SNOSB87D March   2011  – May 2019 LM21215A

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
    1.     Efficiency at 2.5 V, 500 kHz
  3. 3 Description
    1.     Typical Application Circuit
      1.      Device Images
  4. 4 Revision History
  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. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Development Support
        1. 11.1.2.1 Custom Design With WEBENCH® Tools
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Community 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

7.3.3 Soft-Start Capability

When EN exceeds 1.35 V and AVIN is above its UVLO threshold of 2.7 V, the LM21215A begins charging the output linearly to the voltage setpoint dictated by the feedback resistor network. The LM21215A employs a user-adjustable soft-start circuit to set the output voltage ramp time during startup. A capacitor from SS/TRK to GND sets the required soft-start time. Once the enable voltage exceeds 1.35 V, an internal 1.9-µA current source begins to charge the soft-start capacitor. This allows the user to limit inrush currents due to a high output capacitance and avoid an overcurrent condition. Adding a soft-start capacitor also reduces the stress on the input rail. Use Equation 1 to calculate the soft-start capacitance.

Equation 1. LM21215A q_Css_nosb87.gif

where

  • ISS is nominally 1.9 µA
  • tSS is the desired startup time

If VIN is higher than the UVLO level and Enable is toggled high, the soft-start sequence begins. There is a small delay between enable transitioning high and the beginning of the soft-start sequence. This delay allows the LM21215A to initialize its internal circuitry. Once the output has charged to 90% of the nominal output voltage, the PGOOD flag transitions high. This behavior is illustrated in Figure 23.

LM21215A soft_start_nosb87.gifFigure 23. Soft-Start Timing

As shown above, the soft-start capacitance is set by the nominal feedback voltage level 0.6 V, the soft-start charging current ISS, and the desired soft-start time. If a soft-start capacitor is not installed, the LM21215A defaults to a soft-start time of 500 µs. The LM21215A cannot startup faster than 500 µs. When Enable is cycled or the device enters UVLO, the soft-start capacitor is discharged to reset the startup process. This also occurs when the device enters short circuit mode following an overcurrent event.