VOLTAGE REGULATOR Using LDO regulators in power electronics

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The job of a regulator is to keep voltages in a compatible and safe range for all circuit components. Most regulators perform this function at the trade-off between high losses, unnecessary electrical noise, and low efficiency. There isn’t a single regulator type in power electronics — of course, there are many! Unlike other regulators, low-dropout regulators are heroes who won’t generate noise, or what you call EMI, to ruin your electronic system. The article describes LDO regulators and their use in power electronics.

Low-dropout regulators (LDOs) play a crucial role in power electronics, delivering precise voltage regulation with minimal noise, making them indispensable for sensitive electronic applications

What is a low-dropout regulator?

Generally, scientists research semiconductors and electronic devices for years before their invention. Low-dropout regulators debuted directly in a magazine article called “Break Loose from Fixed IC regulators”. It was 1977, and the magazine was Electronic Design, which still runs today.

How does an LDO regulator work?

Just as the name suggests, a low-dropout regulator is capable of operating when the supply voltage is nearly equal to the output voltage. Engineers use LDO regulators when they need a small difference between the input and output voltages. For example, an LDO regulator converts a 5 V input voltage to 3.3 V at the output.

LDO regulator structure

An LDO regulator consists of two main components: a power transistor and a differential amplifier. The power transistor is a power FET (Power Field Effect Transistor), usually a JFET (Junction Field Effect Transistor). In some designs, the power transistor can also be a BJT (Bipolar Junction Transistor). LDOs use only an input capacitor and an output capacitor.

The differential amplifier is known as the error amplifier. The power FET is known as the pass transistor. Incorporating an N-channel power FET-based design reduces the overall power consumption of the LDO. On the other hand, bipolar LDOs are cheaper. In bipolar LDO regulators, Darlington pairs are used. Using a PMOS power FET or P-channel power FET results in effective regulation and lower power loss. Using NPN power BJT offers stable regulation with capacitive loads.

LDO regulator operation

LDOs use a feedback loop to compare the reference voltage to the actual output voltage.

Pass transistor: The input voltage enters the pass transistor. Operating in the linear region, the pass transistor passes current to the output stage.

Voltage divider network: A voltage divider network is at the output. The voltage divider network, consisting of R1 and R2 resistors, is connected to the inverting terminal (negative terminal) of the error amplifier.

The voltage divider network forms a feedback loop with the error amplifier. It scales down the actual output voltage and sends it back to the negative terminal of the error amplifier.

VOLTAGE REGULATORS

The working principles of voltage regulators

Bandgap reference: The non-inverting terminal (positive terminal) of the error amplifier is connected to a voltage reference circuit. The voltage reference circuit is called the bandgap reference. It is a small internal circuit that generates a stable voltage, irrespective of input voltage.

Comparator operation: The error amplifier, also known as a comparator in digital electronics, compares the reference voltage to the scaled-down voltage from the divider network. It generates an output.

Pass transistor action: Based on the output from the error amplifier, the pass transistor opens or closes to allow more or less current flow through the device— adjusting the output voltage. Simply put, the error amplifier adjusts the transistor gate for voltage regulation.

Open-drain configuration: The pass transistor uses an open-drain configuration to perform voltage regulation. In an open-drain configuration, the drain pin is either open or driven by a ground pin.

If the output voltage is too high, the open-drain control circuit opens the gate terminal of the pass transistor. The open-circuited pass transistor turns off to lower the output voltage.

If the output voltage is too low, the open-drain control circuit connects the gate of the pass transistor to the ground. The pass transistor turns on to increase the current flow and raise the output voltage. Practically, this operation is not possible.

Feedback loop: The pass transistor again passes this voltage to the voltage divider network, and the feedback process of regulation continues. Finally, the LDO regulator adjusts the output voltage near the input voltage level, meaning that when the input and output voltages are near each other. The condition is called dropout.

Dropout voltage: Dropout voltage is the minimum difference between the input and output voltages at which an LDO can maintain its regulation. Simply put, it is a measure of the output voltage that can get near the input voltage while the LDO works properly. In practice, datasheets do not list dropout voltage. Engineers have to calculate for themselves.

LDO regulator advantages

Among the benefits of using LDO regulators are several key features that enhance their appeal in tight, low-noise environments.

• LDO regulators use an internal smart control loop to regulate voltages.

• LDO regulators show a small input-output voltage differential.

• LDOs output a steady, stepped-down voltage.

• Under steady-state conditions, LDO regulators behave like simple resistors.

• Non-LDO regulators cannot operate when the input voltage is near the output voltage.

• LDO regulators can operate even when there is a minute difference between the input and output voltages. For example, an LDO regulator can take 3.6 V from a Li-ion battery and convert it into 3.3 V efficiently.

• Non-LDO regulators incorporate emitter follower or source follower configurations. It fails to regulate the voltage.

• LDO regulators do not generate electrical noise.

• LDO regulators make the design compact. Non-LDO regulators need more space between input and output stages, making the design bulkier.

• Using LDO regulators is simple and cost-effective.

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LDO regulator disadvantages

Despite their many advantages, LDO regulators also present some drawbacks that can impact their effectiveness in certain applications.

• LDO regulators are designed only to step down the voltage. They cannot step up voltage or boost voltage. Boost converters are designed for such operations.

• Ground current can cause a spike in bipolar LDO regulators.

• LDOs need additional feedback compensation networks.

• Sometimes LDOs fail to respond quickly to changes in static conditions.

• When LDOs are in dropout, the error amplifier saturates. The LDO cannot regulate properly anymore.

• At dropout, LDOs pass electrical noise.

• LDO regulators still generate a very small amount of heat.

LDO regulator applications

LDOs are called low-dropout regulators because they are capable of dropping the output voltage near the input voltage. Some of their applications are listed below.

Portables

LDO regulators are suitable for battery-powered systems such as smartphones, tablets, small computers, cellular telephones, wearables, and some mobile home appliances.

Radio frequency circuits

LDO regulators offer low noise in RF circuits. Applications include phase-locked loops, voltage-controlled oscillators, analog-to-digital converters, digital-to-analog converters, and many more.

Power electronics

An LDO regulator is a type of linear regulator placed in series between the power supply and load. A linear regulator is a type of regulator that behaves as a variable resistor in a circuit. It changes the output voltage in response to changes in the input and output currents.

In simple words, a linear LDO regulator decreases the output voltage when it goes way above the input voltage. LDOs are used in linear power supplies, SMPS post-regulators, and DC-DC modules.

It is important to note that another famous power regulator, the Zener regulator, is a linear shunt regulator. Unlike LDO regulators, Zener regulators are connected in parallel between the supply and load.

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Zener diode as a voltage regulator

Power ICs

In power management integrated circuits (PMICs), trace resistance causes a significant voltage drop. Crosstalk exists due to capacitive coupling between traces. Such unwanted features degrade the PMIC performance. LDO regulators, in their miniaturized form, are used in PMIC power supply ICs for POL delivery (point of load delivery) and voltage regulation of microprocessors and microcontrollers.

References

• https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/e-learning/basics-of-low-dropout-ldo-regulators/chap1/chap1-3.html

• https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/e-learning/basics-of-low-dropout-ldo-regulators/chap1/chap1-4.html

• https://electronics.stackexchange.com/questions/225368/can-you-step-up-voltage-with-ldo

• https://www.ti.com/lit/ml/slup239a/slup239a.pdf?ts=1751208311207&ref_url=https%253A%252F%252Fwww.google.com%252F

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