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Part 2: Semiconductors
Chapter 7: Voltage Regulator ICs (Preview)

Virtually all power supplies employ semiconductors to provide a regulated output voltage. If the supply has an AC input, it is rectified to be a DC voltage. A power converter IC accepts the DC input and produces a DC output or controls external power output semiconductor switches to produce a DC output. It is a voltage regulator when its output voltage is fed back to a circuit that causes the voltage remains constant. If the output voltage tends to rise or fall, the feedback causes the output to remain the same.

The power converter can operate either as a switch-mode or linear circuit. In a linear configuration, the controlling transistor always dissipates power, which can be minimized by using low dropout regulators (LDOs) that regulate properly even when there is a relatively low voltage differential between their input and output. LDO ICs have simpler circuits than their switch-mode cousins and produce less noise (no switching) but are limited by their current-handling and power dissipation capability. Some LDO ICs are specified at about 200mA and others can handle up to about 1A.

Efficiency of the LDO ICs may be 40-60%, whereas the switch-mode ICs can exhibit up to 95% efficiency. Switch-mode topologies are the primary approach for embedded systems, but LDOs also find use in some applications.

Low Dropout (LDO) Linear Regulator
LDO linear regulators are usually employed in systems that require a low-noise power source instead of a switching regulator that might upset the system. LDOs also find use in applications where the regulator must maintain regulation with small differences between the input supply voltage and output load voltage, such as battery-powered systems. Their low dropout voltage and low quiescent current make them a good fit for portable and wireless applications. LDOs with an on-chip power MOSFET or bipolar transistor typically provide outputs in the 50 to 500mA range.

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An LDO voltage regulator operates in the linear region with the topology shown in Figure 7-1. As a basic voltage regulator its main components are a series pass transistor (bipolar transistor or MOSFET), differential error amplifier, and precise voltage reference.

Key operational factors for an LDO are its dropout voltage, power supply rejection ratio (PSRR), and output noise. Low dropout refers to the differencebetween the input and output voltages that allow the IC to regulate the output load voltage. That is, an LDO can regulate the output load voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible to minimize power dissipation and maximize efficiency. Typically, dropout is considered to be reached when the output voltage has dropped to 100mV below its nominal value. The load current and pass transistor temperature affect the dropout voltage.

An LDO's internal voltage reference is a potential noise source, usually specified as microvolts RMS over a specific bandwidth, such as 30 µV RMS from 1 to 100 kHz. This low-level noise causes fewer problems than the switching transients and harmonics from a switch-mode converter. In Figure 7-1 the LDO has a (voltage-reference) bypass pin to filter reference voltage noise with a capacitor to ground. Adding the datasheet-specified input, output, and bypass capacitors usually results in a non-problematic noise level.

Among their operational considerations are the type and range of the applied input voltage, required output voltage, maximum load current, minimum dropout voltage, quiescent current, power dissipation, and shutdown current.

Controlling the LDO’s frequency compensation loop to include the load capacitor reduces sensitivity to the capacitor’s ESR (equivalent series resistance), which allows a stable LDO with good quality capacitors of any type. In addition, output capacitor placement should be as close as possible to the output.

Additional features in some LDOs are:
An enable input that allows external control of LDO turn-on and turn-off.
Soft-start that limits inrush current and controls output voltage rise time during power-up.
A bypass pin that allows an external capacitor to reduce reference voltage noise.
An error output that indicates if the output is going out of regulation.
Thermal shutdown that turns the LDO off if its temperature exceeds the specified amount.
Overcurrent protection (OCP) that limits the LDO’s output current and power dissipation.

The LT3042 from Linear Technology is a low dropout (LDO) linear regulator that uses a unique architecture to minimize noise effects and optimize Power Supply Ripple Rejection (PSRR).

PSRR describes how well a circuit rejects ripple, injected at its input. The ripple can be either from the input supply such as a 50Hz/60Hz supply ripple, switching ripple from a DC/DC converter, or ripple due to the sharing of an input supply with other circuits.

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