ZHCSCW5B August   2014  – September 2017 LM43600

PRODUCTION DATA.  

  1. 1 特性
  2. 2 应用
  3. 3 说明
  4. 4 简化电路原理图
    1.     5
    2.     辐射发射图VIN = 12V，VOUT = 3.3V，FSW= 500kHz，IOUT = 0.5A
  5. 5 修订历史记录
  6. 6 Pin Configuration and Functions
    1.     Pin Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Timing Requirements
    7. 7.7 Switching Characteristics
    8. 7.8 Typical Characteristics
  8. 8 Detailed Description
    1. 8.1 Overview
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1  Fixed Frequency Peak Current Mode Controlled Step-Down Regulator
      2. 8.3.2  Light Load Operation
      3. 8.3.3  Adjustable Output Voltage
      4. 8.3.4  Enable (ENABLE)
      5. 8.3.5  VCC, UVLO and BIAS
      6. 8.3.6  Soft Start and Voltage Tracking (SS/TRK)
      7. 8.3.7  Switching Frequency (RT) and Synchronization (SYNC)
      8. 8.3.8  Minimum ON-Time, Minimum OFF-Time and Frequency Foldback at Dropout Conditions
      9. 8.3.9  Internal Compensation and CFF
      10. 8.3.10 Bootstrap Voltage (BOOT)
      11. 8.3.11 Power Good (PGOOD)
      12. 8.3.12 Overcurrent and Short-Circuit Protection
      13. 8.3.13 Thermal Shutdown
    4. 8.4 Device Functional Modes
      1. 8.4.1 Shutdown Mode
      2. 8.4.2 Stand-by Mode
      3. 8.4.3 Active Mode
      4. 8.4.4 CCM Mode
      5. 8.4.5 Light Load Operation
      6. 8.4.6 Self-Bias Mode
  9. 9 Applications and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Design Requirements
      2. 9.2.2 Detailed Design Procedure
        1. 9.2.2.1  Custom Design With WEBENCH® Tools
        2. 9.2.2.2  Output Voltage Setpoint
        3. 9.2.2.3  Switching Frequency
        4. 9.2.2.4  Input Capacitors
        5. 9.2.2.5  Inductor Selection
        6. 9.2.2.6  Output Capacitor Selection
        7. 9.2.2.7  Feedforward Capacitor
        8. 9.2.2.8  Bootstrap Capacitors
        9. 9.2.2.9  VCC Capacitor
        10. 9.2.2.10 BIAS Capacitors
        11. 9.2.2.11 Soft-Start Capacitors
        12. 9.2.2.12 Undervoltage Lockout Setpoint
        13. 9.2.2.13 PGOOD
      3. 9.2.3 Application Performance Curves
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
      1. 11.1.1 Compact Layout for EMI Reduction
      2. 11.1.2 Ground Plane and Thermal Considerations
      3. 11.1.3 Feedback Resistors
    2. 11.2 Layout Example
  12. 12器件和文档支持
    1. 12.1 开发支持
      1. 12.1.1 使用 WEBENCH® 工具创建定制设计
    2. 12.2 接收文档更新通知
    3. 12.3 社区资源
    4. 12.4 商标
    5. 12.5 静电放电警告
    6. 12.6 Glossary
  13. 13机械、封装和可订购信息

封装选项

请参考 PDF 数据表获取器件具体的封装图。

机械数据 (封装 | 引脚)
  • PWP|16
散热焊盘机械数据 (封装 | 引脚)
订购信息

8.3.6 Soft Start and Voltage Tracking (SS/TRK)

The LM43600 has a flexible and easy-to-use start up rate control pin: SS/TRK. Soft-start feature is to prevent inrush current impacting the LM43600 and its supply when power is first applied. Soft-start is achieved by slowly ramping up the target regulation voltage when the device is first enabled or powered up.

The simplest way to use the device is to leave the SS/TRK pin open circuit or floating. The LM43600 employw the internal soft-start control ramp and start up to the regulated output voltage in 4.1 ms typically.

In applications with a large amount of output capacitors, or higher VOUT, or other special requirements, the soft-start time can be extended by connecting an external capacitor CSS from SS/TRK pin to AGND. Extended soft-start time further reduces the supply current needed to charge up output capacitors and supply any output loading. An internal current source (ISSC = 2.2 µA) charges CSS and generates a ramp from 0 V to VFB to control the ramp-up rate of the output voltage. For a desired soft start time tSS, the capacitance for CSS can be found by

Equation 2. LM43600 eq02_snvsa13.gif

The soft start capacitor CSS is discharged by an internal FET when VOUT is shutdown by hiccup protection due to excessive load, temperature shutdown due to overheating or ENABLE = logic low. A large CSS cap will take a long time to discharge when ENABLE is toggled low. If ENABLE is toggled high again before the CSS is completely discharged, then the next resulting soft-start ramp will follow the internal soft-start ramp. Only when the soft-start voltage reaches the left-over voltage on CSS, will the output follow the ramp programmed by CSS. This behavior will look as if there are two slopes at startup. If this is not acceptable by a certain application, a R-C low-pass filter can be added to ENABLE to slow down the shutting down of VCC, which allows more time to discharge CSS.

The LM43600 is capable of start up into prebiased output conditions. When the inductor current reaches zero, the LS switch will be turned off to avoid negative current conduction. This operation mode is also called diode emulation mode. It is built-in by the DCM operation in light loads. With a prebiased output voltage, the LM43600 will wait until the soft-start ramp allows regulation above the prebiased voltage and then follow the soft-start ramp to the regulation level.

When an external voltage ramp is applied to the SS/TRK pin, the LM43600 FB voltage follows the ramp if the ramp magnitude is lower than the internal soft-start ramp. A resistor divider pair can be used on the external control ramp to the SS/TRK pin to program the tracking rate of the output voltage. The final voltage seen by the SS/TRK pin should not fall below 1.2 V to avoid abnormal operation.

LM43600 soft_start_track_snvsa13.gifFigure 37. Soft-Start Tracking External Ramp

VOUT tracked to an external voltage ramp has the option of ramping up slower or faster than the internal voltage ramp. VFB always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 38 shows the case when VOUT ramps slower than the internal ramp, while Figure 39 shows when VOUT ramps faster than the internal ramp. Faster start up time may result in inductor current tripping current protection during start-up. Use with special care.

LM43600 tracking_slow_snvsa13.gifFigure 38. Tracking with Longer Start-up Time Than The Internal Ramp
LM43600 tracking_fast_snvsa13.gifFigure 39. Tracking with Shorter Start-up Time Than The Internal Ramp