The low-dropout regulator (LDO) has long been the choice for buck voltage conversion not only where cost is an issue but where noise performance is critical.
The brainchild of Linear Technology co-founder Robert Dobkin, conceived when he worked at National Semiconductor, the core architecture of the regulator is very simple but effective. Dobkin took a fixed-ratio voltage regulator and adapted it so that its output could be adjusted using a voltage divider on the output.¹
In the classic linear regulator, a transistor acts as half of a potential divider. Its output voltage is to control a feedback circuit that has control over the transistor’s gate in the case of a MOSFET, which is normally the case for an LDO regulator. The constant control via feedback over gate voltage provides a stable output voltage at a level set by the reference circuitry. Because of the use of a voltage divider structure, the linear regulator can produce only a voltage that is lower than that of the input. Older regulator circuits could experience a drop of 2 V or more. LDOs were devised to provide easier control over the output voltage and to constrain this dropout voltage to less than 2 V.
The main change in an LDO is from an emitter follower topology to one based on an open-collector or open-drain topology so that the transistor can be operated at saturation, which minimizes the dropout voltage.
One reason why linear regulators are not used in all cases for buck regulation is that their efficiency is poor compared to that of a switched regulator. Even when the regulator is quiescent, it passes a small amount of current that is dissipated as wasted heat. The LDO attacks the efficiency problem on one front by minimizing the difference between input and output voltage, which helps in situations where the input voltage is reasonably constant. For example, this is often the case when a linear regulator follows a switching regulator and is present primarily to provide a stable voltage to the target circuit. However, designers must account for the quiescent current that remains.
Switching regulators have gradually displaced LDO regulators thanks to their higher efficiency. Over time, switching regulators have improved their noise performance, making it possible to use them with more sensitive analog circuitry. But for even lower-noise power, the LDO regulator still provides a major advantage and is continuing to improve thanks to investments made by manufacturers in updated designs that attack the sources of noise in its core circuitry. These improvements suit the LDO regulator to applications such as instrumentation and mobile wireless systems where a stable power input to circuitry, such as phase-locked loops, is important. As with any other component, an LDO regulator is a contributor to the noise injected into to the power supply of its associated circuit. The regulator can pass on noise from the external power supply and generates its own low-level noise. In most designs, an error amplifier used to control the gate voltage of the main pass transistor helps to keep the power supply rejection ratio (PSRR) high to ensure that only a limited amount of noise is allowed to proceed from the voltage input to the LDO regulator’s output.
However, if the transistor is driven into saturation, the PSRR drops significantly. So, if high PSRR is important to a design, the LDO needs to be driven in such a way that it does not move entirely into saturation.
The intrinsic noise generated by an LDO regulator has a number of sources. One is thermal noise, which results from the agitation of carriers in a conductor or semiconductor that is at any temperature above 0 K. It increases with temperature but is random – so it does not vary with frequency.
1/f noise is a characteristic of the surface defects found in semiconductors. It is proportional to the bias current of a device and, as its name suggests, is inversely proportional to frequency although it generally levels off beyond several kilohertz.
Shot noise is the result of quantum fluctuations in current across a potential barrier, such as the PN junctions found in a transistor or diode. As the carriers cross the barrier, a certain amount of shot noise is generated. It does not vary with frequency.
Popcorn noise used to plague semiconductor devices – as it was the result of ionic contamination that has now largely been removed by improvements in manufacturing.
In general, an LDO’s main source of intrinsic noise is the internal reference voltage element. Today’s LDOs operate with bias currents on the order of tens of nanoamps to help achieve low quiescent currents. This implies the use of large bias resistors with values of 1 GΩ or more. In a wideband device, the resulting noise is on the order of hundreds of microvolts. However, by reducing the bandwidth dramatically, it is possible to cut this noise to the single microvolt level.
With a low-noise reference, the error amplifier tends to become the leading contributor to total output noise. But its noise contribution can be limited by improvements in circuit design that limit the AC gain.
Generally, measurements of noise produced by an LDO are over the 10 Hz to 100 kHz, expressed as root-mean-square (rms) volts or noise spectral density in V per root-Hertz, because the sum of noise components from the LDO are typically at their highest in this range. A combination of filtering and advanced error amplifier design techniques contribute to reductions in noise over this range in specialist low-noise LDOs.
Figure 1: The basic structure of a low-dropout regulator and the noise sources associated with it (Source: Analog Devices).
An example of a low-noise LDO is the Linear Technology LT1763. This is a micropower LDO able to supply 500 mA of output current with a dropout voltage of 300 mV designed for use in battery-powered systems thanks to a low quiescent current of 30 µA that does not rise with dropout. A quiescent current dependence on dropout is common in many regulator designs.
By adding an external 0.01 µF capacitor to help provide a filter, the noise produced by the LT1763 drops to 20 µVrms over the 10 Hz to 100 kHz range. The LT1763 includes internal protection circuitry that guards against reverse battery connection, overcurrent and overtemperature conditions. The LDOs are available in fixed and variable output-voltage forms. The MAX8887 LDO regulator made by Maxim Integrated Products operates from a 2.5 V to 5.5 V input and can deliver current levels up to 500 mA. Designed primarily for battery-powered equipment, the LDO regulator employs a p-channel MOS pass transistor to avoid the need for a base drive, which helps bring quiescent current down to less than 60 µA. The device also sports a shutdown mode that reduces current draw to 0.1 µA. The addition of a 0.01 µF capacitor creates a low-pass filter for noise reduction, bringing the output noise down to 42 µVrms.
With a dropout of just 100 mV at a load of 200 mA, the MAX8887 has both thermal overload and current-limit protection. The PSRR is 60 dB below 1 kHz, dropping to around 20 dB above 100 kHz. Using bipolar rather than MOSFET technology, the Texas Instruments’ TPS7A30xx series of LDO regulators can source a load current of up to 200 mA and reduce the output noise to just 15.1 µVrms over the bandwidth 10 Hz to 100 kHz. The regulator is designed for high-accuracy, high-precision instrumentation that demands high-quality voltage rails, using a 0.01 µF to provide noise filtering.
The dropout voltage of the TPS7A30xx is 216 mV with a 100 mA load and includes current-limit and thermal shutdown protection. The regulator offers a PSRR of more than 55 dB over the 10 Hz to 700 kHz range and has a CMOS compatible shutdown control pin.
The Analog Devices ADP223 is a dual-output LDO regulator with a PSRR of more than 60 dB for frequencies as high as 100 kHz. It offers a noise output of 27 µVrms with an output voltage of 1.2 V and 50 µVrms at 2.8 V. The dropout voltage of the device is 170 mV at a load of 300 mA. The ADP223 offers both overcurrent and thermal protection.
The dropout voltage of the MIC5205 fabricated by Micrel can fall to as low as 17 mV at light loads and is maintained at 165 mV at a load of 150 mA. Using a CMOS-compatible shutdown pin, the power consumption of the device drops to almost zero. The device’s noise spectral density is typically 1 µV/root-Hz from 10 Hz to 100 kHz and its PSRR ranges from 20 dB to 100 dB depending on frequency and current output.
The SC560 from Semtech is a dual-output LDO regulator intended for battery-powered applications that can supply a maximum output current of 300 mA and which provides a dropout of 200 mV at 200 mA. Offering a PSRR of up to 65 dB at 1 kHz, the output noise on each output is less than 50 µVrms when used in combination with a filter capacitor.
Versions of the SC560 allow the two outputs to be controlled separately or to hold a processor in reset if the main output is not in regulation. Quiescent current is 100 µA with both LDO stages enabled, falling to 100 nA in shutdown mode. The device has circuitry to guard against over-current and undervoltage conditions as well as thermal protection.
ams has used a p-channel MOSFET in its AS1358 to reduce quiescent current to 40 µA, delivering a dropout of 140 mV for a load current of 300 mA. With a PSRR of better than 80 dB up to 10 kHz and 92 dB at 1 kHz, the regulator is optimized for low-noise applications. It generates just 9 µVrms of output voltage noise over the 100 Hz to 100 kHz range, although its noise spectral density increases from 100 Hz to 10 Hz.
The variety of LDO regulators available for low-noise applications is an indicator of the continuing health of this class of device and the continued demand for improved noise performance in a variety of low-power, mobile and sensitive analog applications.
Summary
This article has provided an overview of LDO regulator design techniques and their role in reducing system noise.
