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Buck Regulator Architectures

Buck Regulator Architectures. 4.6 Voltage Mode Buck Regulators. Voltage-Mode Regulator. Output Filter. Modulator. Error Amplifier. Advantages and Disadvantages. Advantages Stable modulation/less sensitive to noise Single feedback path Can work over a wide range of duty cycles

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Buck Regulator Architectures

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  1. Buck Regulator Architectures 4.6 Voltage Mode Buck Regulators

  2. Voltage-Mode Regulator Output Filter Modulator Error Amplifier

  3. Advantages and Disadvantages • Advantages • Stable modulation/less sensitive to noise • Single feedback path • Can work over a wide range of duty cycles • Disadvantages • Loop gain proportional to VIN • LC double pole often drives Type III compensation • CCM and DCM differences - a compensation challenge • Slow response to input voltage changes • Current limiting must be done separately

  4. Modulator Gain

  5. Output Filter dB ZA ZB (L, Co) (Resr, Co) Hz * (Rx, Cy) indicate the components that drive the locations of the pole and the zero, detailed equations are in the notes

  6. Error Amplifier The easiest place to compensate the entire loop is to adjust the compensation around the error amplifier. Several different approaches are possible.

  7. ZF Type II Compensation dB ZI ZF - ZI Hz (Rc, Cc1) (Rc, RFB2) (Rc, Cc2) * (Rx, Cy) indicate the components that drive the locations of the pole and the zero (and k), detailed equations are in the notes

  8. Design Guidelines for Type II Compensation • Choose a large value for RFB2, between 2-200 kW • Set the mid-band gain k to give desired bandwidth • Set wP equal to half the switching frequency: wP = 2p*Fsw/2 • Set wz equal to the output filter double pole wO • Use the following equations to solve for the remaining variables

  9. Type III Compensation dB ZI ZF (RFB2, Cc3) Hz (Rc1, Cc1) (Rc1, Cc2) (Rc2, Cc3) * (Rx, Cy) indicate the components that drive the locations of the poles and zeros, detailed equations are in the notes

  10. Design Guidelines for Type III Compensation • Choose a large value for RFB2, between 2-200 kW • Set the mid-band gain k to shift the open-loop gain up to give desired bandwidth • Set wP1 equal to half the switching frequency: wP1 = 2p * Fsw/2 • Set wP2 equal to the output filter zero, wESR • Set wZ1 and wZ2 equal to cancel out the output filter double pole • Use the following equations to solve for the remaining variables

  11. Internal Type III Voltage Mode Compensation (LM367x) Internal Block Diagram Typical Application Circuit

  12. Internally Compensated Internal Type III Voltage Mode Compensation (LM285x) Internal Block Diagram Typical Application Circuit

  13. Thank you!

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