A low dropout (LDO) voltage regulator is often used in complex SoC systems to provide a stable and highly accurate low-noise output voltage. To obtain acceptable transient response, power supply noise rejection (PSR), and stability of the LDO, LDO voltage regulators normally require an external capacitor. Traditional LDO voltage regulators use off-chip output capacitors in the range of a few microfarads. The external capacitor, however, has four major concerns that must be considered when selecting one for applications where they are required.
1. External, discrete capacitors have non-idealities that can be critical.
There is a parasitic Effective Series Inductance (ESL), and a parasitic Effective Series Resistance (ESR). Both ESL and ESR decrease the capacitor’s performance at high frequency.
The reactance of an ideal capacitor decreases as frequency increases. At very high frequencies, the reactance of the capacitor can become very small. When conducting a loop stability analysis of a traditional regulator, the dynamic response can be degraded if the impedance is too small. The total impedance, however, includes both the reactance of the capacitance and the ESR. Therefore, a minimum ESR is usually required so that the total impedance doesn’t go to zero at high frequencies.
Unfortunately, a large ESR can also create a zero that extends the unity gain frequency in the closed loop, yielding a larger unity gain frequency but severely deteriorating the phase margin, making the system unstable. While the minimum ESR is a concern, the maximum ESR is the more critical value to ensure stable operation.
To satisfy stability, LDO voltage regulator manufacturers impose restrictions on the ESR of the output capacitance, requiring it to be bounded within a specific range of minimum and maximum values.
Furthermore, these stability analyses should consider that the capacitor non-idealities are sensitive to mechanical effects, and vary with bias and temperature. Capacitance variation over temperature can be ±20% or more. ESL and ESR are usually not specified versus temperature or given a target tolerance; however, they can vary widely, causing their impact over temperature on loop dynamics to be extreme. The lack of specification adds risk to the LDO voltage regulator system operation because it becomes difficult for the user to guarantee that the LDO voltage regulator will work over all operating conditions. You can see that the capacitor non-idealities must be carefully considered in conventional LDO voltage regulator designs. This makes selection of a capacitor for use in the application a difficult challenge.
2. An incorrect selection of the type of external load capacitor might lead to stability problems, excessive power dissipation, and noise, which will all reduce the battery and product life.
LDO voltage regulator applications use three main types of capacitors: ceramic, tantalum, and in some larger power applications, aluminum electrolytic. Different capacitor technologies have different performance capabilities and costs. Ceramic capacitors are the most common capacitors used in LDO voltage regulator applications for their low cost and reduced footprint size. Ceramic capacitors typically have much lower ESL and ESR (in the range of several 10s of milliohms, compared to tantalum capacitors which typically have an ESR in the range of 100s of milliohms) and therefore work better at high frequencies, but they are also usually available only in smaller capacitance values. For this reason, it is common, to use a large-value electrolytic or tantalum capacitor for bulk capacitance, and to add a small ceramic capacitor in parallel to allow high-frequency bypassing of the bulk capacitor, thus extending the useful frequency range of the external capacitance. Where a tantalum capacitor is used, its ESR is more sensitive to temperature variations, forcing designers to anticipate how to compensate for this perturbation.
3. Sometimes you won’t know what load capacitance you need until AFTER selecting the LDO.
In practice, most LDO voltage regulators can only guarantee stability for a specific output capacitance range. However, there are often real, practical situations where the load capacitance of the device to be powered by the LDO voltage regulator is not known in advance. In such cases, LDO voltage regulators are required to properly operate within a wide range of capacitance loads, from near-zero to several microfarads. Finding an LDO that remains stable across a range of capacitance loads is difficult.
4. Capacitors have board real estate and economic cost implications.
External capacitors are usually bulky, occupy valuable board space, decrease long-term reliability, and consume valuable pins in SoC devices.
TO AVOID THESE CONCERNS, CONSIDER A CAPLESS LDO
For multiple reasons explained above, selecting output capacitors for reliable operation can be very difficult. An ideal LDO solution would not require an external capacitor. Therefore, you should explore the use of capless LDO voltage regulators and their advantages.
To learn more about the advantages of using capless LDOs, download our full white paper: