■ Linear regulators for ultra-low noise applications in RF and instrumentation

A new product type has emerged in the electronics industry: the digital low dropout (LDO) linear regulator. This product provides telemetry and parameter adjustability of linear power supplies in a very small area. These solutions are used in ultra-low noise applications in RF and instrumentation.

■ Converts high input voltage to lower output voltage

A linear regulator is a simple voltage converter that can convert a high input voltage to a lower output voltage. It acts like a dynamic resistor, constantly adjusting the resistance value to the required value to maintain a constant output voltage at a given current flow.

The availability of a wide variety of easy-to-use switch-mode power converters with much higher efficiency for most applications has led to a decline in the use of linear regulators.

However, there is one place where linear regulators are still very important. It is used to filter the voltage generated by the switch mode power converter. Figure 1 shows such an example.

Switch mode power supplies (SMPS) produce output voltages with voltage ripple due to their technical characteristics. Linear regulators typically have good power supply rejection ratio (PSRR). This indicates how well a device can reduce output voltage interference caused by power supply noise at various frequencies.


For applications of linear regulators such as those in Figure 1, a special type of linear regulator has been developed, which is called an ultra-low noise linear regulator. Such linear regulators not only provide high PSRR values, but also generate very low noise, especially in the low frequency band between 1 Hz and 100 kHz.

Typical applications for such low-noise power supplies include high- and low-bandwidth precision circuits such as phase-locked loops (PLLs), voltage-controlled oscillators (VCOs), mixers, low-noise amplifiers (LNAs), and power amplifiers (PAs), as well as high-precision instrumentation applications.

Some of these applications benefit from being able to detect the state of the power supply. The system can use information such as the input voltage, output voltage, load current, and temperature of the linear regulator to determine whether the circuit is functioning properly. A new solution for these applications has emerged: digital LDOs.

Digital LDOs are designed similarly to conventional ultra-low noise LDOs, but feature a digital interface called PMBus®. Figure 2 shows a similar system to Figure 1, but is used with a digital LDO, the LT3074.


Digital LDOs are a new product category. In the past, a digital interface device such as the LTC2972 could be combined with an ultra-low noise linear regulator such as the LT3041.

However, this approach takes up more circuit space and makes the design more complex. With new digital LDOs such as the LT3074, all functions are integrated into a single IC.

This integrated circuit (IC) is a linear regulator with a digital interface and includes telemetry functions for setting LDO parameters such as current limit, output voltage, etc., and obtaining operating status information.

For example, in this system, the microcontroller (MCU) communicates with the LDO as shown in Figure 2. Evaluation can be performed using LTpowerPlay®, a free software download available from analog.com. This program provides a graphical user interface (GUI) to communicate with the digital LDO via the PMBus interface over USB.

You might wonder why this technology, the combination of digital interconnection and ultra-low noise LDOs, was not possible in the past. The reason is that it was difficult to combine digital clock circuit components with the ultra-low noise technology of LDOs without the digital part interfering with the analog part. Now, however, this combination is possible with the LT3074, which has noise of 1.2μV rms (10Hz to 100kHz).

■ Application miniaturization with high level of integration

New developments in linear regulators have led to the first digital LDOs. These devices will help build more advanced systems in RF, instrumentation, and medical applications. Systems will become more robust due to integrated telemetry, more functional due to parameter adjustment, and smaller due to high levels of integration.


※ About the author
Frederik Dostal is a power management expert with over 20 years of experience in the industry. He graduated from the University of Erlangen in Germany with a degree in microelectronics and joined National Semiconductor in 2001 where he gained extensive experience implementing power management solutions on customer projects as a FAE. While at NS he worked in Phoenix, Arizona for four years as an applications engineer working on switch mode power supplies (SMPS). He joined Analog Devices in 2009 and has since held various positions in product lines and European technical support. He is currently based in Munich, ADI as a power management expert with extensive design and application knowledge.