LDOs are one of the oldest and most commonly used devices for voltage regulation; however, many of their key performance parameters are not so well understood or at least not utilized to their full potential. While cost is a very important factor, the use of LDOs is primarily driven by the system’s power requirement and acceptable noise level of the load being powered. LDOs are also used for noise reduction, and to fix problems caused by electromagnetic interference (EMI) and PCB routing.
For very low current loads, an LDO’s power dissipation is very minuscule, so they become an obvious choice for their simplicity, cost and ease of use. However, for high current loads greater than 500mA, other factors become more important and sometimes even critical. In these types of applications, it is important for system designers to look at performance parameters that grow in importance at higher current levels, such as the dropout voltage, load regulation and transient performance.
Since LDOs are a type of linear regulator, they are often compared with traditional linear regulators, especially in terms of cost. It is important to understand that the pass element is the core of an LDO, and this core and its surrounding circuits dictate the LDO’s performance.
Inside the LDO
An LDO comprises three basic functional elements: a reference voltage, a pass element and an error amplifier, as shown in Figure 1. During normal operation, the pass element behaves as a voltage controller current source. The pass element is driven by a compensated control signal from the error amplifier, which senses the output voltage and compares it with the reference voltage. All of these function blocks affect the LDO’s performance. LDO manufacturers’ datasheets always include specifications that indicate the performance of these functional elements.
Figure 1: Block diagram of the LDO
As you can see in Figure 2, four different kinds of pass