A Low Dropout regulator (LDO) is generally thought of as a simple and inexpensive way to regulate and control an output voltage which is produced from a higher input-voltage supply. However, cost and simplicity are not the only reasons for their widespread use. In fact, today’s systems are getting more complex, noise-sensitive and power-hungry. The widespread use of switching power supplies at all power levels means that designers must spend more time avoiding noise-coupling and interference, while improving system efficiency, so cost and simplicity cannot be the only factors affecting power component choices.
For most applications, a datasheet’s specifications of basic parameters are clear and easy to understand. Unfortunately, datasheets do not list the parameters for every possible circuit condition. Therefore, to make the best use of an LDO, it is necessary to understand the key performance parameters and their impact on any given load. Designers will need to determine whether the LDO is suitable for a specific load by closely analysing the prevailing circuit conditions.
An LDO is comprised of three basic functional elements: a reference voltage, a pass element and an error amplifier. During normal operation, the pass element behaves as a voltage-controlled 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. Each of these functional blocks affects the LDO’s performance. LDO manufacturers’ datasheets always include specifications that indicate the performance of these functional elements.
Dropout voltage is defined as the difference between the input and output voltages at the point when a further decrease in input voltage causes output voltage regulation to fail. In the dropout condition, the pass element operates in the linear region and behaves like a resistor. For the modern LDO, the pass element is commonly implemented with PMOS or NMOS FETs, which can typically achieve a dropout voltage ranging between 30mV and 500mV. Figure 1 shows the dropout voltage of the ISL80510 LDO, which uses a PMOS FET as the pass element.
Load regulation is defined as the output voltage change for a given load change. This is typically from no load to full load:
Load regulation indicates the performance of the pass element and the closed-loop DC gain of the regulator. The higher the closed-loop DC gain, the better the load regulation.
Line regulation is the output voltage change for a given input voltage change, as shown in the following equation:
Since line regulation is also dependent on the performance of the pass element and closed-loop DC gain, dropout operation is often not included when considering line regulation. Hence, the minimum input voltage for line regulation must be higher than the dropout voltage.
Power-supply rejection ratio
The Power-Supply Rejection Ratio (PSRR) is an indication of the LDO’s ability to attenuate fluctuations in the output voltage caused by the input voltage. While line regulation is only considered at DC, PSRR must be considered over a wide frequency range:
The PSRR consists of the closed-loop gain, T(s), and the inverse of the open-loop transfer function from input to output voltage, 1/Gvg, as shown in Figure 2. While the closed-loop transfer function dominates at lower frequencies, the openloop transfer function from input to output voltage dominates at higher frequencies.
This parameter normally refers to the noise on the output voltage generated by the LDO itself, which is an inherent characteristic of the bandgap voltage reference. Most low-noise LDOs need an additional filter to prevent noise from entering the closed loop.
As in all closed-loop systems, the transient response mainly depends on the bandwidth of the closed-loop transfer function. To achieve the best transient response, the closed-loop bandwidth has to be as high as possible while ensuring sufficient phase margin to maintain stability. Although an LDO’s conversion efficiency is lower than that of a Switch-Mode Power Supply (SMPS), in many applications the LDO is to be preferred. In noisesensitive applications, it is difficult for an SMPS to achieve the necessary output ripple to meet a tight noise specification. Consequently, it is not uncommon for an LDO to be added as an active filter to the output of an SMPS. This LDO must have high PSRR at the SMPS’ switching frequency.
LDOs are particularly well suited to applications that require an output voltage regulated to slightly below the input voltage.
Favourable parameters of ISL80510
For mid- to high-current applications, Intersil’s ISL80510/05 provides balanced performance across all the important LDO performance parameters: low dropout, transient performance, voltage accuracy and a near flat PSRR response across a wide range of frequencies, as shown in Figure 3.