Tag Archives: Low Dropout Regulator

Why Low Dropout Regulators?

In this era of high-efficiency switching power supplies and voltage regulators, low dropout (LDO) regulators seem almost out of place. Contrary to popular belief, low dropout regulators are small components, simple to use, and cost-effective for obtaining an output of regulated voltage from an input of higher voltage.

For system designers, low dropout regulators offer a simple method of obtaining a voltage from a source that is very close to the output voltage. This is one major reason designers use LDO regulators widely. The second reason is LDO regulators are analog devices, and unlike switching regulators, introduce very low noise into the system.

Small LDO regulator devices such as those from Diodes Incorporated offer a variety of features such as high-power supply rejection ratio, ultra-low quiescent current, wide input voltage handling capability, physically small footprint, and high output current supply capability.

Keeping in line with other SMT components, manufacturers are making LDO regulators in smaller form factors, enabling designers to use PCB space more effectively. Designers can make better use of the newer families of LDO regulators in highly dense PCBs as these components are of very small size, and occupy the minimum space, while they offer the same high-quality performance.

Not all power supply sources offer clean and regulated outputs. LDO regulators help filter out most of the noise from unregulated power sources with their high-power supply rejection ratio specifications. By rejecting the noise from the power source, LDO regulators provide noiseless and spike-free DC power to ensure the system operates reliably.

Many systems do not require continuous power. In remote areas, where it is difficult to deliver power, engineers rely on batteries to power their equipment. LDO regulators with ultra-low quiescent current consumption are a boon, as they consume the minimum amount of power when the system is idle, resulting in a significant increase in the life of the battery.

LDO regulators can handle a wide range of input voltages, in some cases, up to as high as 40 VDC. In multi-voltage systems, which are now common-place, such LDO regulators are very cost-effective, and they make the design more robust and reliable.

Sensors and related electronics work better with clean power supplies. Noise from switching regulators can limit the sensitivity of sensors drastically, resulting in reduced coverage or misleading measurements. LDO regulators supplying clean and efficient power with high current output allow using components for sensitive measurements, without the introduction of ripple and noise. Even with their high current output, LDO regulators work with voltage differentials as low as 350 mVDC.

Automotive applications require high-temperature reliability, and LDO regulators are available that cover a wide temperature range of -40 ºC to +125 ºC. This is a necessary feature in an automobile, as many applications must work concurrently to keep the vehicle operational.

The new family of LDO regulators are ideal for portable and small consumer devices, such as smartwatches, smartphones, wearables, wireless earphones, smart homes, smart offices, and different sensor applications. The industry uses these LDO regulators for other applications such as healthcare devices, smart meters, and other devices powered by batteries.

What is an LDO and How Does it Work?

When you need a voltage regulator for your circuit and do not have much of a voltage head room, the trick is to use an LDO or a low-dropout regulator. Normal regulators need voltage headroom of roughly around 3V to allow good regulation, but LDOs can do with a lot less – of the order of a few 100 millivolts. However, there are other considerations as well.

To regulate and control an output voltage, it is necessary to source it from a higher input voltage supply. For normal regulators, the voltage headroom or the difference between the output regulated voltage and the minimum input unregulated voltage must be more than 3V. For example, if you need a regulated voltage of 5V, it must be sourced from a minimum input voltage of 8V. That ensures the regulated output voltage never dips below 5V. With circuits getting more complex and noise sensitive, new designs must deal with higher currents and lower voltages. Hence, headroom voltages of 3V or more may not be available in all cases, and it is necessary to use LDOs.

Although manufacturers offer datasheet specifications for basic parameters of regulators, they cannot list all parameters for every possible circuit conditions. Therefore, to use the LDO in the best possible manner, designers must necessarily understand the key performance parameters of the LDO and their impact on given loads. A close analysis of the surrounding circuit conditions helps to determine the suitability of a specific LDO.

In applications, LDOs primarily isolate a sensitive load from a noisy power source. The pass transistor or the MOSFET regulating and maintaining the output voltage accurately is always on and dissipates continuous power. This is different from switching regulators, which work as on-off switches. That makes LDOs less efficient and designers must handle the thermal issues related. System power requirements primarily drive the use of LDOs as voltage regulators. Since they are linear devices, they are also used for noise reduction and for fixing problems related to EMI and PCB routing.

As the power dissipation of an LDO is primarily governed by the current through it, LDOs are an obvious choice for very low current loads, bringing with their use simplicity, cost economics, and ease of use. For load currents of more than 500mA, designers must consider other parameters also, such as the dropout voltage, load regulation, and transient performance.

LDOs comprise three basic functional elements – a pass element, a reference voltage, and an error amplifier. Under normal operation, the pass element behaves as a voltage controlled current source. A compensated control signal from the error amplifier drives the pass element. The error amplifier senses the output voltage and compares it with the reference voltage. LDO regulator designs use four different kinds of pass elements – PNP transistor based regulators, NPN transistor based regulators, P-channel MOSFET-based regulators and N-channel MOSFET-based regulators.

While using a specific LDO in their circuits, designers need to consider the performance of the LDO with respect to its dropout voltage, load regulation, line regulation, and the power supply rejection ratio or PSRR.