SNVA559C September   2012  – February 2019 LM2574 , LM2575-N , LM2575HV , LM2576 , LM2576HV , LM2577

 

  1.   Switching regulator fundamentals
    1.     Trademarks
    2. 1 Switching Fundamentals
      1. 1.1 The Law of Inductance
      2. 1.2 Transformer Operation
      3. 1.3 Pulse Width Modulation (PWM)
    3. 2 Switching Converter Topologies
      1. 2.1  Buck Regulator
      2. 2.2  Continuous vs Discontinuous Operation
      3. 2.3  Boost Regulator
      4. 2.4  Output Current and Load Power
      5. 2.5  Buck-Boost (Inverting) Regulator
      6. 2.6  Flyback Regulator
      7. 2.7  Generating Multiple Outputs
      8. 2.8  Push-Pull Converter
      9. 2.9  Half-Bridge Converter
      10. 2.10 Full-Bridge Converter
    4. 3 Application Hints for Switching Regulators
      1. 3.1 Capacitor Parasitics Affecting Switching Regulator Performance
        1. 3.1.1 Input Capacitors
        2. 3.1.2 Output Capacitor ESR Effects
        3. 3.1.3 Bypass Capacitors
      2. 3.2 Proper Grounding
      3. 3.3 Transformer/Inductor Cores and Radiated Noise
      4. 3.4 Measuring Output Ripple Voltage
      5. 3.5 Measuring Regulator Efficiency of DC/DC Converters
      6. 3.6 Measuring Regulator Efficiency of Offline Converters
    5. 4 Application Circuits
      1. 4.1 LM2577: A Complete Flyback/Boost Regulator IC
        1. 4.1.1 Increasing Available Load Power in an LM2577 Boost Regulator
      2. 4.2 LM2577 Negative Buck Regulator
      3. 4.3 LM2577 Three-Output, Isolated Flyback Regulator
      4. 4.4 LM2575 and LM2576 Buck Regulators
      5. 4.5 Low Dropout, High Efficiency 5-V/3-A Buck Regulator
    6. 5 References and Related Products
  2.   Revision History

4.2 LM2577 Negative Buck Regulator

The LM2577 can be used in a Buck regulator configuration that takes a negative input voltage and produces a regulated negative output voltage (see Figure 27).

negative_buck_regulator_snva559.gifFigure 27. Negative Buck Regulator

The LM2577 is referenced to the negative input, which means the feedback signal coming from the regulated output must be DC level shifted. R1, D1, and Q1 form a current source that sets a current through R2 that is directly proportional to the output voltage (D1 is included to cancel out the VBE of Q1).

Neglecting the base current error of Q1, the current through R2 is equal to:

Equation 14. IR2 = VOUT / R1 (which is 300 μA for this example.)

The voltage across R2 provides the 1.23-V feedback signal which the LM2577 requires for its feedback loop.

The operation of the power converter is similar to what was previously described for the Buck regulator:

  • When the switch is ON, current flows from ground through the load, into the regulator output, through the inductor, and down through the switch to return to the negative input. The output capacitor also charges during the switch ON time.
  • When the switch turns OFF, the voltage at the diode end of the inductor flies positive until the Schottky diode turns on (this allows the inductor current to continue to flow through the load during the OFF time). The output capacitor also discharges through the load during the OFF time, providing part of the load current.