What is an LDO?
Power management is an important design consideration for many products, especially for devices that rely on a battery for their power.
A voltage regulator is an electronic component that automatically provides a regulated and controlled output voltage to the various circuits inside an electronic device. The two main categories of voltage regulators are low dropout (LDO) linear voltage regulators and switching regulators. There is certainly a significant usage of switch-mode power supplies for the benefits they provide in power density and overall efficiency; however, LDOs are also used in a wide variety of applications today, and are an important component for you to understand and consider. For example, LDOs are used due to their low-noise output voltage, small size, low shutdown current, and low cost when compared to switching regulators. With so many possible LDO applications and a large number of parameters that can be optimized for each LDO, it’s not easy to determine which LDO is best suited for a particular application.
9 Basic AC Parameters
To select an LDO for your system, it’s important to understand the basic AC parameters required. The LDO you choose must operate within a range of required AC parameters to provide the required functionality for a particular application. Following are nine typical AC parameters for you to consider.
DC Line Regulation
DC line regulation is defined as the resulting change in the output voltage for a given change in the input voltage. This measurement is made under conditions of low power dissipation.
DC Load Regulation
The DC load regulation is the change in output voltage for a static change in output (load) current.
Output Noise Voltage
This parameter is the rms output noise voltage generated only by the LDO (including the voltage reference-noise as it is assumed that the LDO has an integrated reference) over a given frequency range (typically 10 Hz to 100 kHz) under a constant output current and a clean input voltage. Most of the noise comes from flicker noise in the internal LDO voltage reference.
Power Supply Rejection Ration (PSRR)
The PSRR is a measure of how well the LDO rejects electrical noise at the power-supply input by measuring the change in output noise voltage.
Quiescent current, also called ground current, is the current used to operate the LDO, and is not delivered to the load. It is measured when the LDO is enabled and the output/load current is zero (0). A small quiescent current is needed to maximize LDO output efficiency, reduce heat, and extend battery life in battery-operated applications.
Shutdown current is leakage current through the LDO, when the LDO is disabled or powered down.
Start-up or Turn-on Time
Start-up time is the time between the rising edge of the enable signal and the output voltage reaching 90% of its nominal value.
Line Transient Regulation
The line transient regulation parameter provides a measure of the ability of the LDO to maintain a constant output voltage with a transient step at the input.
Load Transient Regulation
The load transient regulation is defined as a change in the output voltage resulting from a given dynamic (step) change in the output current. Load transient regulation includes overshoot (difference between the maximum VOUT and the initial VOUT during a load transient regulation test when the output current changes to a lower value), as well as undershoot (difference between the minimum VOUT and the initial VOUT during a load transient regulation test when the output current changes to a larger value).
Low dropout linear voltage regulators (LDOs) are a key component used in multiple applications. In addition to this basic understanding of the typical AC parameters you should consider, you should also explore the different DC parameters and features required of your LDO. For a comprehensive look at all of the DC, AC, and other optional parameters and features to consider, download our full LDO guide: