SNVS089P July   2000  – September 2026 LM3488

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings: LM3488
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics
    6. 5.6 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Slope Compensation Ramp
      2. 6.3.2 Frequency Adjust, Synchronization, Shutdown
      3. 6.3.3 Short-Circuit Protection
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
    2. 7.2 Typical Applications
      1. 7.2.1 Boost Converter
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
          1. 7.2.1.2.1 Custom Design With WEBENCH® Tools
          2. 7.2.1.2.2 Power Inductor Selection
          3. 7.2.1.2.3 Programming the Output Voltage
          4. 7.2.1.2.4 Setting the Current Limit
          5. 7.2.1.2.5 Current Limit With External Slope Compensation
          6. 7.2.1.2.6 Power Diode Selection
          7. 7.2.1.2.7 Power MOSFET Selection
          8. 7.2.1.2.8 Input Capacitor Selection
          9. 7.2.1.2.9 Output Capacitor Selection
        3. 7.2.1.3 Application Curve
      2. 7.2.2 Designing SEPIC Using LM3488
        1. 7.2.2.1 Design Requirements
        2. 7.2.2.2 Detailed Design Procedure
          1. 7.2.2.2.1 Power MOSFET Selection
          2. 7.2.2.2.2 Power Diode Selection
          3. 7.2.2.2.3 Selection Of Inductors L1 and L2
          4. 7.2.2.2.4 Sense Resistor Selection
          5. 7.2.2.2.5 SEPIC Capacitor Selection
          6. 7.2.2.2.6 Input Capacitor Selection
          7. 7.2.2.2.7 Output Capacitor Selection
    3. 7.3 Power Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Example
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
        1. 8.1.1.1 Custom Design With WEBENCH® Tools
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

6.3.1 Slope Compensation Ramp

The LM3488 uses a current mode control scheme. The main advantages of current mode control are inherent cycle-by-cycle current limit for the switch, and simpler control loop characteristics. Paralleling power stages using current mode control is also easy as current sharing is automatic.

Current mode control has an inherent instability for duty cycles greater than 50%, as shown in Figure 6-2. In Figure 6-2, a small increase in the load current causes the switch current to increase by ΔIO. The effect of this load change, ΔI1, is:

Equation 1. LM3488

From the above equation, when D > 0.5, ΔI1 is greater than ΔIO. In other words, the disturbance is divergent. So a very small perturbation in the load causes the disturbance to increase.

To prevent the sub-harmonic oscillations, a compensation ramp is added to the control signal, as shown in Figure 6-3.

With the compensation ramp,

Equation 2. LM3488
LM3488 Sub-Harmonic Oscillation for D>0.5 Figure 6-2 Sub-Harmonic Oscillation for D>0.5
LM3488 Compensation Ramp Avoids Sub-Harmonic Oscillation Figure 6-3 Compensation Ramp Avoids Sub-Harmonic Oscillation

The compensation ramp has been added internally in LM3488. The slope of this compensation ramp has been selected to satisfy most of the applications. The slope of the internal compensation ramp depends on the frequency. This slope can be calculated using the formula:

Equation 3. MC = VSL.FS Volts/second

In the above equation, VSL is the amplitude of the internal compensation ramp. Limits for VSL have been specified in the electrical characteristics.

To provide the user additional flexibility, a patented scheme has been implemented inside the IC to increase the slope of the compensation ramp externally, if the need arises. Adding a single external resistor, RSL(as shown in Figure 6-4) increases the slope of the compensation ramp, MC by :

Equation 4. LM3488

In this equation, ΔVSL is equal to 40.10-6RSL. Hence,

Equation 5. LM3488

ΔVSL versus RSL has been plotted in Figure 6-5 for different frequencies.

LM3488 Increasing the Slope of the Compensation RampFigure 6-4 Increasing the Slope of the Compensation Ramp
LM3488 ΔVSL vs RSLFigure 6-5 ΔVSL vs RSL