The Basics of LDOs and How to Apply Them to Extend Battery Life in Portables and Wearables

Modern electronic devices are getting smaller and more portable. Smartwatches, fitness trackers, security systems, and Internet of Things (IoT) devices are increasingly battery-powered. As such, they require high-efficiency power regulators that squeeze every milliwatt of power from each charge to keep the device working longer. They must also operate with a minimal temperature rise. Traditional linear regulators and switched-mode power regulators cannot easily reach the efficiencies required for these portable devices. Additionally, switched-mode power regulators also suffer from noise and transient voltages.

The low-dropout voltage regulator (LDO), the most recent addition to the line of linear and switching regulators, capitalizes on operating with very low voltage drops across the regulator to improve efficiency and lower the thermal dissipation. Variations of LDOs are well-suited to low-to-medium power applications for which they can come in packages as small as 3 × 3 x 0.6 millimeters (mm). Versions with fixed or adjustable output voltages are available, as well as some versions with on-off control via an output enable line.

This article examines the basics of low-dropout regulators and their key characteristics relative to traditional linear and switched-mode power regulators. It then introduces real-world LDO devices from Diodes Incorporated and shows how they’re applied.

What is an LDO regulator?

The function of a voltage regulator is to maintain a constant output voltage in the presence of changes in load and source voltage. Traditional voltage regulator circuits use either linear or switched-mode designs. LDO regulators belong to the class of linear regulators but operate with very low voltages between the input and output terminals. Like all linear voltage regulators, the LDO is based on a feedback control loop .

Image of LDO regulator is based on a voltage-controlled feedback circuit

Figure 1: An LDO regulator is based on a voltage-controlled feedback circuit. The series pass device, which can be a PMOS, NMOS, or PNP bipolar transistor, acts like a voltage-controlled resistor. (Image source: Diodes Incorporated)

The LDO regulator senses the output voltage through a resistive voltage divider which scales the output level. The scaled output voltage is applied to an error amplifier, where it is compared to a reference voltage. The error amplifier drives the series pass device to maintain the desired voltage at the output terminal. The difference between the input and output voltage is the dropout voltage, which appears across the pass device.

The series pass device in an LDO acts like a voltage-variable resistor. The series pass device can be a P-channel metal oxide semiconductor (PMOS), an N-channel metal oxide semiconductor (NMOS), or a PNP bipolar transistor. PMOS and PNP devices can be driven into saturation, minimizing the dropout voltage. In the case of a PMOS field effect transistor (FET), the dropout voltage is approximately the channel ON resistance (RDSON) times the output current. While each of these devices has advantages and disadvantages, the PMOS device proves to have the lowest implementation cost. The Diodes Incorporated AP7361EA series of positive output LDO regulators uses a PMOS pass device and achieves a dropout voltage of about 360 millivolts (mV) for a 3.3-volt output at a load current of 1 ampere (A), and a voltage accuracy of ±1% .

Graph of dropout voltage of the Diodes AP7361EA series 3.3 volt LDO

Figure 2: Shown are plots of the dropout voltage of the AP7361EA series 3.3 volt LDO as a function of the output current at three different temperatures. (Image source: Diodes Incorporated)

The plot of the dropout voltage as a function of the output current shows a constant slope for each temperature, indicative of its resistive nature. The dropout voltage is somewhat temperature dependent, with the level increasing with increasing temperature. Note that the LDO dropout voltage is much lower than that of a conventional linear power regulator, which would have a dropout voltage of about 2 volts.

Notice that the output capacitor in Figure 1 is shown with its inherent effective series resistance (ESR), which affects the stability of the regulator. The selected capacitor should have an ESR below 10 ohms (Ω) in order to guarantee stability over the full operating temperature of -40° to +85°C. Suggested capacitor types include multilayer ceramic capacitors (MLCC), solid-state E-CAPs, and tantalum capacitors with values over 2.2 microfarads (mF).

The quiescent current, IQ, represents the current drawn from the power source by the LDO with no load. The quiescent current supplies power to the LDO internal circuits, like the error amplifier and the output voltage divider. In battery-powered devices, the quiescent current affects the discharge rate of the battery and is generally designed to be as low as possible. The Diodes Incorporated AP7361EA series has a typical IQ of 68 mA.

The AP7361EA series LDOs

The AP7361EA series includes three alternative circuit configurations.

Diagram of Diodes AP7361EA series fixed or adjustable output voltage devices (click to enlarge)

Figure 3: The AP7361EA series offers fixed or adjustable output voltage devices, with or without an enable control. (Image source: Diodes Incorporated)

The AP7361EA series includes versions with fixed or adjustable output voltages. The fixed voltage versions have internal voltage dividers and offer output voltage levels of 1.0, 1.2, 1.5, 1.8, 2.5, 2.8, or 3.3 volts. The adjustable output devices require a user-supplied external voltage divider and have an output voltage range of 0.8 to 5 volts. The output voltage accuracy specification for all versions is ± 1%, along with an input voltage range of 2.2 to 6 volts.

The fixed or adjustable versions can include an enable control line (EN). The AP7361EA is turned on by setting the EN pin high and is turned off by pulling it low. If this feature is not used, the EN pin should be tied to the input pin (IN) to keep the regulator output on at all times. Response time for the enable line is approximately 200 microseconds (ms) for turn on and about 50 ms for turn off.

The other significant difference between AP7361EA devices is the physical package. It is available in the U-DFN3030-8 (Type E), SOT89-5, SOT223, TO252 (DPAK), and SO-8EP packages.

A comparison of a few examples of the AP7361EA products, including both fixed (AP7361EA-33DR-13, AP7361EA-10ER-13) and adjustable (AP7361EA-FGE-7, AP7361EA-SPR-13) versions, is shown in Table 1.

Part number Fixed/Adjustable Output voltage Output current Output enable Package

AP7361EA-33DR-13 Fixed 3.3 V 1 A No TO-252, (D-Pak)

AP7361EA-10ER-13 Fixed 1.0 V 1 A No SOT-223-3

AP7361EA-FGE-7 Adjustable 0.8 V to 5.0 V 1 A No U-DFN3030-8

AP7361EA-SPR-13 Adjustable 0.8 V to 5.0 V 1 A Yes 8-SO-EP

Table 1: A sample of the AP7361EA fixed and adjustable voltage configurations. (Table source: Art Pini, using data from Diodes Inc.)

The AP7361EA series devices are all protected against short circuits and overcurrent. The short circuit and overcurrent protection features a foldback current limit of 400 milliamperes (mA) if the output current exceeds the current limit, typically 1.5 A. Thermal shutdown occurs when the device’s junction temperature increases to nominally 150°C, and operation is restored when it drops below about 130°C.

Load and line regulation

Load regulation describes the LDO’s ability to maintain its output voltage despite changes in the output load current. This is important in battery-powered portable devices, where the controllers often shut down subsystems when they are not in use. The AP7361EA LDO series has a maximum specified load regulation of 1.5% for output levels of 1 to 1.2 volts and 1% for outputs of 1.2 to 3.3 volts .

Image of load regulation graph for a 3.3-volt output

Figure 4: An example of load regulation graph for a 3.3-volt output. The maximum output variation is approximately 0.15% or about 5.0 mV for a load change of from 100 to 500 mA for the 3.3-volt nominal output. (Image source: Diodes Incorporated)

The load regulation is calculated as the ratio of the maximum output voltage variation to the nominal output voltage. In the above example, the maximum output variation is about 5.0 mV for a load change of 100 mA to 500 mA. So, the load regulation is 0.005/3.3 or 0.15%

Line variation specifies the variation in the output for a change in the source voltage per volt of output. The AP7361EA series has a maximum line regulation specification of 0.1% per volt (%/V) at room temperature and 0.2%/V over the full temperature range. For a 3.3-volt output, an input level change of 1 volt should have an output level change of less than 0.33% of the nominal 3.3-volt output.

Image of graph of line regulation for an Diodes AP7361EA

Figure 5: Shown is a graph of line regulation for an AP7361EA operating with a 3.3 volt output. A change in input voltage from 4.3 to 5.3 volts results in a 0.05% change in output voltage. (Image source: Diodes Incorporated)

Figure 5 shows the line regulation characteristic of the LDO. A change in the source voltage, from 4.3 to 5.3 volts, results in a 0.05% change in the output level, or about 1.65 mV.

Note, that under both the line and load variation conditions, the output shows a rapid recovery from the transient events. This is important when restarting processes in portable equipment where the power bus must be up and functioning before the silenced circuits can be restarted.

Power supply rejection ratio

LDOs, being linear circuits, produce much less noise than switched-mode power supplies (SMPS) or power converters. In many applications, an LDO is used locally on the circuit board, but the power source is an SMPS. Due to the control system within an LDO, it tends to suppress noise and ripple from the input power source. The measure of this noise suppression is the power supply rejection ratio (PSRR).

Diagram of PSRR is calculated based on the alternating current signals (click to enlarge)

Figure 6: PSRR is calculated based on the alternating current signals measured at the input and output of the LDO. (Image source: Diodes Incorporated)

PSRR is calculated based on the ratio of the AC components of the input to those of the output, as shown in Figure 6. PSRR in the AP7361EA series is frequency dependent, decreasing with increasing frequency. The PSRR is 75 decibels (dB) at 1 kilohertz (kHz) and drops to 55 dB at a frequency of 10 kHz. 75 dB represents an attenuation of over 5600:1. A 10 mV ripple or noise signal at 1 kHz would be attenuated to about 1.7 microvolts (µV).

Application example

A typical application of an adjustable output LDO is shown in Figure 7. It includes an output enable similar to the AP7361EA-SPR-13, as well as an external output voltage divider.

Diagram of using an adjustable output LDO requiring an external output voltage divider

Figure 7: An example of using an adjustable output LDO requiring an external output voltage divider. The equation (lower right) shows the relationship between resistors R1 and R2 for the desired output voltage and the internal reference voltage. (Image source: Diodes Incorporated)

The resistor divider resistor values can be calculated using the equations shown in the lower right of Figure 7. The value of R2 should be kept at less than 80 kilohms (kΩ) to ensure the stability of the internal voltage reference. For an output of 2.4 volts with a reference voltage of 0.8 volts and R2 equal to 61.9 kΩ, the value of R1 works out to be 123.8 kΩ. A 124 kΩ, 1% resistor would be suitable.

Conclusion

LDOs are linear voltage regulators that operate with low voltage differences across the input and output, and with low quiescent currents. They offer high power efficiency with low noise and small size. They are especially well-suited for battery-operated, portable devices where they extend battery life and improve reliability.