Using ultra-low noise LDOs to provide “clean” power

Linear regulator integrated circuits (ICs) step down voltage from a higher voltage to a lower voltage without the need for an inductor. A low dropout (LDO) linear regulator is a special type of linear regulator whose dropout voltage (the difference between the input and output voltages required to maintain regulation) is typically less than 400 mV. Early linear regulator designs provided a dropout voltage of approximately 1.3 V, which meant that for an input voltage of 5 V, the maximum output that the device could regulate was only about 3.7 V. However, with today's more sophisticated design techniques and wafer fabrication processes, "low" is roughly defined as <100 mV to around 300 mV.

Furthermore, while an LDO regulator is typically one of the lowest cost components in any given system, it is often one of the most valuable components from a cost/benefit perspective. In addition to output voltage regulation, another critical task of an LDO regulator is to protect the expensive downstream load from harsh environmental conditions, such as voltage transients, power supply noise, reverse voltage, current surges, etc. In short, its design must be rugged and include all the protection features to suppress the performance impact caused by the environment while protecting the load. Many low-cost LDO linear regulators fail due to the lack of necessary protection features, which not only causes damage to the regulator itself, but also damages the downstream load.

1. Comparison between LDO regulator and other regulators

Step-down conversion and regulation can be accomplished by a variety of methods. Switching regulators operate efficiently over a wide voltage range, but require external components (such as inductors and capacitors) to operate, and therefore occupy a relatively large board area. Inductorless charge pumps (or switched capacitor voltage converters) can also be used to achieve lower voltage conversion and generally operate more efficiently (depending on the conversion region), but have limited output current capability, poor transient performance, and require more external components than linear regulators.

The new generation of high-current, low-voltage fast digital ICs (such as FPGAs, DSPs, CPUs, GPUs, and ASICs) has placed more stringent requirements on the power supply for the core and I/O channels. In the past, these devices have been powered by efficient switching regulators because charge pumps could not provide sufficient output current and transient response. However, switching regulators have potential noise interference issues, sometimes their transient response is slow, and layout is restricted. Therefore, in these applications and other low-voltage systems, LDO regulators can be used instead. Thanks to recent product innovations and feature enhancements, LDO regulators have some more popular performance advantages.

Furthermore, when it comes to noise-sensitive analog/RF applications (common in test and measurement systems, where the measurement accuracy of the machine or equipment needs to be several orders of magnitude higher than the entity being measured), LDO regulators are often preferred over switching regulators. Low-noise LDO regulators power a variety of analog/RF designs, including frequency synthesizers (PLL/VCOs), RF mixers and modulators, high-resolution, high-speed data converters (ADCs), and precision sensors. However, the sensitivity of these applications has reached the test limit of traditional low-noise LDO regulators. For example, in many high-end VCOs, power supply noise directly affects the VCO output phase noise (jitter). In addition, to meet the overall system efficiency requirements, LDO regulators are often used to post-regulate the output of relatively noisy switching converters, so the high-frequency power supply ripple rejection (PSRR) performance of the LDO becomes critical. Furthermore, compared with industry-standard switching regulators, the noise level of LDO regulators can be reduced by two to three orders of magnitude, from the mV (rms) range to the μV (rms) range. The GM1203 series products launched by Common Mode Semiconductor, with the lowest voltage drop of only 150mV, are more efficient than switching regulators and are particularly suitable for low input voltage applications. They can effectively replace low voltage, low noise, and high current LDO products such as TPS7A84 and TPS74401. When the input voltage is high, products such as GM1204, GM12041, and GM12046 can increase the load current up to 3A and achieve an output voltage noise of less than 10μVRMS, effectively replacing high performance LDO products such as LT1764, LT1963, and ADM7172.

Common Mode Ultra-Low Noise LDO Product Series

2. LDO Design Challenges

Some integrated circuits, such as operational amplifiers, instrumentation amplifiers, and digital converters such as digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), are called bipolar because they require two input supplies: a positive supply and a negative supply. The positive rail is usually powered by a switching regulator, or better yet, a linear or low dropout regulator. The negative rail is traditionally powered by a negative switching regulator or inverter. However, inductor-based switching regulators can easily introduce noise into the system. With the advent of negative output regulators, it is more advantageous to use negative output LDO regulators for negative system rail powering, which can take advantage of all the features of LDO regulators (no inductor, low noise, higher PSRR, fast transient response, and multiple protections). Older, older LDO regulators have much worse PSRR and noise performance, and while it is still possible to use them to create these low-noise supplies, it requires a lot of extra components, board space, and a lot of design time to put the system together. These extra components can also negatively impact the power budget depending on their characteristics (such as parasitic resistance, etc.). Common Mode Semiconductor's 20V and 40V positive voltage LDO series such as GM1207 and GM1400, -10V and -40V negative voltage LDO series such as GM1206 and GM1402 can provide clean power for bipolar precision signal chain systems, saving circuit debugging optimization time and reducing circuit board space.

Customers using op amps, ADCs, or other signal chain components will also face another system performance challenge: these ICs have limited power supply rejection capabilities, and worse, the power supply rejection capabilities can be significantly reduced at high frequencies. In the past, this meant using additional filtering components on the board, which increased the size of the solution. In addition, if designers try to achieve higher accuracy, more trouble may arise if the regulator power supply noise is too high, which will cause undesirable changes in the measurement scenario.

Many industry-standard linear regulators perform low-dropout operation with a single voltage supply, but most cannot simultaneously achieve low output noise, very low voltage conversion, wide range input/output voltages, and extensive protection features. P-type tube LDO regulators can achieve voltage drop and operate on a single power supply, achieving low dropout, high input voltage, low noise, high PSRR, and very low voltage conversion. While N-type tube-based devices provide fast transient response, they require two bias supplies to power the device.

In order to obtain good overall efficiency, many high-performance analog and RF circuits use LDO regulators to post-regulate the output of the switching converter to provide power. This requires the LDO regulator to have high PSRR and low output voltage noise when the input-to-output voltage difference is small. LDO regulators with high PSRR can easily filter and suppress the output noise from the switching regulator without bulky filtering components. In addition, the low output voltage noise of the device over a wide bandwidth is very beneficial to today's power rails, where noise sensitivity is a key consideration. Low output voltage noise at high current is obviously a must-have specification requirement. Common Mode's GM1200, GM1205 and GM12051 can provide 120dB PSRR at low frequencies of 1kHz and more than 60dB PSRR at high frequencies of 1MHz, greatly suppressing the ripple of the previous switching regulator and providing clean power to the subsequent load. They can effectively replace high-performance low-noise LDOs such as LT3042, LT3045, LT3041 and ADM7151.