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Buck Regulator Architectures

Buck Regulator Architectures. 4.1 Overview. LM22676. LM3102. LM2747. LM3489. Buck-Switching Converters. Synchronous . Non-Synchronous . (Internal-FET) Regulators. (External-FET) Controllers. Non-Synchronous Buck Converters. L. S. Is. IL. +. Io. Id. Ic. C. Vin. D. Vout. -.

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Buck Regulator Architectures

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  1. Buck Regulator Architectures 4.1 Overview

  2. LM22676 LM3102 LM2747 LM3489 Buck-Switching Converters Synchronous Non-Synchronous (Internal-FET) Regulators (External-FET) Controllers

  3. Non-Synchronous Buck Converters L S Is IL + Io Id Ic C Vin D Vout - ΔIL, Ton = ΔIL, Toff Vout = D*Vin ΔIL, Ton = (Vin – Vout ) * Ton / L D = Ton / (Toff +Ton) ΔIL, Toff = -Vout * Toff/ L

  4. Buck Topology: Current and Voltage Waveforms Input current Low side switch or diode Output and Inductor Switch

  5. Light-Load Operation: CCM and DCM D = VOUT/VIN Full Synchronous Mode. Stays in Continuous Conduction Mode (CCM) t Inductor current reverses polarity at light loads D ≠ VOUT/VIN Diode or Diode Emulation t Inductor current drops to zero before the end of the cycle: “Discontinuous Conduction Mode” (DCM)

  6. Cross Conduction with Synchronous Buck • Direct Connection between VIN and Ground • High – Side and Low – Side must not be in ON state at the same time • A time in which both MOSFET are Turned OFF is required: DEAD - TIME

  7. DEAD – TIME • FIXED DEAD – TIME • Fixed time between Turn-OFF and Turn-ON • No flexibility in MOSFET choice • ADAPTATIVE DEAD – TIME • High-Side turns ON only if LS is OFF and vice versa • Full flexibility in MOSFET choice • It is necessary to detect the Turn-OFF of both MOSFET

  8. Control Mode • Voltage Mode Control (VMC) • Current Mode Control (CMC) • Peak Current Mode Control (PCMC) • Valley Current Mode Control (VCMC) • Average Current Mode Control (ACMC) • Hysteretic Mode Control (HMC)

  9. Voltage Mode ControlAdvantages 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

  10. Current Mode ControlAdvantages and Disadvantages • Advantages • Power plant gain offers a single-pole roll-off • Line rejection • Cycle-by-cycle current limiting protection • Current sharing • Disadvantages • Noise • Minimum ON-time • Sense resistor

  11. Hysteretic Mode ControlAdvantages and Disadvantages • Advantages • Ultra fast transient response (preferred to use in power hungry load) • No phase compensation required; In other words, Hysteretic Mode is a kind of large signal control • Disadvantages • Noise Jitter susceptible • Very layout sensitive • Large switching frequency variation; Minimum ripple requirement

  12. Thank you!

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