Several key knowledge points of LDO regulator are here!
Latest update time：2025-03-12
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This article will introduce the basics of low-dropout (LDO) regulators and their key characteristics compared to traditional linear and switch-mode power regulators . It will then introduce true resistor devices from Diodes Incorporated and their uses .
Modern electronic devices are becoming increasingly smaller and more portable. Smartwatches, fitness trackers, security systems, and Internet of Things (IoT) devices are increasingly powered by batteries. Consequently, these devices require highly efficient power regulators that can extract every milliwatt of power from each charge to ensure extended device operation. Furthermore, these devices must maintain minimal overtemperature rise. Traditional linear regulators and switch-mode power regulators cannot easily meet the efficiency requirements of these portable devices. Furthermore, switch-mode power regulators are susceptible to noise and transient voltages.
Low-dropout regulators (LDOs) are the latest addition to the family of linear and switching regulators. They operate with an extremely low voltage drop to improve efficiency and reduce heat dissipation. A wide variety of LDOs are available, with packages as small as 3×3×0.6 mm, making them ideal for low- to medium-power applications. Versions are available with either a constant or adjustable output voltage, as well as some with on/off control via an output enable line.
What is an LDO voltage regulator?
The function of a voltage regulator is to maintain a constant output voltage despite variations in load and source voltage. Traditional voltage regulator circuits utilize either linear or switch-mode designs.
Low-dropout (LDO) regulators are linear regulators, but operate with a very low voltage between their input and output terminals.
Like all linear voltage regulators, LDOs are based on a feedback control loop (Figure 1).
Figure 1: LDO regulators are based on a voltage-controlled feedback circuit. The series pass device can be a PMOS, NMOS, or PNP bipolar transistor, which acts like a voltage-controlled resistor. (Image source: Diodes Incorporated)
An LDO regulator senses the output voltage using a resistor divider, which scales the output level. The scaled output voltage is applied to an error amplifier, which then compares it to a reference voltage
. The error amplifier drives the series pass device to maintain the desired voltage at the output. The difference between the input and output voltages is the dropout voltage, which appears across the pass device.
In an LDO, the series pass device acts like a voltage-variable
resistor
.
It 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 to minimize dropout voltage. In the case of a PMOS field-effect transistor (FET), the dropout voltage is approximately equal to the channel on-resistance
(
RDSON) multiplied by the output current. While each device has advantages and disadvantages, PMOS devices have proven to be the least expensive to implement. Diodes Incorporated's
AP7361EA
series of positive-output
LDO regulators uses a PMOS pass device to achieve a 3.3 V output dropout voltage of approximately 360 mV at a 1 A load current with ±1% voltage accuracy (Figure 2).
Figure 2: The AP7361EA series 3.3 V LDO shows a graph of dropout voltage as a function of output current at three different temperatures. (Image source: Diodes Incorporated)
A plot of dropout voltage as a function of output current shows a constant slope at every temperature, indicating its resistive nature. Dropout voltage is somewhat temperature-dependent, increasing in level as temperature rises. Note that the dropout voltage of an LDO is much lower than that of a traditional linear power regulator, which has a dropout voltage of approximately 2 V.
Note that the output capacitors shown in Figure 1 are expressed in terms of their inherent effective series resistance (ESR), which affects the stability of the regulator.
The selected capacitors should have an ESR below 10 Ω to guarantee stability over the full operating temperature range of -40°C to +85°C. Recommended capacitor types include multilayer ceramic capacitors (MLCCs), solid-state E-CAPs, and tantalum capacitors with values greater than 2.2 mF.
Quiescent current (
IQ)
represents the supply current consumed by an LDO when there is no load. This quiescent current powers the LDO's internal circuitry, such as the error amplifier and output voltage divider. In battery-powered devices, quiescent current affects the battery's discharge rate and is typically kept as low as possible. The typical IQ for the AP7361EA series from Diodes Incorporated
is 68 mA.
AP7361EA Series LDO
The AP7361EA series includes three alternative circuit configurations, as shown in Figure 3.
Figure 3: The AP7361EA family offers devices with constant or adjustable output voltage, with or without enable control. (Image source: Diodes Incorporated)
The AP7361EA
series is available in versions with either constant or adjustable output voltage.
Constant-voltage versions feature an internal voltage divider, providing output voltage levels of 1.0 V, 1.2 V, 1.5 V, 1.8 V, 2.5 V, 2.8 V, or 3.3 V. Adjustable-output devices require a user-provided external voltage divider and offer an output voltage range of 0.8 V to 5 V. All versions offer an output voltage accuracy specification of ±1%, with an input voltage range of 2.2 V to 6 V.
Constant-voltage or adjustable-output-voltage versions can include an enable control line (EN). The AP7361EA is turned on by setting the EN pin high and shuts down 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. The enable line has a response time of approximately 200 ms for turn-on and approximately 50 ms for turn-off.
Another unique feature of the
AP7361EA
device is its packaging. The device is available in U-DFN3030-8 (Type E), SOT89-5, SOT223, TO252 (DPAK), and SO-8EP packages.
Table 1 compares different versions of the AP7361EA product, including constant voltage types (
AP7361EA-33DR-13
,
AP7361EA-10ER-13
) and adjustable types (
AP7361EA-FGE-7
,
AP7361EA-SPR-13
).
|
Part Number
|
Constant/adjustable
|
Output voltage
|
Output current
|
Output Enable
|
Package
|
|
AP7361EA-33DR-13
|
Constant
|
3.3 V
|
1 A
|
no
|
TO-252, (D-Pak)
|
|
AP7361EA-10ER-13
|
Constant
|
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: Samples of the AP7361EA in constant voltage and adjustable voltage configurations. (Source: Art Pini, using data provided by Diodes Inc.)
The AP7361EA series devices feature short-circuit and overcurrent protection. If the output current exceeds the current limit (typically 1.5A), both short-circuit and overcurrent protections include a 400mA foldback current limit. Thermal shutdown occurs when the device's junction temperature rises to a nominal 150°C and resumes operation when the junction temperature drops to approximately 130°C.
Load and line regulation
Load regulation describes an LDO's ability to maintain its output voltage despite variations in output load current. This characteristic is crucial for battery-powered portable devices, as controllers often shut down subsystems when not in use. The AP7361EA LDO family has a maximum specified load regulation of 1.5% for output levels from 1 V to 1.2 V and 1% for outputs from 1.2 V to 3.3 V (Figure 4).
Figure 4: Example of a load regulation plot for a 3.3 V output. For a nominal 3.3 V output, the maximum output change is approximately 0.15% or approximately 5.0 mV when the load varies from 100 mA to 500 mA. (Image source: Diodes Incorporated)
Load regulation is the ratio of the maximum output voltage change to the rated output voltage. In the example above, when the load changes from 100 mA to 500 mA, the maximum output change is approximately 5.0 mV. Therefore, the load regulation is 0.005/3.3, or 0.15%.
Line regulation specifies the change in output for each volt of output source voltage change. The AP7361EA series has a maximum line regulation of 0.1%/V at room temperature and 0.2%/V over the entire temperature range. For a 3.3 V output, a 1 V input level change should result in a change in output level of less than 0.33% of the nominal 3.3 V output (Figure 5).
Figure 5: Shown is the line regulation of the AP7361EA at 3.3 V output. An input voltage change between 4.3 V and 5.3 V results in a 0.05% change in output voltage. (Image source: Diodes Incorporated)
The line regulation of the LDO is shown in Figure 5. A source voltage change between 4.3 V and 5.3 V results in a 0.05% or approximately 1.65 mV change in output level.
Note that the output recovers quickly from transient events under varying line and load conditions. This feature is important during the restart process of portable equipment, as the power bus must be up and functioning before the noise suppression circuit can be re-activated.
Power Supply Rejection Ratio
As a linear circuit, an LDO generates much less noise than a switch-mode power supply (SMPS) or power converter. In many applications, an LDO is used on a circuit board, but the power supply is an SMPS. Due to the control system within the LDO, it tends to reject noise and ripple from the input power supply.
A measure of this noise rejection is the power supply rejection ratio
(PSRR) (Figure 6).
Figure 6: PSRR is calculated from alternating current signals measured at the input and output of an LDO. (Image source: Diodes Incorporated)
As shown in Figure 6, PSRR is calculated as the ratio of the input AC component to the output AC component. The AP7361EA series' PSRR is frequency-dependent, decreasing as frequency increases. At 1 kHz, the PSRR is 75 dB; at 10 kHz, it drops to 55 dB. 75 dB represents an attenuation exceeding 5600:1. A 10mV ripple or noise signal at 1kHz will be attenuated to approximately 1.7μV.
Application Examples
A typical application of an adjustable-output LDO is shown in Figure 7. This application includes an output enable similar to the AP7361EA-SPR-13 and an external output voltage divider.
Figure 7: Example of using an adjustable-output LDO that requires an external output voltage divider. The equation on the lower right shows the relationship between resistors R1 and R2 to meet the desired output voltage and the internal reference. (Image source: Diodes Incorporated)
Resistors The resistor
values for
the voltage
divider
can be calculated using the formula shown in the lower right corner of Figure 7. R2 should be kept less than 80 kΩ to ensure stability of the internal voltage reference. For a 2.4 V output with a 0.8 V reference voltage, R2 is 61.9 kΩ and R1 is 123.8 kΩ. A 124 kΩ, 1%
resistor
is the required
value
.
Conclusion
LDOs are linear regulators that operate with a low voltage difference between input and output and low quiescent current. They offer high power efficiency, low noise, and a small size. They are particularly well-suited for battery-powered portable devices, extending battery life and improving reliability.
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