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

4.5 Low Dropout, High Efficiency 5-V/3-A Buck Regulator

A circuit was developed which provides a 5-V/3-A regulated output voltage with very high efficiency and very low dropout voltage (see Figure 31). The customer required that the circuit be able to operate with an input voltage range of 6 V to 12 V, allowing only 1 V of dropout at the lowest input voltage.

low_dropout_5v3a_regulator_snva559.gifFigure 31. Low-Dropout 5-V/3-A regulator

An unusual feature of this circuit is that it can stay in regulation with only 300 mV across the regulator. Also, the efficiency is highest (89%) at the lowest input voltage (buck converters are typically more efficient at higher input voltages).

The low (< 300 mV) dropout voltage is achieved by using an external PNP power transistor (Q2) as the main switching transistor (the other transistors in the circuit are drivers for Q2). With components values shown, Q2 has a saturation voltage of 200 mV at 3 A, which allows the 300 mV input-output differential requirement for the regulator to be met.

The switch inside the LM2575 drives the base of Q1 through R2. Note that the maximum collector current of Q1 (and the maximum base drive available for Q2) is limited by Z1 and R4. When Z1 clamps at 5V, the maximum current through Q1 is:

Equation 16. IQ1 (MAX) = (5 – VBE) / R4 = 215 mA

The maximum Q1 current (215 mA) limits the amount of base drive available to Q2, forcing the collector current of Q2 to beta limit as the output is overloaded (this means the maximum collector current of Q2 will be limited by the gain of the transistor and the base drive provided). Although this is not a precise current limiter, it is adequate to protect Q2 from damage during an overload placed on the output.

If the regulator output is shorted to ground, the output short-circuit current flows from the output of the LM2575 (through D1 and the inductor), which means the regulator short-circuit current is limited to the value set internally to the LM2575 (which is about 2 A).

Note also that when the regulator output is shorted to ground, the cathode of D1 will also be near ground. This allows D1 to clamp off the base drive to Q1 off, preventing current flow in the switch transistor Q2.

If the input voltage does not exceed 8 V, R2 and Z1 are not required in the circuit.

This circuit was tested with 6-V input and was able to deliver more than 4 A of load current with 5 Vout. Other test data taken are:

Table 2. Summary of Performance Data

MEASURED PERFORMANCE DATA
LINE REGULATION
5.3 V to 12 V at 1 A 32 mV
5.3 V to 12 V at 3 A 45 mV
LOAD REGULATION
0.3 A to 3 A at 5.3-V Input 10 mV
0.3 A to 3 A at 12-V Input 17 m
EFFICIENCY AT 3-A LOAD
VIN = 5.3 V 89%
VIN = 12 V 80%
OUTPUT RIPPLE VOLTAGE
VIN = 7.2 V, IL = 3 A 35 mV(p-p)