Low dropout regulator
Encyclopedia
Linear regulator
In electronics, a linear regulator is a voltage regulator based on an active device operating in its "linear region" or passive devices like Zener diodes operated in their breakdown region...
which can operate with a very small input–output differential voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
.
The advantages of a low dropout voltage include a lower minimum operating voltage, higher efficiency operation and lower heat dissipation.
The main components are a power FET
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
and a differential amplifier
Differential amplifier
A differential amplifier is a type of electronic amplifier that amplifies the difference between two voltages but does not amplify the particular voltages.- Theory :Many electronic devices use differential amplifiers internally....
(error amplifier). One input of the differential amplifier monitors the fraction of the output determined by the resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference (bandgap reference
Bandgap voltage reference
A bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits, usually with an output voltage around 1.25 V, close to the theoretical 1.22 eV bandgap of silicon at 0 K. This circuit concept was first published by David Hilbiber in 1964...
). If the output voltage rises too high relative to the reference voltage, the drive to the power FET changes to maintain a constant output voltage.
Overview
The adjustable low-dropout regulator debuted on April 12, 1977 in an Electronic DesignElectronic Design Magazine
First published 55 years ago, Electronic Design is the largest published print magazine for the electronic design industry published in the USA by Penton Media....
article entitled "Break Loose from Fixed IC Regulators". The article was written by Robert Dobkin, an IC
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
designer then working for National Semiconductor
National Semiconductor
National Semiconductor was an American semiconductor manufacturer, that specialized in analog devices and subsystems,formerly headquartered in Santa Clara, California, USA. The products of National Semiconductor included power management circuits, display drivers, audio and operational amplifiers,...
. Because of this, National Semiconductor claims the title of "LDO inventor". Dobkin later left National in 1981 to found Linear Technology
Linear Technology
Linear Technology Corporation designs, manufactures and markets a broad line of standard high performance analog integrated circuits. Applications for the company's products include telecommunications, cellular telephones, networking products, notebook and desktop computers, video/multimedia,...
where he is currently chief technology officer.
Current advances in semiconductor technology made possible to significantly reduce voltage drop on all linear voltage regulators, so producers today advertise almost everything as low-dropout. For example, the AS1117 is advertised as low drop out even if it uses same topology as 78xx series.
Regulation
Low-dropout (LDO) regulators work in the same way as all linear voltage regulatorsLinear regulator
In electronics, a linear regulator is a voltage regulator based on an active device operating in its "linear region" or passive devices like Zener diodes operated in their breakdown region...
. The main difference between LDO and non-LDO regulators is their schematic topology
Topology (electronics)
The topology of an electronic circuit is the form taken by the network of interconnections of the circuit components. Different specific values or ratings of the components are regarded as being the same topology....
. Instead of an emitter follower topology, Low-dropout regulators utilize open collector
Open collector
An open collector is a common type of output found on many integrated circuits . Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized on a pin of the IC. The emitter of the...
or open drain topology. This enables transistor saturation, which limits the voltage drop to only the saturation voltage.
If a bipolar transistor is used however, significant additional power is lost to control it, when non-LDO regulators take that power from voltage drop itself. For high voltages under very low In-Out difference there will be significant power loss in control circuit.
Because the power control element is working as an inverter, another inverting amplifier is required to control it, which increases schematic complexity compared to a simple voltage stabilizer.
Power FET
Field-effect transistor
The field-effect transistor is a transistor that relies on an electric field to control the shape and hence the conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are sometimes called unipolar transistors to contrast their single-carrier-type operation with...
s may be preferable to reduce power consumption, but this significantly increases price and poses problems when the regulator is used for low input voltage, because FETs usually require 5 to 10V to open completely.
Quiescent current
Among other important characteristics is the quiescent current (the current flowing through the system when no load is present), which creates a difference between the input and output currents. The series pass element, topologiesTopology (electronics)
The topology of an electronic circuit is the form taken by the network of interconnections of the circuit components. Different specific values or ratings of the components are regarded as being the same topology....
, and ambient temperature are the primary contributors to quiescent current. Quiescent current and input–output drop limit the efficiency of LDO regulators and should thus be minimized.