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. But the linear regulator has limited efficiency and cannot boost voltage (to make V out > V in ) 2

Disadvantages Pros and Cons 3 Advantages Low efficiency, especially when V in to V out 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. V out has to be less than V in 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 V out on large load step Easy to implement short circuit protection

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

Dropout Voltage Dropout voltage - the smallest possible difference between V IN and V OUT for the linear regulator to remain inside the regulator's intended operating range 5

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. 6

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. 7

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 8

LDO Selection 9 Applications Low Iq Low dropout High PSRR Low noise High CurrentNotes LNA, PLLYes Defines the system noise floor. Quiet LDO is required Baseband - digital YesAlways on Baseband- analog Yes Always on. Needs to reject input ripple and low dropout TCXOYes Used in IF section for low noise in system Real Time Clock YesAlways on AudioYes High PSRR from 20Hz to 200kHz

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

Thank you! 11