“The main reason that linear regulators can achieve these characteristics is that the internal pass transistor uses P-channel field effect transistors, rather than the usual PNP transistors in linear regulators. P-channel FETs do not need base current drive, so the power supply current of the device itself is greatly reduced. On the other hand, in the structure using the PNP transistor, in order to prevent the PNP transistor from entering a saturated state and reducing the output capability, a large input-output voltage difference must be guaranteed. The voltage difference of the P-channel FET is roughly equal to the product of the output current and its on-resistance, and the extremely small on-resistance makes the voltage drop very low.
Low dropout linear regulators, so the name implies linear regulators, can only be used in step-down applications, that is, the output voltage must be less than the input voltage.
Advantages: good stability, fast load response, small output ripple.
Disadvantages: Low efficiency, the voltage difference between input and output should not be too large, and the load should not be too large. Currently, the largest LDO is 5A, but there are many restrictions to ensure the output of 5A.
DC voltage is converted to DC voltage. Strictly speaking, LDO is also a kind of DC/DC, but at present DC/DC multi-finger switching power supply has many topological structures, such as buck, boost and so on.
Advantages: High efficiency, wide input voltage range.
Disadvantages: The load response is worse than that of LDO, and the output ripple is larger than that of LDO.
So, what is the difference between DC/DC and LDO?
The DC/DC converter is generally composed of a control chip, a pole coil, a diode, a triode, and a capacitor. The DC/DC converter is a voltage converter that effectively outputs a fixed voltage after converting the input voltage. DC/DC converters are divided into three categories: step-up DC/DC converters, step-down DC/DC converters, and buck-boost DC/DC converters.
Three types of controls can be used according to requirements:
• PWM control type has high efficiency and good output voltage ripple and noise;
• The PFM control type has the advantage of low power consumption even if it is used for a long time, especially when the load is small;
• PWM/PFM conversion type PFM control is implemented at light loads, and automatically switched to PWM control at heavy loads.
At present, DC-DC converters are widely used in mobile phones, MP3, digital cameras, portable media players and other products.
Brief description of DC-DC principle
In fact, the inside is to first convert the DC power supply to the AC power supply, which is usually a self-excited oscillation circuit, so discrete components such as inductors are required outside. Then at the output end, it is filtered by the integral and then back to the DC power supply. Since the AC power supply is generated, it can be easily boosted and stepped down. Two conversions will inevitably produce losses, which is the problem of how to improve the DC-DC efficiency that everyone is working hard to study.
DCtoDC includes boost (step-up), buck (step-down), Boost/buck (step-up/step-down) and inverting structures, with high efficiency, high output current, low quiescent current, etc. With the improvement of integration, many The peripheral circuit of the new DC-DC converter only needs an Inductor and a filter capacitor, but the output ripple and switching noise of this type of power controller are relatively high, and the cost is relatively high.
The outstanding advantages of LDO low dropout linear regulators are the lowest cost, lowest noise and lowest quiescent current. It also has few peripheral components, usually only one or two bypass capacitors. The new LDO can achieve the following indicators: 30μV Output noise, 60dBPSRR, 6µA quiescent current, and 100mV dropout.
A brief description of the principle of LDO
The main reason that linear regulators can achieve these characteristics is that the internal pass transistor uses P-channel field effect transistors, rather than the usual PNP transistors in linear regulators. P-channel FETs do not need base current drive, so the power supply current of the device itself is greatly reduced. On the other hand, in the structure using the PNP transistor, in order to prevent the PNP transistor from entering a saturated state and reducing the output capability, a large input-output voltage difference must be guaranteed. The voltage difference of the P-channel FET is roughly equal to the product of the output current and its on-resistance, and the extremely small on-resistance makes the voltage drop very low.
When the input voltage and output voltage in the system are close, LDO is the best choice and can achieve high efficiency. Therefore, LDOs are mostly used in applications that convert Li-ion battery voltage to 3V voltage. Although 10% of the battery’s final discharge energy is not used, LDOs can still provide long battery life in a low-noise structure.
Whether the portable Electronic device is powered by AC mains after rectification (or AC adapter), or powered by a battery pack, the power supply voltage will vary within a wide range during operation. For example, when a single lithium-ion battery is fully charged, the voltage is 4.2V, and the voltage after discharge is 2.3V, which varies widely.
The output voltage of various rectifiers is not only affected by the change of the mains voltage, but also by the change of the load. In order to ensure the stability of the power supply voltage, almost all electronic devices are powered by a voltage stabilizer. Small and precise electronic equipment also requires a very clean power supply, without ripples and noise, so as not to affect the normal operation of the electronic equipment. In order to meet the requirements of precision electronic equipment, a linear regulator should be added to the input end of the power supply to ensure a constant power supply voltage and achieve active noise filtering.
01 The basic principle of LDO
The basic circuit of a low dropout linear regulator (LDO) is shown in Figure 1-1. The circuit consists of a series regulator VT (PNP transistor, Note: In practical applications, P-channel field effect transistors are commonly used here), sampling Resistors R1 and R2, and the comparator amplifier A are composed.
Figure 1-1 Basic circuit of low dropout linear regulator
The sampling voltage Uin is added to the non-inverting input terminal of the comparator A, and is compared with the reference voltage Uref (Uout*R2/(R1+R2)) added to the inverting input terminal. After the difference between the two is amplified by the amplifier A, Uout =(U+-U-)*A Note A is the multiple of the amplifier, which controls the voltage drop of the series adjustment tube, thereby stabilizing the output voltage. When the output voltage Uout decreases, the difference between the reference voltage Uref and the sampling voltage Uin increases, the drive current output by the comparator increases, and the voltage drop of the series regulator decreases, thereby increasing the output voltage.
Conversely, if the output voltage Uout exceeds the required set value, the pre-drive current output by the comparator amplifier decreases, thereby reducing the output voltage. During the power supply process, the output voltage correction is performed continuously, and the adjustment time is only limited by the response speed of the comparator amplifier and the output transistor loop. It should be noted that the actual linear regulator should also have many other functions, such as load short-circuit protection, overvoltage shutdown, thermal shutdown, reverse connection protection, etc., and the series pass transistor can also use MOSFET.
02 Main parameters of low dropout linear regulator
1.The output voltage
The output voltage is the most important parameter of a low dropout linear regulator, and it is also the first parameter that electronic equipment designers should consider when selecting a regulator. There are two types of low dropout linear voltage regulators: fixed output voltage and adjustable output voltage. The fixed output voltage regulator is more convenient to use, and because the output voltage is precisely adjusted by the manufacturer, the voltage regulator has high precision. However, the set output voltage values are all common voltage values, which cannot meet all application requirements, but the change in the value of external components will affect the stability accuracy.
2.Maximum output current
The power of the electrical equipment is different, and the maximum current required by the voltage stabilizer is also different. Generally, the voltage stabilizer with a larger output current costs more. In order to reduce the cost, in the power supply system composed of multiple voltage stabilizers, the appropriate voltage stabilizer should be selected according to the current value required by each part.
3.Input and output voltage difference
The input and output voltage difference is the most important parameter of the low dropout linear regulator. Under the condition of ensuring the stability of the output voltage, the lower the voltage difference, the better the performance of the linear regulator. For example, a 5.0V low dropout linear regulator can stabilize the output voltage at 5.0V as long as the input voltage is 5.5V.
The grounding circuit IGND refers to the working current of the regulator provided by the input power supply when the output current of the series regulator is zero. This current is sometimes called quiescent current, but when a PNP transistor is used as a series regulator element, this customary name is incorrect, and the ground current of an ideal low-dropout regulator is usually very small.
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 interference.
△Vload-load regulation rate;
Imax—LDO maximum output current;
Vt – when the output current is Imax, the output voltage of the LDO;
Vo—When the output current is 0.1mA, the output voltage of the LDO;
△V—The difference between the output voltages when the load current is 0.1mA and Imax, respectively.
6.Linear adjustment rate
The linear adjustment rate can be defined by Figure 2-2 and Equation 2-2. The smaller the linear adjustment rate of the LDO, the smaller the influence of the input voltage change on the output voltage, and the better the performance of the LDO.
△Vline—LDO linear adjustment rate;
Vo—LDO nominal output voltage;
Vmax—LDO maximum input voltage;
△V—LDO input Vo to Vmax’ the difference between the maximum and minimum output voltage.
7.power supply rejection ratio
The input source of the LDO often has many interference signals, and the PSRR reflects the LDO’s ability to suppress these interference signals.
03 Typical application of LDO
The typical application of low-dropout linear regulator is shown in Figure 3-1. The circuit shown in Figure 3-1(a) is one of the most common AC/DC power supplies. The AC power supply voltage is transformed into the required voltage, which is rectified into a DC voltage. In this circuit, the role of the low dropout linear regulator is to stabilize the output voltage when the AC power supply voltage or load changes, suppress the ripple voltage, and eliminate the AC noise generated by the power supply.
The working voltage of various batteries varies within a certain range. In order to ensure a constant output voltage of the battery pack, a low-dropout linear voltage regulator should usually be connected to the output end of the battery pack, as shown in Figure 3-1(b).
The power of the low-dropout linear regulator is low, so it can prolong the service life of the battery. At the same time, since the output voltage of the low-dropout linear regulator is close to the input voltage, the output voltage can still be guaranteed to be stable when the battery is nearly discharged. . As we all know, the efficiency of switching regulated power supply is very high, but the output ripple voltage is high, the noise is large, and the voltage regulation rate and other performance are also poor, especially when the analog circuit is powered, it will have a greater impact.
Connecting a low-dropout linear regulator to the output end of the switching regulator, as shown in Figure 3-1(c), can realize active filtering, and can also greatly improve the voltage regulation accuracy of the output voltage, while the power system efficiency will not be significantly reduced.
In some applications, such as radio communication equipment, there is usually only one battery to supply power, but each part of the circuit often uses different voltages that are isolated from each other, so it must be powered by multiple voltage regulators. In order to save the power of the common battery, usually when the device is not working, the low dropout linear regulator is expected to work in the sleep state. For this reason, the linear regulator is required to have an enable control terminal. Figure 3-1(d) shows the power supply system with multiple outputs powered by a single battery and with on-off control function.
Figure 3-1 Low dropout linear regulator (LDO) typical application
04 DC-DC should be understood like this
DC-DC means DC to (to) DC, that is, the conversion of different DC power values. As long as this definition is met, it can be called a DCDC converter, including LDO. But the general saying is that the device that converts DC to (to) DC by switching is called DCDC. DC-DC converters include step-up, step-down, step-up/step-down and inverse equalization circuits. The advantages of DC-DC converters are high efficiency, high output current and small quiescent current. With increased integration, many new DC-DC converters require only a few external inductors and filter capacitors.
However, the output ripple and switching noise of this type of power controller are relatively large, and the cost is relatively high. In recent years, with the development of semiconductor technology, surface-mounted inductors, capacitors, and highly integrated power control chips Costs continue to decrease and volumes become smaller. Since a MOSFET with a small on-resistance can output a large power, an external high-power FET is not required. For example, for an input voltage of 3V, an output of 5V/2A can be obtained by using the NFET on the chip. Secondly, for small and medium power applications, a low-cost small package can be used. In addition, if the switching frequency is increased to 1MHz, the cost can also be reduced, and smaller inductors and capacitors can be used. Some new devices also add many new functions, such as soft start, current limit, PFM or PWM mode selection.
In general, DCDC must be selected for boost, and DCDC or LDO for step-down should be compared in terms of cost, efficiency, noise and performance.
05 Comparison of LDO and DC/DC
First of all, in terms of efficiency, the efficiency of DC/DC is generally much higher than that of LDO, which is determined by its working principle. Secondly, DC/DC has Boost, Buck, and Boost/Buck. Some people also classify ChargePump into this category, while LDO has only step-down type.
Once again, it is also a very important point. Due to the switching frequency of DC/DC, its power supply noise is very large, which is much larger than that of LDO. You can pay attention to the parameter of PSRR. Therefore, when considering the more sensitive analog circuits, it may be necessary to sacrifice efficiency to ensure the purity of the power supply and choose LDO.
Also, usually the peripheral devices required by LDO are simple and occupy a small area, while DC/DC generally requires inductors, diodes, large capacitors, and some MOSFETs. Especially the Boost circuit needs to consider the maximum working current of the inductor, the reverse recovery time of the diode, the ESR of the large capacitor, etc., so the selection of peripheral devices is more complicated than the LDO, and the area will be much larger accordingly.
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