Thermal Shutdown Protection
    4. 7.4 Device Functional Modes
      1. 7.4.1 Device Status
        1. 7.4.1.1 Shutdown Mode
        2. 7.4.1.2 Configuration Mode
        3. 7.4.1.3 Active Mode
        4. 7.4.1.4 Bypass Mode
          1. 7.4.1.4.1 Bypass DE mode
          2. 7.4.1.4.2 Bypass FPWM
      2. 7.4.2 Light Load Switching Mode
        1. 7.4.2.1 Forced PWM (FPWM) Mode
        2. 7.4.2.2 Diode Emulation (DE) Mode
        3. 7.4.2.3 Forced Diode Emulation Operation in FPWM 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
      3. 8.2.3 Application Ideas
      4. 8.2.4 Application Curves
    3. 8.3 System Example
    4. 8.4 Power Supply Recommendations
    5. 8.5 Layout
      1. 8.5.1 Layout Guidelines
      2. 8.5.2 Layout Example
  9. 9 Device and Documentation Support
    1. 9.1 接收文档更新通知
    2. 9.2 支持资源
    3. 9.3 Trademarks
    4. 9.4 Electrostatic Discharge Caution
    5. 9.5 术语表
  10. 10Mechanical, Packaging, and Orderable Information

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7.3.5 Line Undervoltage Lockout (UVLO Pin)

When UVLO is greater than the UVLO threshold (VUVLO), the device enters active mode if the device configuration is finished. UVLO hysteresis is accomplished with an internal 25-mV voltage hysteresis (VUVLO-HYS) at the UVLO pin, and an additional 10-μA current sink (IUVLO-HYS) that is switched on or off. When the UVLO pin voltage exceeds VUVLO, the current sink is disabled to quickly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below VUVLO or during the device configuration time, the current sink is enabled, causing the voltage at the UVLO pin to fall quickly.

The external UVLO resistor voltage divider (RUVLOT, RUVLOB) must be designed so that the voltage at the UVLO pin is greater than VUVLO when VSUPPLY is in the desired operating range. The values of RUVLOT and RUVLOB can be calculated as follows.

Equation 2. RUVLOT=VSUPPLY_ON-VUVLO_RISINGVUVLO_FALLING×VSUPPLY_OFFIUVLO_HYS
Equation 3. RUVLOB=VUVLO_FALLING×RUVLOTVSUPPLY_OFF-VUVLO_FALLING

A UVLO capacitor (CUVLO) is required in case VSUPPLY drops below VSUPPLY-OFF momentarily during the start-up or during a severe load transient at the low input voltage. If the required UVLO capacitor is large, an additional series UVLO resistor (RUVLOS) can be used to quickly raise the voltage at the UVLO pin when IUVLO-HYS is disabled.

The UVLO pin can be connected to the BIAS pin if not used. Drive the UVLO pin through a minimum of a 5-kΩ resistor if the BIAS pin voltage is less than the UVLO pin voltage in any conditions.