How Do LDO and Buck Regulators Work?

• Input voltage
• Regulator
• Output voltage

What you will learn

• Explain how an LDO lowers voltage and why it warms up.
• Describe the parts of a buck converter.
• Understand how duty cycle relates to the output voltage.

Before you start

It helps to know what a transistor, a capacitor and a diode are.

How does an LDO hold the voltage?

A linear regulator uses a pass element between its input and output to hold a regulated DC voltage. An LDO turns excess power—mainly determined by the input-to-output voltage difference and load current—into heat.

The pass element operates in its linear region and dissipates power as heat. A larger voltage difference or load current increases heat; an LDO can suit low-power uses or a small voltage difference.

• Series transistor
• Heat given off
• Lower output voltage

The source example uses an input capacitor, an output capacitor and feedback resistors to set the output voltage; the parts required by a specific regulator depend on its design.

What parts does a buck converter have?

The buck regulator is designed to efficiently reduce a DC voltage from a higher voltage to a lower one. It is a switch mode power supply, using semiconductor switching techniques rather than standard linear methods.

One basic buck circuit uses a series transistor switch, a diode, an inductor and a smoothing capacitor; a synchronous buck uses a second switch instead of the diode. In the ideal model, the switching transistor acts like an ON/OFF switch.

• Switching transistor
• Inductor
• Smoothing capacitor
• Diode

How does duty cycle relate to the output voltage?

The average output voltage is related to the duty cycle, the fraction of each switching cycle during which the transistor conducts. Voltage regulation is obtained by varying the duty cycle.

When operating as a step-down converter, a buck produces an output below its input; changing the duty cycle changes the output level.

• Lower duty ratio
• Higher duty ratio
• Low output voltage
• Higher output voltage

Real-world example

With an adult, look at a phone charger or a small board. There is often a small part that lowers the voltage for the microcontroller. It may be an LDO or a buck converter, depending on the design.

Common misunderstandings

Any LDO heat means something is wrong. Some heat is part of linear regulation, but the amount depends on voltage difference and load. Check the regulator’s specified thermal limits; normal operation does not make excessive heat safe.

A buck converter can give an output higher than its input. A buck is a step-down converter: within its step-down operating range, its output is below its input.

Frequently asked questions

What is an LDO?

An LDO is a linear regulator that regulates voltage by turning excess power into heat.

What is a buck converter used for?

It efficiently reduces a DC voltage from a higher level to a lower one.

What is duty cycle?

It is the fraction of a switching cycle during which the transistor conducts.

What parts does a buck circuit need?

One basic buck circuit uses a series transistor switch, a diode, an inductor and a smoothing capacitor; a synchronous buck uses a second switch instead of the diode.

Summary

• An LDO uses a series pass element in its linear region while regulating and turns excess power into heat.
• A buck converter switches a transistor and uses a diode or second switch, an inductor and a smoothing capacitor to step voltage down efficiently.
• Changing the duty cycle changes the output voltage; a buck operates as a step-down converter.