LDOs – What are they and when should they be used??

Low dropout (LDO) regulators have been around for some time, but today’s electronic designs have seen lower-noise LDOs become more commonly used for a greater range of applications. This is because there is currently a growing requirement for stable, low-noise power supplies as more electronic systems incorporate highly sensitive components. For example, sensors intended for use in the internet of things are often required to measure tiny signals, and any noise introduced from the power supply can distort the results. The same situation applies throughout the signal chain as the measured signal is conditioned and amplified. RF systems also require minimal noise to function effectively. LDOs are the ideal components to minimize noise and ripple from the power supply.

LDOs also work very well in applications where the input voltage may vary, and a stable output voltage is required. For example, if a microcontroller requires a 3.3V power supply, an LDO regulator could be used to regulate the input voltage to a stable 3.3V, even if the input voltage varies between 3.5V and 4.5V. This makes LDOs especially suitable for battery-powered applications, as they can regulate the voltage of a battery as it discharges, providing a stable output voltage as the input voltage drops.

However, LDOs can’t be used in every application. They are only normally useful in applications where the input voltage is slightly higher than the desired output voltage. For this reason, they often work beside a normal switching regulator that brings the initial input voltage down to a level that the LDO can use. Circuits with higher currents are also less suitable for the use of LDOs, due to the higher dissipated power, but manufacturers are working to push those current capabilities higher. For example by using packages with lower thermal resistance.

As their use grows, many companies are also updating their LDO designs to make them more useful for modern applications. These improvements include new features, higher accuracy and a better power supply rejection ratio (PSRR). The PSRR is one of the most important metrics for LDO regulators as it describes the ability of the system to sustain its output voltage as its DC power-supply voltage is varied by the noise on the input supply propagating to the output. In LDOs the main PSRR value measures how well the regulator rejects an AC signal, such as the ripple introduced by a switching power supply, riding on the DC voltage. The PSRR of the LDO is normally expressed in decibels and is defined over a wide frequency range.

Frederik Dostal of Analog Devices explains the importance of PSRR, “Some applications require a generated voltage with the lowest noise possible. Any input voltage ripple or input noise will show up on the output side if the LDO doesn’t have an excellent PSRR. It is just as important in these applications as the internal noise the regulator generates, which is around 1 microvolt RMS between 10Hz and 100kHz. This is already much lower than battery noise, for example, a lithium ion battery produces noise measuring a few microvolts. Typically people think the battery is the lowest noise component possible, but a good LDO is much lower.”

A typical example of a modern LDO is the Analog Devices LT3041 which was launched last year. It is a high-performance, low-dropout linear regulator with ultra-low noise and ultra-high PSRR architecture which can output up to 1 Amp, which is very high for LDO operation. Its PSRR is measured at 80dB at 1MHz – a typical frequency for today’s power supplies. – a typical frequency for today’s power supplies. The device is designed as a precision-current reference followed by a high-performance voltage buffer with an operating quiescent current around 4.3 mA and 18 μA in shutdown.

As well as the usual features found in similar devices, such as reverse-battery protection and reverse-current protection, the LT3041 also incorporates Analog Devices’ VIOC control to reduce power dissipation. Dostal explains its operation, “VIOC is a variable pin that can be connected to the feedback pin or feedback resistive divider of the switching regulator that supplies the LDO. It automatically detects if the LDO input voltage is a little low, which makes it more difficult to stabilize the output voltage. If the input voltage is too low, it tells the switching regulator to increase the output voltage so that the LDO can operate normally. Traditionally, to solve this problem, the output voltage would be set a little bit higher give a bit more headroom. However, it would then dissipate more heat, losing energy. VIOC dynamic control tells the switching regulator what to do keep the output voltage just high enough for the LDO to operate correctly and eliminates that excess power dissipation”.