Linear Regulator Block Symbol
Definition: The Linear Regulator Block symbol represents a three-terminal voltage-regulation circuit or IC that maintains a fixed or adjustable DC output voltage (VOUT) by dissipating excess input-to-output voltage difference as heat, with pins VIN (input), GND (ground), and VOUT (output), drawn in schematics as a labelled rectangular block per general IC conventions since neither IEC 60617 nor IEEE 315 defines a dedicated linear-regulator glyph.
Also known as: LDO regulator, low-dropout regulator, linear voltage regulator, series voltage regulator, VReg, series-pass regulator.
What the Linear Regulator Block symbol means
The Linear Regulator Block symbol denotes a DC–DC voltage-regulation element that uses a series-pass transistor (MOSFET or BJT) in its linear region to drop the difference between the input voltage VIN and the regulated output voltage VOUT as heat. The output voltage remains constant (within the regulator's specified load and line regulation specifications) regardless of variation in the input voltage or output current, provided VIN exceeds VOUT by at least the minimum dropout voltage (Vdropout).
In circuit diagrams the linear regulator block symbol marks the voltage domain boundary: everything upstream of the VIN pin operates at the higher, unregulated supply voltage, while everything downstream of the VOUT pin operates at the stable, noise-free regulated voltage. The GND pin connects to the common ground reference. The linear regulator is preferred over a switching regulator wherever low output noise, fast transient response, and circuit simplicity are more important than power conversion efficiency.
How to identify the Linear Regulator Block symbol
The linear regulator block symbol is a rectangle labelled with the regulator type — 'Linear Regulator', 'LDO', or the specific part number (e.g. LM317, LM7805, AMS1117). Three pins are drawn: VIN on the left edge (input), GND also on the left edge or bottom (ground), and VOUT on the right edge (output). Some representations show VIN and GND entering from the left with VOUT exiting to the right, mirroring the physical pinout of common TO-220 package regulators. The block may include the output voltage specification (e.g. '5 V', '3.3 V') as a text annotation inside or beside the rectangle.
Function in a circuit
A linear voltage regulator operates by continuously adjusting the resistance of an internal series-pass element (a BJT emitter follower or a MOSFET source follower) to maintain the output voltage at the target value set by an internal voltage reference and error amplifier. The error amplifier compares a scaled fraction of the output voltage to the precision reference and drives the pass element's gate or base to correct any deviation. Any difference between VIN and VOUT appears across the pass element as a voltage drop, and the power dissipated equals (VIN - VOUT) × IOUT. For low-dropout (LDO) regulators the minimum VIN - VOUT dropout is typically 100–600 mV; for standard regulators (e.g. 78xx series) the dropout is 2–3 V.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated schematic symbol for linear voltage regulator ICs. In IEC-style schematics the regulator is drawn as a general rectangular function block (per IEC 60617-02 conventions) with VIN, GND, and VOUT pins labelled; the block carries the part number or generic label 'LDO' or 'VREG'. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) does not define a specific symbol for linear regulators. The device is represented as a labelled rectangular IC block with inputs on the left (VIN, GND) and output on the right (VOUT), following the general IC drawing convention. |
| Key difference | IEC and ANSI representations are functionally identical for linear regulator blocks. No standard-specific glyph distinguishes them; the only variation is the pin arrangement convention: IEC may place all supply pins at the top and bottom while ANSI places inputs on the left and outputs on the right. |
Terminals / pins
| Pin | Name |
|---|---|
| vin | VIN |
| gnd | GND |
| vout | VOUT |
Typical values
Input voltage VIN: typically 2–40 V (device specific); Output voltage VOUT: fixed (1.2–24 V, device series dependent) or adjustable (1.25–37 V for LM317); Dropout voltage: 2–3 V (standard), 100–600 mV (LDO); Output current: 100 mA–5 A (TO-220 package); Line regulation: 0.01–0.1%/V; Load regulation: 0.01–0.1%/mA; Output noise: 10–100 µV RMS (LDO); Junction-to-ambient thermal resistance: 5–50 °C/W (package dependent).
Where the Linear Regulator Block symbol is used
- Microcontroller and digital logic supply: LDO regulators convert a 5 V or higher supply to the 3.3 V or 1.8 V required by ARM, PIC, or AVR microcontrollers
- Analog and RF circuits: linear regulators provide low-noise, well-regulated supplies for op-amp, ADC, and RF front-end circuits where switching regulator ripple would degrade performance
- Battery-powered devices: LDOs with low quiescent current (1–10 µA) regulate battery voltage down to the required rail while minimising standby power drain
- Post-regulator filtering: a linear regulator placed after a switching regulator removes residual switching noise, combining the efficiency of a switcher with the clean output of an LDO
- Reference voltage generation: precision LDOs with temperature-compensated references (e.g. LT3042) supply stable voltage references for DACs, ADCs, and measurement instruments
- USB-powered device supply: 3.3 V or 5 V LDOs regulate the 5 V USB bus voltage to the exact supply required by the device circuitry
Example
In a 3.3 V Arduino-compatible sensor shield schematic, an AMS1117-3.3 LDO regulator (U1) is shown with VIN connected to the 5 V USB rail, GND to the board ground plane, and VOUT supplying the 3.3 V rail for an ESP8266 Wi-Fi module. A 10 µF ceramic capacitor is placed at VIN and a 22 µF at VOUT per the datasheet recommendation. The linear regulator block symbol is labelled 'AMS1117-3.3 / LDO 3.3 V / U1' and the VIN-to-VOUT dropout voltage annotation reads '≥ 1 V required'.
Key facts
- A linear voltage regulator maintains a stable DC output voltage by dissipating the excess input-to-output voltage difference as heat via a series-pass transistor operating in its linear region.
- The linear regulator block symbol has three pins: VIN (input supply), GND (ground reference), and VOUT (regulated output); the block is a labelled rectangle following general IC conventions since no dedicated IEC 60617 or IEEE 315 symbol exists.
- Low-dropout (LDO) regulators require only 100–600 mV of headroom between VIN and VOUT, versus 2–3 V for standard 78xx-series regulators, making LDOs preferred for battery-powered applications where supply margin is limited.
- Power dissipation in a linear regulator equals (VIN - VOUT) × IOUT; at high currents and large VIN–VOUT differences this can be substantial (e.g. dropping 12 V to 5 V at 1 A dissipates 7 W), requiring a heat sink on the TO-220 package.
- Linear regulators provide significantly lower output noise (10–100 µV RMS) than switching regulators (1–10 mV ripple), making them essential for analogue, audio, and RF supply rails.
- The reference designator for a linear regulator IC in schematics is U followed by a number (e.g. U1, U3), following the standard IC reference designator convention of both IEC and IEEE 315.
- Efficiency of a linear regulator equals VOUT/VIN; at VIN = 12 V and VOUT = 3.3 V the maximum efficiency is 27.5%, far below a switching regulator's typical 85–95% — so linear regulators are avoided in high-current, large-differential applications where thermal management is a constraint.
Frequently asked questions
What does the linear regulator symbol look like in a schematic?
The linear regulator block symbol is a rectangle labelled 'Linear Regulator', 'LDO', or the specific part number (e.g. LM317, AMS1117). Three pins are shown: VIN (input voltage, left or top), GND (ground, left or bottom), and VOUT (regulated output, right). The output voltage value may be annotated on or beside the block (e.g. '3.3 V' or '5 V').
What does a linear regulator do in a circuit?
A linear regulator converts an unregulated or higher DC input voltage (VIN) to a stable, lower DC output voltage (VOUT) by continuously adjusting an internal series-pass transistor. The output voltage remains constant despite variations in input voltage or load current, within the device's specified regulation limits. The difference between VIN and VOUT is dissipated as heat in the pass transistor.
What is the difference between a linear regulator and an LDO?
A standard linear regulator (e.g. LM7805, 78xx series) requires 2–3 V of headroom between VIN and VOUT to maintain regulation; it uses a bipolar NPN pass transistor configured as an emitter follower. An LDO (low-dropout regulator) requires only 100–600 mV of dropout voltage; it uses a PNP transistor or PMOS FET as the pass element, which saturates at a lower voltage drop. LDOs are preferred for battery-powered applications where the supply voltage is close to the desired output.
How is a linear regulator different from a switching regulator?
A linear regulator uses a pass transistor in its linear region and dissipates all excess input power as heat; its efficiency equals VOUT/VIN. A switching regulator converts power using a rapidly switching transistor and energy storage elements (inductor, capacitor), achieving 85–95% efficiency regardless of the VIN-to-VOUT ratio. Linear regulators produce much lower output noise and are simpler; switching regulators are more efficient and generate heat only from conduction losses.
What standard defines the linear regulator schematic symbol?
Neither IEC 60617 nor IEEE 315-1975 (ANSI Y32.2) defines a dedicated symbol for linear voltage regulator ICs. The device is drawn as a general rectangular function block in both IEC-style and ANSI/IEEE-style schematics, with VIN, GND, and VOUT pins labelled. The specific part number or generic label 'LDO' or 'VREG' identifies the component type.
What is the reference designator for a linear regulator?
The reference designator for a linear regulator IC is U followed by a sequential number, e.g. U1, U2, U4, following the standard IC reference designation convention used in both IEC and IEEE 315 schematics. Some legacy drawings use VR (Voltage Regulator) followed by a number, e.g. VR1.
Why does a linear regulator need a heat sink?
A linear regulator dissipates power equal to (VIN - VOUT) × IOUT as heat. For example, a 5 V regulator supplied from 12 V at 500 mA dissipates (12 - 5) × 0.5 = 3.5 W. The junction-to-ambient thermal resistance of a TO-220 package without a heat sink is approximately 50 °C/W, raising the junction temperature by 3.5 × 50 = 175 °C above ambient — well above the maximum junction temperature of 125–150 °C. A heat sink is required to reduce junction temperature to a safe operating level.
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