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.