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DC-DC Fundamentals

DC-DC Fundamentals. 1.2 Linear Regulator. What is a Linear Regulator?. The linear regulator is a DC-DC converter to provide a constant voltage output without using switching components. The linear regulator is very popular in many applications for its low cost, low noise and simple to use.

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DC-DC Fundamentals

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  1. DC-DC Fundamentals 1.2 Linear Regulator

  2. What is a Linear Regulator? • The linear regulator is a DC-DC converter to provide a constant voltage output without using switching components. • The linear regulator is very popular in many applications for its low cost, low noise and simple to use. • But the linear regulator has limited efficiency and cannot boost voltage (to make Vout > Vin)

  3. Pros and Cons Advantages Disadvantages • Low output ripple & noise, no EMI concern as no switching • Low cost (for low power, at least) • Simplicity – few external components required, easy to configure and design • Fast transient response at Vouton large load step • Easy to implement short circuit protection • Low efficiency, especially when Vin to Vout difference is high • Thermal issue – for high power and/or a large dropout voltage, there can be substantial amount of heat generated due to the losses inherent to the regulator. • Vout has to be less than Vin

  4. How Does a Linear Regulator Work? • The linear regulator and the output impedance together form a voltage divider network. • The linear regulator behaves like controlled variable resistor adjusting itself according to the output load to maintain a steady output. - +

  5. Dropout Voltage • Dropout voltage - the smallest possible difference between VIN and VOUT for the linear regulator to remain inside the regulator's intended operating range

  6. Types of Linear Regulators • The pass element in a linear regulator can be bipolar transistors or MOSFETs. Different configuration gives different dropout voltages • The bipolar linear regulators have higher drop out voltages, and can support higher input voltages and have better transient response. • MOSFET LDO can support very low dropouts, low quiescent current, improved noise performance and low power supply rejection.

  7. More About LDO Dropout • In an LDO datasheet, dropout is only specified under max output current condition. At other operating condition the dropout can be calculated. • The FET used in an LDO operates in the linear region. The FET has a minimum resistance at the saturation line. LDO cannot operate at the left side of the saturation line.

  8. Other Key Specifications • Quiescent current – the current consumed by the regulator and doesn’t flow to the output load. • Important to applications that needs running all the time like baseband, real time clock • Power supply rejection ratio (PSRR) – the ratio of the regulated output voltage ripple to the input voltage ripple. • Important to applications with high noise restriction like low noise amplifier, audio, RF and wireless • Broadband noise – total noise energy over a specific frequency range • Important to applications with high noise restriction like PLL, TCXO, RF and wireless

  9. LDO Selection

  10. Summary • Introduction to linear regulator • Types of linear regulator • Dropout of LDO • Considerations in selecting LDO

  11. Thank you!

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