Introduction to Low Dropout Linear Voltage Regulators (LDO)

A linear voltage regulator is a circuit that takes in a variable input voltage and provides a continuously controlled, steady, low-noise DC output voltage. Generally, linear voltage regulators require a large voltage drop between the input and the output to function correctly. This requires a relatively high-voltage input power supply and results in low power efficiency. A low dropout (LDO) linear voltage regulator is a type of linear voltage regulator circuit that works well even when the output voltage is very close to the input voltage, improving its power efficiency.

LDOs have two main functions, the first obviously being the reduction of an incoming supply voltage to the lower voltage that is needed by the load. A second function is the supply of a very low-noise voltage source, even in the presence of noise on the incoming power supply or transients in the load. In fact, this is their main advantage over switching converters, where noise isolation and emissions are major system concerns.

Fundamentals of Circuit Operation

The fundamental concept of the linear voltage regulator is shown in Figure 1.

Figure 1: Linear Voltage Regulator Fundamental Concept

In simple terms, the linear voltage regulator is an effective impedance (R_LDO) that is presented to the load (R_LOAD) so that the excess voltage is dropped across R_LDO in order to maintain the required voltage levels at the load.

In many systems, the power supply providing the input voltage varies considerably, which, in the above implementation, would cause the output voltage to also vary by a corresponding amount. For this reason, it is necessary to add a closed-loop control system to ensure the output voltage remains constant, independent of the input line voltage. Such closed-loop feedback networks usually regulate by using a fixed voltage reference, typically provided by a bandgap reference circuit. Figure 2 depicts the closed-loop system.

Figure 2: Simplified Linear Voltage Regulator

The linear voltage regulator output voltage (V_OUT) is now independent of the input source voltage (V_IN) and is directly related to the voltage reference. If this voltage reference is stable and clean, the output is generally immune to any line changes. If the control loop is fast enough, then the regulator can also maintain a steady output voltage during abrupt transient changes in load current.

When the design of the regulator is such that the minimum required voltage drop across R_LDO is small (a few hundred millivolts or less), then this is known as a low dropout linear voltage regulator or simply LDO.

Additional Features

In addition to the above-mentioned fundamental functionality of an LDO, several other useful features can be included.

Enable/Disable

Many LDOs have a control input to enable the regulator or shut it down.

Soft-Start, Soft-Shutdown

Powering up the LDO is an important event and care must be taken to ensure that the output voltage rises at the optimal speed. If the voltage builds up too fast, it can trigger ESD clamps at the output and/or drive more than the rated current while trying to charge up the load. To prevent such unwanted occurrences, which can cause temporary or permanent damage to circuitry, the rate at which the output comes up is controlled. This is called Soft-Start. Vidatronic’s IP cores provide Soft-Start functionality, ensuring a smooth, monotonic startup.

Similarly, when the LDO is disabled, the output voltage will decay as a function of output capacitance and load current. Vidatronic’s IP cores have an additional feature normally not found on most LDOs. Should the part be re-enabled before the output fully decays, a slew rate function within the IP core will first bring the output voltage to zero before restarting, thus eliminating issues with startup glitches or non-monotonicity.

Over-Current/Over-Temperature Shutdown

In the event of prolonged exposure to conditions over and beyond the rated temperatures and load current levels, immediate or long-term damage to the device can occur. To safeguard against these risks, power systems usually have built-in over-current and over-temperature protection circuitry.

For Vidatronic’s IP cores, in the case of an over-current, the device will set a flag and initiate a Soft-Shutdown. After the current is decreased to zero, the part will wait a turn-on delay time of some milliseconds and then proceed with a slew-rate-controlled startup. This operation will continue indefinitely until the fault disappears.

Undervoltage Detection and Overvoltage Detection

Input voltages can undergo sudden excursions that can have catastrophic consequences if they are outside acceptable values. An undervoltage (sometimes called a “brownout”) condition can force the LDO output to go below its regulation window, causing the load to fail functionally. An overvoltage is cause for reliability or Safe Operating Area (SOA) concerns.

Detecting the occurrence of these abnormal conditions is useful for designing failsafe limits in the overall system. Depending on the application space, many LDO manufacturers offer one or both voltage excursion detection features. One of the common responses to such faulty conditions is shutting down the regulator and flagging the system.

The circuit that responds to the undervoltage or brownout condition is typically called an undervoltage lockout, or UVLO. Should the input voltage fall below the required threshold to maintain regulation for Vidatronic’s IP cores, the regulator will set an internal flag and proceed with a Soft-Shutdown.

Loop Compensation

As with any closed-loop control system, the LDO feedback loop needs to be frequency compensated for the output voltage to be stable. A careful analysis of the various poles and zeros of the loop filter is required, and this analysis is typically the most complex part of the design of an LDO, requiring a significant amount of design time.

Previous Solutions

One of the common approaches to frequency compensation of LDO loops is to use a very large capacitor at the LDO output. Such LDOs are called output-compensated regulators. The exact capacitor values depend on several factors such as load current, headroom available, and others. Typical values are in the range of hundreds of nF to a few F. These capacitors help with sourcing or sinking surge currents due to fast load changes. The main drawbacks with such systems are the additional cost for these external capacitors and the extra space on the PCB for mounting them. Typical Internet of Things (IoT) systems and other similar applications require several LDOs. A capacitor for each LDO output will very quickly become prohibitive.

Another factor is that the range of required values for the load capacitors usually spans a few decades or more, depending on the circuit. It is impractical to design custom LDO cores that work for every application, even though it is often desirable to re-use an existing core for different applications. Working out a frequency compensation scheme that can support such a wide range of output capacitors is an extremely tough requirement to meet. Conventional solutions to such problems result in over-design in terms of silicon area and additional circuitry, which invariably get passed on to the end user as additional cost and size.

Vidatronic’s Solution

Vidatronic has a unique solution for such problems. Vidatronic’s LDOs use a novel load-detection scheme that allows the LDO to sense the amount of capacitive load the regulator sees at the output and automatically configure itself to the most stable internal loop compensation. This “calibration” is done one time, at startup and then is done. The advantage of this is two-fold:

• Vidatronic’s LDOs do NOT require an external capacitor to be stable. Vidatronic’s patented Noise Quencher^® technology ensures good transient response even for fast load transients, WITHOUT the need for an external capacitor.
• Vidatronic’s LDOs will AUTOMATICALLY adjust internal compensation to be stable if the customer decides to place an external capacitor for specific performance reasons (for instance to achieve very high noise suppression).

This is a guest blog by Anand Veeravalli and Stephen M. Nola of Vidatronic. For more information on our patented LDO technology, please visit our website. To read a more in depth introduction to LDO linear voltage regulators that includes electrical characteristics definitions and application considerations, download the full white paper.