When is it best to use LDO regulators?  

LDOs can offer the low noise and PSRR performance to be used in wide voltage range applications such as in test and measurement systems, write Amit Patel and Steve Knoth

Linear Regulators are integrated circuits that step a voltage down from a higher voltage to a lower voltage without the need of an inductor. The low dropout linear regulator (LDO) is a special type of linear regulator in which the dropout voltage – the differential between the input-to-output voltages needed to maintain regulation – is typically below 400mV.

Early linear regulator designs offered dropouts on the order of ~1.3V, meaning for a 5V input, the maximum achievable output was only ~3.7V for the device to stay in regulation. More recently, “low” dropout is considered to be 500mV or less.

Nevertheless, in these days of more sophisticated design techniques and wafer fabrication processes, “low” dropout is typically <100-300mV or so.

Furthermore, although the LDO is rarely the most expensive system component in any given system, it is often one of the most valuable components from a cost/benefit basis.

Figure 1. LT3045 Typical Application Schematic & Features

Figure 1. LT3045 Typical Application Schematic & Features

One of the LDO’s tasks is to protect expensive downstream loads from harsh environmental conditions such as voltage transients, power supply noise, reverse voltage, current surges, etc. In short, its design must be robust and also contain all of the protection features needed to “absorb the punishment” from its environment while protecting the load.

Many low-cost LDO linear regulators do not have the necessary protection features and thus fail, often causing damage not only to the regulator itself, but also the downstream load.

LDOs vs other regulators 

Low voltage step-down conversion and regulation can be achieved via a variety of methods.

Switching regulators operate with high efficiency over a wide range of voltages but require external components such as inductors and capacitors for operation, thus taking up a relatively larger board space.

Inductorless charge pumps (or switched capacitor voltage converters) can also be used to achieve lower voltage conversion but are limited in output current capability, suffer from poor transient performance, and require more external components versus a linear regulator.

When it comes to powering noise-sensitive analogue/RF applications (such as commonly found in test and measurement systems, where the measurement accuracy of the machine or equipment needs to be orders of magnitude better than the entity being measured), LDOs are generally preferred over their switching counterparts.

Low noise LDO regulators power a wide range of analogue/RF designs, including frequency synthesisers (PLLs/VCOs), RF mixers and modulators, high speed and high resolution data converters (ADCs and DACs) and precision sensors.

Nevertheless, these applications have reached sensitivities that are testing the limits of conventional low noise LDOs. For instance, in many high-end VCOs, power supply noise directly affects the VCO output phase noise (jitter).

Moreover, to meet overall system efficiency requirements, the LDO usually post-regulates the output of a relatively noisy switching converter, so the high frequency power supply ripple rejection (PSRR) performance of the LDO becomes paramount.

LDO design challenges

Plenty of industry standard linear regulators perform the low dropout operation with a single voltage supply, yet most cannot achieve the combination of very low voltage conversion with low output noise, wide ranging input/output voltages and extensive protection features.

PMOS LDOs achieve the dropout and run on a single supply but are limited at low input voltages by the pass transistor’s Vgs characteristics as well as lack the many protection features from high performance regulators. NMOS-based devices offer fast transient response but require two supplies to bias the device.

NPN regulators offer wide input and output voltage range but either require two supply voltages or have higher dropout.

By contrast, with the proper design architecture, a PNP regulator can achieve low dropout, high input voltage, low noise, high PSRR and very low voltage conversion with bulletproof protection and all from a single supply rail.

For best overall efficiency, many high performance analog and RF circuits are powered from LDOs post-regulating the output of a switching converter. This requires high power supply ripple rejection (PSRR) and low output voltage noise at low input-to-output differentials across the LDO.

LDOs used in parallel

LDOs used in parallel

An LDO with high PSRR easily filters and rejects noise from the switcher’s output without requiring bulky filtering components. Further, a device with low output voltage noise across a wide bandwidth is beneficial for today’s modern rails where noise-sensitivity is a key consideration. Low output voltage noise at high currents is clearly a necessary specification.

To address this, Linear Technology introduced its LT304x family of LDO regulators. The LT3045 is a higher-output current version of the previously released 200mA LT3042 LDO. The LT3045 has spot noise of only 2nV/√Hz at 10kHz and 0.8µVRMS integrated output noise across a 10Hz to 100kHz bandwidth.

Low frequency PSRR exceeds 90dB out to 10kHz and high frequency PSRR exceeds 70dB out to 2.5MHz, thereby quieting noisy or high ripple input supplies.

The firm employs a proprietary LDO architecture – a precision current source reference followed by a unity gain buffer, resulting in virtually constant bandwidth, noise, PSRR and load regulation performance, independent of output voltage.

The devices can also be used in parallel to further decrease noise, increase output current and spread heat across the circuit board.

The LT3045 delivers up to 500mA output current with a 260mV dropout voltage at full load, across a wide 1.8V to 20V input voltage range. Output voltage range is 0V to 15V and output voltage tolerance is highly accurate at ±2% over line, load and temperature.

A future high temperature H-grade version will be rated from –40°C to 150°C.

 

 

Amit P. Patel Senior Design Engineer, Power Products & Steve Knoth Senior Product Marketing Engineer at Linear Technology

 


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