LDO:
Low dropout linear regulators, as the name suggests, are linear regulators and can only be used in step-down applications, meaning the output voltage must be lower than the input voltage.
Advantages: Good stability, fast load response, and low output ripple.
Disadvantages: Low efficiency, the voltage difference between input and output cannot be too large, the load cannot be too large, and the current maximum LDO is 5A, but there are still many limitations to ensure a 5A output.
DC/DC:
DC to DC voltage conversion. Strictly speaking, LDO is also a type of DC/DC converter, but currently DC/DC converters mostly refer to switching power supplies, which have many topological structures, such as BUCK and BOOST.
Advantages: High efficiency and wide input voltage range.
Disadvantages: The load response is worse than that of an LDO, and the output ripple is larger than that of an LDO.
So, what is the difference between a DC/DC converter and an LDO?
A DC/DC converter typically consists of a control chip, a coil, diodes, transistors, and capacitors. A DC/DC converter is a voltage converter that transforms an input voltage to effectively output a fixed voltage. DC/DC converters are classified into three types: boost DC/DC converters, buck DC/DC converters, and buck-boost DC/DC converters.
Three types of control can be adopted depending on the requirements:
PWM control offers high efficiency and good output voltage ripple and noise reduction.
PFM control type has the advantage of low power consumption even during long-term use, especially under low load;
The PWM/PFM switching type implements PFM control under light loads and automatically switches to PWM control under heavy loads.
Currently, DC-DC converters are widely used in products such as mobile phones, MP3 players, digital cameras, and portable media players.
DC-DC Brief Explanation of Principles
Internally, the DC power supply is first converted to AC power, typically using a self-oscillating circuit, requiring discrete components such as inductors. Then, at the output, it undergoes integration and filtering, returning to DC power. Because AC power is generated, boosting and bucking can be easily achieved. These two conversions inevitably incur losses, which is why improving DC-DC efficiency is a key research area.
contrast
DC-DC converters include boost, buck, boost/buck, and inverting structures, featuring high efficiency, high output current, and low quiescent current. With the increase in integration, many new DC-DC converters only require inductors and filter capacitors in their peripheral circuits. However, these power controllers have relatively high output ripple and switching noise, and are relatively expensive.
The outstanding advantages of LDO (Low Dropout Linear Regulator) are its lowest cost, lowest noise, and lowest quiescent current. It also requires very few external components, typically only one or two bypass capacitors. Newer LDOs can achieve the following specifications: 30μV output noise, 60dBPSRR, 6μA quiescent current, and 100mV dropout voltage.
LDO (Low Displacement) principle explained
The main reason linear regulators can achieve these characteristics is that they use a P-channel MOSFET (P-channel field-effect transistor) as the internal regulating transistor, instead of the PNP transistor typically found in linear regulators. P-channel MOSFETs do not require base current drive, thus significantly reducing the device's own supply current. On the other hand, in structures using PNP transistors, a large input-output voltage difference must be maintained to prevent the PNP transistor from saturating and reducing its output capability. However, the voltage difference of a P-channel MOSFET is approximately equal to the product of its output current and its on-resistance; its extremely low on-resistance results in a very low voltage difference.
When the input and output voltages of a system are close, an LDO is the best choice, achieving high efficiency. Therefore, LDOs are often used in applications that convert lithium-ion battery voltage to 3V, even though about 10% of the battery's final discharge energy is not used; LDOs can still provide a longer battery life in a low-noise configuration.
Whether portable electronic devices are powered by AC mains power after rectification (or an AC adapter) or by a battery pack, the power supply voltage will vary significantly during operation. For example, the voltage of a single lithium-ion battery is 4.2V when fully charged and 2.3V when fully discharged, showing a large range of variation.
The output voltage of various rectifiers is affected not only by changes in mains voltage but also by changes in load. To ensure a stable supply voltage, almost all electronic devices use voltage regulators. Small, precision electronic devices also require a very clean power supply, free of ripple and noise, to avoid affecting the normal operation of the electronic equipment. To meet the requirements of precision electronic devices, a linear regulator should be added to the input of the power supply to ensure a constant power supply voltage and achieve active noise filtering.
Basic principles of LDO
The basic circuit of a low dropout linear regulator (LDO) is shown in Figure 1-1. The circuit consists of a series regulating transistor VT (PNP transistor; note: in practical applications, a P-channel MOSFET is commonly used here), sampling resistors R1 and R2, and a comparator amplifier A.
Figure 1-1 Basic circuit of a low dropout linear regulator
The sampled voltage Uin is applied to the non-inverting input of comparator A and compared with the reference voltage Uref(Uout*R2/(R1+R2)) applied to the inverting input. The difference between the two is amplified by amplifier A, Uout = (U+-U-)*A (Note: A is the multiplier of the comparator amplifier), which controls the voltage drop across the series regulating transistor, thereby stabilizing the output voltage. When the output voltage Uout decreases, the difference between the reference voltage Uref and the sampled voltage Uin increases, the drive current of the comparator amplifier increases, the voltage drop across the series regulating transistor decreases, and thus the output voltage increases.
Conversely, if the output voltage Uout exceeds the required set value, the pre-drive current of the comparator amplifier decreases, thereby lowering the output voltage. During power supply, output voltage correction is continuous, and the adjustment time is limited only by the response speed of the comparator amplifier and the output transistor circuit. It should be noted that a practical linear regulator should also have many other functions, such as load short-circuit protection, overvoltage shutdown, overheat shutdown, reverse connection protection, etc., and the series regulating transistor can also be a MOSFET.
Main parameters of low dropout linear regulator
1. Output voltage
Output voltage is the most important parameter of a low-dropout linear regulator (LDL) and should be the first parameter that electronic equipment designers consider when selecting a regulator. LDLs come in two types: fixed output voltage and adjustable output voltage. Fixed output voltage regulators are more convenient to use, and because the output voltage is precisely adjusted by the manufacturer, the regulator has high accuracy. However, the set output voltage values are common values and cannot meet all application requirements; variations in the values of external components will affect the stability accuracy.
2. Maximum output current
Different electrical devices have different power requirements, which in turn require different maximum output currents from voltage regulators. Generally, voltage regulators with higher output currents are more expensive. To reduce costs, in power supply systems consisting of multiple voltage regulators, appropriate voltage regulators should be selected based on the current requirements of each component.
3. Input-output voltage difference
The input-output voltage difference is the most important parameter of a low-dropout linear regulator. The lower this voltage difference, the better the performance of the linear regulator, while ensuring stable output voltage. For example, a 5.0V low-dropout linear regulator can maintain a stable output voltage of 5.0V with an input voltage of only 5.5V.
4. Grounding current
The grounding circuit IGND refers to the regulator's operating current supplied by the input power supply when the output current of the series regulating transistor is zero. This current is sometimes called the quiescent current, but this terminology is incorrect when using a PNP transistor as the series regulating element. Typically, the grounding current of an ideal low-dropout regulator is very small.
5. Load regulation rate
The load regulation rate can be defined by Figure 2-1 and Equation 2-1. The smaller the load regulation rate of the LDO, the stronger the LDO's ability to suppress load disturbances.
Figure 2-1OutputVoltage&OutputCurrent
In the formula:
△Vload—Load regulation rate;
Imax—Maximum output current of the LDO;
Vt—The output voltage of the LDO when the output current is Imax;
Vo—The output voltage of the LDO when the output current is 0.1mA;
△V—The difference in output voltage when the load current is 0.1mA and Imax, respectively.
6. Linear adjustment rate
The line regulation rate can be defined by Figure 2-2 and Equation 2-2. The smaller the line regulation rate of the LDO, the smaller the impact of input voltage changes on the output voltage, and the better the performance of the LDO.
In the formula:
△Vline—LDO linear adjustment rate;
Vo—Nominal output voltage of LDO;
Vmax—Maximum input voltage of the LDO;
△V—LDO input Vo to Vmax' is the difference between the maximum and minimum output voltage values.
7. Power Supply Rejection Ratio
LDO input sources often contain many interference signals, and PSRR reflects the LDO's ability to suppress these interference signals.
Typical applications of LDO
A typical application of a low-dropout linear regulator is shown in Figure 3-1. The circuit shown in Figure 3-1(a) is a common AC/DC power supply. The AC power supply voltage is transformed into the required voltage by a transformer, and then rectified into a DC voltage. In this circuit, the function of the low-dropout linear regulator is to stabilize the output voltage when the AC power supply voltage or load changes, suppress ripple voltage, and eliminate AC noise generated by the power supply.
The operating voltage of various batteries varies within a certain range. In order to ensure that the battery pack outputs a constant voltage, a low-dropout linear regulator should usually be connected to the output terminal of the battery pack, as shown in Figure 3-1(b).
Low-dropout linear regulators have lower power ratings, thus extending battery life. Furthermore, because their output voltage is close to the input voltage, they maintain stable output voltage even when the battery is nearly fully discharged. As is well known, switching power supplies are highly efficient, but they suffer from higher output ripple voltage, greater noise, and poorer voltage regulation, which can have a significant impact, especially when powering analog circuits.
By connecting a low-dropout linear regulator to the output of a switching regulator, as shown in Figure 3-1(c), active filtering can be achieved, which can also greatly improve the voltage regulation accuracy of the output voltage, while the efficiency of the power supply system will not be significantly reduced.
In some applications, such as radio communication equipment, a single battery is often sufficient for power supply, but different circuits frequently use isolated voltages, necessitating the use of multiple voltage regulators. To conserve battery power, it is desirable for the low-dropout linear regulator to operate in sleep mode when the equipment is not in use. Therefore, the linear regulator must have an enable control terminal. A power supply system with multiple outputs powered by a single battery and on/off control functionality is shown in Figure 3-1(d).
DC-DC can be understood in this way.
DC-DC means direct current to direct current conversion, that is, the conversion between different DC power supply values. Any device that meets this definition can be called a DC-DC converter, including LDOs. However, generally speaking, a DC-DC converter is a device that uses a switching mechanism to achieve the DC-DC conversion. DC-DC converters include boost, buck, boost/buck, and inverting circuits. The advantages of DC-DC converters are high efficiency, high current output, and low quiescent current. With the increase in integration, many new DC-DC converters only require a few external inductors and filter capacitors.
However, these types of power controllers have relatively high output ripple and switching noise, and are relatively expensive. In recent years, with the development of semiconductor technology, the cost of surface-mount inductors, capacitors, and highly integrated power control chips has been continuously decreasing, and their size has become smaller and smaller. Because MOSFETs with very low on-resistance can output high power, external high-power FETs are no longer needed. For example, for a 3V input voltage, a 5V/2A output can be obtained using an on-chip NFET. Furthermore, for low-to-medium power applications, low-cost, small packages can be used. In addition, increasing the switching frequency to 1MHz can further reduce costs and allow the use of smaller inductors and capacitors. Some new devices also add many new features, such as soft-start, current limiting, and PFM or PWM mode selection.
In general, for boost converters, DC-DC converters are a must. For buck converters, the choice between DC-DC and LDO depends on a comparison of cost, efficiency, noise, and performance.
LDO vs. DC/DC Comparison
First, in terms of efficiency, DC/DC converters are generally much more efficient than LDOs, which is determined by their working principle. Second, DC/DC converters include Boost, Buck, and Boost/Buck types, and some people also classify Charge Pumps as this type, while LDOs only have step-down types.
Secondly, and very importantly, DC/DC converters have significantly higher power supply noise due to their switching frequency, much higher than LDOs. You should pay attention to the PSRR parameter. Therefore, when considering sensitive analog circuits, it may be necessary to sacrifice efficiency to ensure power supply purity when choosing an LDO.
Furthermore, LDOs typically require simpler, smaller external components, while DC/DC converters generally require inductors, diodes, large capacitors, and sometimes MOSFETs. Boost circuits, in particular, need to consider the maximum operating current of the inductor, the reverse recovery time of the diode, the ESR of the large capacitor, and so on. Therefore, the selection of external components is more complex than for LDOs, and the area occupied is correspondingly much larger.
