One of the major parameters that affects the low dropout regulator (LDO) is its dc performance and accuracy, and specifying this performance is one of the first design requirements. Whether the load needs high accuracy on the output voltage, or the dropout voltage of the overall circuit is very low, can be deciding factors in which regulator is chosen for the application.
DC accuracy refers to the total deviation in output voltage that is acceptable for the application. Many processors require fairly tight dc regulation, at around 5 percent. For mobile applications in particular, the accuracy of the output voltage is critical; this is more for performance than for maintaining the required tolerance specified by the IC manufacturers.
When optimizing a mobile system for best performance over the operating life of a typical lithium-ion battery, a system designer may find that the system works best at full battery charge (4.2V) with the RF section operating at 3.0V supply voltages. However, the full operating range of a lithium-ion battery is as low as 3.0V, meaning that the RF must operate at a lower voltage in order to maintain regulation and provide reasonable performance. Most RF sections of mobile phones today regulate at 2.85V and must maintain an overall accuracy of 3 to 5 percent.
The LDO that regulates voltage should maintain tight initial accuracy of 1 percent, with a worst-case temperature variation of 2 percent. This allows for design margin and prevents the output voltage from exceeding the maximum tolerance limits for the circuit design.
Line regulation also impacts the overall accuracy of dc performance. For example, in a mobile application, if the input voltage changes from 4.2V (maximum charge) to 3.2V (minimum charge), the output voltage will change because the internal bias settings of the regulator will change. Most regulators have excellent line regulation because the control loops are designed with sufficient gain to correct the output voltage for these variations on the input supply. However, some LDO regulators with low loop gain may exhibit poor line regulation.
Load regulation is also a consideration. If the regulator is providing 100 μA and the load changes to 150 mA, the output voltage will show a dc change. This is due to changes in biasing that are related to the higher current level. Once again, the amount of change in the output voltage is directly proportional to the amount of open-loop gain in the error amplifier. Achieving the correct balance between loop gain and stability is critical.
Micrel's MIC5305 is one regulator with excellent regulation characteristics and extremely high levels of accuracy. Figure 1 shows the breakdown of its total accuracy into the sum of three parts: initial accuracy and accuracy over temperature, with load regulation and line regulation.
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Figure 1: Contributors to LDO inaccuracy.
Most regulators have load regulation such that when the load increases, the output voltage goes down. The load-regulation term will almost exclusively pull the output voltage down from whatever nominal voltage it is regulating to. However, most manufacturers specify in both directions, so the example shows both positive and negative load regulation.
Dropout voltage is defined as the minimum voltage between output and input voltage required to keep the LDO in proper regulation. With the majority of low dropout regulators, the pass transistor is a PNP bipolar transistor or a P-channel MOSFET. The PNP pass transistors have a dropout voltage defined by the Vce (collector-emitter voltage) saturation characteristics.
Most PNP LDOs will saturate at their maximum designed output currents with a dropout voltage (saturation voltage) between 200 and 300 mV. The P-channel MOSFET-equivalent LDOs have a dropout voltage dependent upon the Rds(on) of the P-channel device. To fully enhance the PMOS device, the gate has to be driven as close as possible to ground. The dropout voltage of the PMOS-based LDO regulator will vary directly with the input voltage.
The process technology for PMOS transistors is much smaller than the closest PNP equivalent, and very low Rds(on) values are achievable in very small areas for PMOS regulators. A very low dropout voltage can be achieved within a reasonable silicon area with PMOS; in contrast, the PNP device would have to be significantly larger.
The benefit of lower dropout voltage in a mobile application is that a much lower input voltage is required to maintain the same level of performance on the output. In order to maintain good performance, the regulator needs some additional headroom on top of the dropout voltage to keep the pass transistor in a region where it can reject input noise and respond to input and output transients.
Ground current is the measure of current required by the LDO to function. Ground current is the only parameter directly affected by the choice of pass transistor. The main characteristic in choosing a PNP transistor for the output device of a linear regulator is its gain characteristic, or beta. The beta factor for a PNP pass transistor determines the amount of current required to drive the base of the PNP while supplying load current.
If beta is around 100, then base current (or ground current in an LDO) for a 100 mA load will be 1 mA. The overall ground current of the LDO will be the sum of the bias current plus the base current. In a PMOS-based LDO, the pass transistor is voltage controlled and doesn't have the same current requirements the PNP. The current at no load condition could be equal to the current at full load, because there is no requirement to drive the gate of the PNP with current, only voltage. Therefore, PMOS-based LDOs have lower ground current at full load, compared to the PNP-based LDOs.
The bias current is usually proportional to performance. An LDO with very low bias current will usually have lower loop gain and, therefore, worse accuracy. This type of LDO will typically also have worse ac performance for parameters such as power-supply ripple rejection, transient response and self noise.
About the author
John McGinty is with Micrel, Inc, San Jose, CA. Mr. McGinty has more than eight years of high-tech experience in the semiconductor industry. He began his career at the Company as an Applications Engineer and has also worked for Micrel in the capacity as a Product Marketing Engineer and a Field Applications Engineer. Currently, Mr. McGinty has responsibility for designing in Micrel's analog products throughout Europe as well as responsibility for new product definitions.
He is the author of numerous articles in various high tech publications worldwide, and holds a Bachelor of Science Degree in Engineering Physics from John Carroll University, Ohio. His first high-tech patent is pending.