By: Sitthipong Angkititrakul and Dhananjay Singh, Intersil Corporation
Low-dropout regulators, commonly known as LDOs, are used extensively in a wide variety of electronic applications across many different industries. An LDO is generally perceived as a simple and inexpensive way to regulate and control an output voltage that is delivered from a higher input voltage supply. However, cost and simplicity are not the only reason, for their widespread use. In fact, today’s systems are getting more complex, noise sensitive and power hungry with every new design. 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 drivers.
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 given loads. Designers will need to determine whether the LDO is suitable for a specific load by closely analyzing the surrounding circuit conditions.
An LDO comprises three basic functional elements: a reference voltage, a pass element and an error amplifier. 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.
Key LDO Performance Parameters :
1) Dropout Voltage
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 typically implemented with PMOS or NMOS FETs, which can achieve a dropout voltage as low as 30mV to 500mV. Figure 1 shows the dropout voltage of the ISL80510 LDO, which uses a PMOS FET as the pass element.
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.
3) Line Regulation
Line regulation is the output voltage change for a given input voltage change, as defined in Equation 2:
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.
4) Power Supply Rejection Ratio (PSRR)
PSRR is an indication of the LDO’s ability to attenuate fluctuations in the output voltage caused by the input voltage, as expressed in Equation 3. While line regulation is only considered at DC, PSRR must be considered over a wide frequency range. Equation 3:
Considered a conventional closed-loop system, the small signal output voltage, VOUT, can be expressed as shown in Equation 4:
Where VIN is the small signal input voltage, Gvg is the open-loop transfer function from input to output voltage, kv is the output voltage sensing gain, GC is the compensator’s transfer function, Goc is the open-loop transfer function from the control signal to the output voltage, and kv×GCC×Goc is the closed-loop transfer function, T(s).
As we can see in Equations 3 and 4, it is clear that 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 open-loop 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 band-gap voltage reference. Equation 4 shows the relation of the reference voltage to the output voltage. Unfortunately, the closed-loop transfer function is not effective at rejecting the noise from the reference voltage to the output voltage. Hence, most low-noise LDOs need an additional filter to prevent noise from entering the closed-loop.
6) Transient Response
LDOs are commonly used in applications where point-of-load regulation is important, such as powering digital ICs, DSPs, FPGAs and lowpower CPUs. The load in such applications has multiple modes of operation, which require different supply currents. As a result, the LDO has to respond quickly to keep the supply voltage within the required limits. This makes the transient behavior of an LDO one of the critical performance parameters.
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.
7) Quiescent Current
The quiescent current (or ground current) of an LDO is the combination of the bias current and drive current of the pass element, and is normally kept as low as possible. Additionally, when PMOS or NMOS FETs are used as the pass element, the quiescent current is relatively unaffected by the load current. Since the quiescent current doesn’t pass through to the output, it influences the LDO’s efficiency, which can be calculated as follows in Equation 5:
The power dissipation inside the LDO is defined by: VIN x (Iq x IOUT) – VOUT x IOUT. To optimize the LDO’s efficiency, both quiescent current and the difference between the input and output voltages must be minimized. The difference between the input and output voltages have a direct impact on efficiency and power dissipation, so the lowest dropout voltage is generally preferred.
Even though an LDO cannot deliver high efficiency conversion compared to a switching mode power supply (SMPS), it is still a necessary voltage regulator for many modern applications. In noise sensitive applications, it is very challenging 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 suited to applications that require an output voltage regulated to slightly below the input voltage. While buck and boost converters have limitations on the maximum/minimum duty cycle, their output voltage will lose regulation with an input voltage that is close to the output voltage.
Though simple in concept and implementation, widely used LDOs perform a vital function in system power design. There are many factors that need to be considered to optimize a design, particularly at higher current levels. For mid to high current applications, Intersil’s ISL80510/05 provide balanced performance across all key LDO performance parameters: low dropout, transient performance, voltage accuracy and a near flat PSRR response across a wide range of frequencies.