SNVS952F December   2012  – May 2021 LM25019

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  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 Switching Characteristics
    7. 6.7 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  Control Overview
      2. 7.3.2  VCC Regulator
      3. 7.3.3  Regulation Comparator
      4. 7.3.4  Overvoltage Comparator
      5. 7.3.5  On-Time Generator
      6. 7.3.6  Current Limit
      7. 7.3.7  N-Channel Buck Switch and Driver
      8. 7.3.8  Synchronous Rectifier
      9. 7.3.9  Undervoltage Detector
      10. 7.3.10 Thermal Protection
      11. 7.3.11 Ripple Configuration
      12. 7.3.12 Soft Start
    4. 7.4 Device Functional Modes
  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 RFB1, RFB2
        2. 8.2.2.2 Frequency Selection
        3. 8.2.2.3 Inductor Selection
        4. 8.2.2.4 Output Capacitor
        5. 8.2.2.5 Type III Ripple Circuit
        6. 8.2.2.6 VCC and Bootstrap Capacitor
        7. 8.2.2.7 Input Capacitor
        8. 8.2.2.8 UVLO Resistors
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Documentation Support
      1. 11.1.1 Related Documentation
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Support Resources
    4. 11.4 Trademarks
    5. 11.5 Electrostatic Discharge Caution
    6. 11.6 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

Mechanical Data (Package|Pins)
Thermal pad, mechanical data (Package|Pins)
Orderable Information

7.3.12 Soft Start

A soft-start feature can be implemented with the LM25019 device using an external circuit. As shown in Figure 7-4, the soft-start circuit consists of one capacitor C1, two resistors, R1 and R2, and a diode, D. During the initial start-up, the VCC voltage is established before the VOUT voltage. Capacitor C1 is discharged and D is thereby forward biased. The FB voltage is pulled up above the reference voltage (1.225 V) and switching is thereby disabled. As capacitor C1 charges, the voltage at node B gradually decreases and switching commences. VOUT gradually rises to maintain the FB voltage at the reference voltage. Once the voltage at node B is less than a diode drop above the FB voltage, the soft-start sequence is finished and D is reverse-biased.

During the initial part of the start-up, the FB voltage can be approximated as shown in Equation 8.

Equation 8. GUID-5F252F4C-5264-4EB3-A29F-D86D975F24A6-low.gif

C1 is charged after the first start-up. Diode D1 is optional and can be added to discharge C1 and initialize the soft-start sequence when the input voltage experiences a momentary drop.

To achieve the desired soft start, the following design guidance is recommended:

  1. R2 is selected so that VFB is higher than 1.225 V for a VCC of 4.5 V, but is lower than 5 V when VCC is 8.55 V. If an external VCC is used, VFB must not exceed 5 V at maximum VCC.
  2. C1 is selected to achieve the desired start-up time that can be determined from Equation 9.
    Equation 9. GUID-6022C435-B6CF-4148-9123-7A8D38A31639-low.gif
  3. R1 is used to maintain the node B voltage at zero after the soft start is finished. A value larger than the feedback resistor divider is preferred. The effect of resistor R1 is ignored in Equation 9.

With component values from the applications schematic shown in Figure 8-1, selecting C1 = 1 µF, R2 = 1 kΩ, and R1 = 30 kΩ results in a soft-start time of about 2 ms.

GUID-53D77B8A-17FE-444A-8FCA-A0CCBF408500-low.gifFigure 7-4 Soft-Start Circuit