SNVS617H April   2009  – November 2014 LM25011 , LM25011-Q1

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Typical Application
  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 Handling Ratings: LM25011
    3. 6.3 Handling Ratings: LM25011-Q1
    4. 6.4 Recommended Operating Conditions
    5. 6.5 Thermal Information
    6. 6.6 Electrical 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 Circuit Overview
      2. 7.3.2 On-Time Timer
      3. 7.3.3 Current Limit
      4. 7.3.4 Ripple Requirements
      5. 7.3.5 N-Channel Buck Switch and Driver
      6. 7.3.6 Soft-Start
      7. 7.3.7 Power Good Output (PGD)
      8. 7.3.8 Thermal Shutdown
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Function
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 LM25011 Example Circuit
        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 External Components
        3. 8.2.1.3 Application Curves
      2. 8.2.2 Output Ripple Control
        1. 8.2.2.1 Option A: Lowest Cost Configuration
        2. 8.2.2.2 Option B: Intermediate VOUT Ripple Configuration
        3. 8.2.2.3 Option C: Minimum VOUT Ripple Configuration
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
    3. 10.3 Power Dissipation
  11. 11Device and Documentation Support
    1. 11.1 Custom Design with WEBENCH Tools
    2. 11.2 Receiving Notification of Documentation Updates
    3. 11.3 Related Links
    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

7.3.1 Control Circuit Overview

The LM25011 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an internal reference (2.51 V). If the FB voltage is below the reference, the internal buck switch is switched on for the one-shot timer period which is a function of the input voltage and the programming resistor (RT). Following the on-time, the switch remains off until the FB voltage falls below the reference, but never less than the minimum off-time forced by the off-time one-shot timer. When the FB pin voltage falls below the reference and the off-time one-shot period expires, the buck switch is then turned on for another on-time one-shot period.

When in regulation, the LM25011 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous conduction mode, the inductor current is always greater than zero and the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half of the ripple current amplitude of the inductor. The approximate operating frequency is calculated as follows:

Equation 1. LM25011 LM25011-Q1 LM25011A LM25011A-Q1 30094619.gif

The buck switch duty cycle is approximately equal to:

Equation 2. LM25011 LM25011-Q1 LM25011A LM25011A-Q1 30094620.gif

When the load current is less than one-half of the ripple current amplitude of the inductor, the circuit operates in discontinuous conduction mode. The off-time is longer than in continuous conduction mode while the inductor current is zero, causing the switching frequency to reduce as the load current is reduced. Conversion efficiency is maintained at light loads because the switching losses are reduced with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows:

Equation 3. LM25011 LM25011-Q1 LM25011A LM25011A-Q1 30094621.gif

where RL = the load resistance, and L1 is the inductor in the circuit.

The output voltage is set by the two feedback resistors (RFB1, RFB2 in the Functional Block Diagram). The regulated output voltage is calculated as follows:

Equation 4. VOUT = 2.51 V × (RFB1 + RFB2) / RFB1

Ripple voltage, which is required at the input of the regulation comparator for proper output regulation, is generated internally in the LM25011, and externally when the LM25011A is used. In the LM25011 the ERM (emulated ripple mode) control circuit generates the required internal ripple voltage from the ripple waveform at the CS pin. The LM25011A, which is designed for higher frequency operation, requires additional ripple voltage which must be generated externally and provided to the FB pin. This is described in the Application and Implementation section.