SNVS209G November   2002  – May 2019 LM2733

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
    1.     Device Images
      1.      Typical Application Circuit
      2.      Efficiency vs. Load Current
  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 Switching Frequency
    4. 7.4 Device Functional Modes
      1. 7.4.1 Shutdown Pin Operation
  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  Selecting the External Capacitors
        2. 8.2.2.2  Selecting the Output Capacitor
        3. 8.2.2.3  Selecting the Input Capacitor
        4. 8.2.2.4  Feedforward Compensation
        5. 8.2.2.5  Selecting Diodes
        6. 8.2.2.6  Setting the Output Voltage
        7. 8.2.2.7  Switching Frequency
        8. 8.2.2.8  Duty Cycle
        9. 8.2.2.9  Inductance Value
        10. 8.2.2.10 Maximum Switch Current
        11. 8.2.2.11 Calculating Load Current
        12. 8.2.2.12 Design Parameters VSW and ISW
        13. 8.2.2.13 Thermal Considerations
        14. 8.2.2.14 Minimum Inductance
        15. 8.2.2.15 Inductor Suppliers
      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 Trademarks
    2. 11.2 Electrostatic Discharge Caution
    3. 11.3 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

Package Options

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

8.2.2.14 Minimum Inductance

In some applications where the maximum load current is relatively small, it may be advantageous to use the smallest possible inductance value for cost and size savings. The converter will operate in discontinuous mode in such a case.

The minimum inductance should be selected such that the inductor (switch) current peak on each cycle does not reach the 1-A current limit maximum. To understand how to do this, an example will be presented.

In the example, the LM2733X will be used (nominal switching frequency 1.6 MHz, minimum switching frequency 1.15 MHz). This means the maximum cycle period is the reciprocal of the minimum frequency:

Equation 12. TON(max) = 1/1.15M = 0.870 µs

We will assume the input voltage is 5 V, VOUT = 12 V, VSW = 0.2 V, VDIODE = 0.3 V. The duty cycle is:

Duty Cycle = 60.3%

Therefore, the maximum switch ON time is 0.524 µs. An inductor should be selected with enough inductance to prevent the switch current from reaching 1A in the 0.524 µs ON time interval (see below):

LM2733 20055413.pngFigure 30. Discontinuous Design, 5V–12V Boost (LM2733X)

The voltage across the inductor during ON time is 4.8V. Minimum inductance value is found by:

Equation 13. V = L X dl/dt, L = V X (dt/dl) = 4.8 (0.524µ/1) = 2.5 µH

In this case, a 2.7 µH inductor could be used assuming it provided at least that much inductance up to the 1A current value. This same analysis can be used to find the minimum inductance for any boost application. Using the slower switching “Y” version requires a higher amount of minimum inductance because of the longer switching period.