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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 Vout > Vin)
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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 Vout on
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
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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.
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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
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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.
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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.
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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
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LDO Selection
Applications
Low Iq
Low
dropout
LNA, PLL
Baseband digital
Yes
Basebandanalog
Yes
TCXO
Real Time
Clock
Audio
High
PSRR
Low
noise
Yes
Yes
High
Current
Notes
Defines the system noise
floor. Quiet LDO is required
Always on
Yes
Always on. Needs to reject
input ripple and low dropout
Yes
Yes
Used in IF section for low
noise in system
Yes
Yes
Always on
Yes
Yes
Yes
High PSRR from 20Hz to
200kHz
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Summary
• Introduction to linear regulator
• Types of linear regulator
• Dropout of LDO
• Considerations in selecting LDO
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Thank you!
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