John UM
Ldo: LOW DROPOUT VOLTAGE LDO (short for low dropout voltage regulator, rectifier) low-dropout linear regulator, hence the name, a linear regulator, can only be used in buck applications. That is, the output voltage must be less than the input voltage
Advantages: Good stability and fast load response. The output ripple is small
Disadvantages: Low efficiency, the input and output voltage difference can not be too large. The load should not be too large, and the current maximum LDO is 5A (but there are still many restrictions on the 5A output).
DC/DC: DC voltage to DC voltage. Strictly speaking, LDO is also a kind of DC/DC, but currently DC/DC multi-finger switching power supply. Has a variety of topological structures, such as BUCK, BOOST and so on
Advantages: High efficiency and wide input voltage range
Disadvantages: The load response is worse than LDO, and the output ripple is larger than LDO
What is the difference between DC/DC and LDO?
DC/DC converters are generally composed of control chips, pole coils, diodes, transistors, and capacitors
The DC/DC converter is a voltage converter that effectively outputs a fixed voltage after the input voltage is changed. There are three types of DC/DC converters: step-up DC/DC converters, step-down DC/DC converters, and buck-boost DC/DC converters. Three types of controls are available depending on the requirements. The PWM control type is highly efficient and has good output voltage ripple and noise. The PFM control type has the advantage of low power consumption even when used for a long time, especially at a small load. PFM control is implemented during PWM/PFM conversion type small load, and is automatically switched to PWM control at heavy load. At present, DC-DC. converters are widely used in mobile phones, MP3 players, digital cameras, portable media players and other products
DC-DC (Summary Principle).
In fact, the internal is to first convert the DC power supply to AC power AC. It is usually a self-excited oscillating circuit, so discrete components such as inductors are required outside
Then go through the integral filter at the output and go back to the DC power supply. Since the AC power is generated, it is easy to boost and step down. Two conversions will inevitably lead to loss, which is how everyone is trying to improve the efficiency of DC-DC
Compared:
1, DCtoDC includes boost (buck), buck (buck), Boost/buck (l/b) and inverting structure, featuring high efficiency, high output current, low quiescent current, etc. The degree of improvement, the peripheral circuits of many new DC-DC converters only need inductors and filter capacitors; however, the output ripple and switching noise of this type of power controller is relatively large and relatively high cost
2, LDO: The outstanding advantages of low dropout linear regulators are the lowest cost, lowest noise and lowest quiescent current. It also has few peripheral components, usually with only one or two bypass capacitors. The new LDO achieves the following specifications: 30μV output noise, 60dBPSRR, 6μA quiescent current, and 100mV dropout
LDO Brief Principle:
The main reason why linear regulators can achieve these characteristics is that the internal trim tube uses a P-channel FET instead of a PNP transistor in a typical linear regulator. The P-channel field effect transistor does not need to be driven by the base current, so the power supply current of the device itself is greatly reduced. On the other hand, in the structure using the PNP tube, in order to prevent the PNP transistor from entering the saturation state and reducing the output capability, it is necessary to ensure The large input-output voltage difference; the P-channel FET voltage difference is roughly equal to the product of the output current and its on-resistance, and the small on-resistance makes the voltage difference very low. LDO is the best choice when the input voltage and output voltage are close in the system, achieving high efficiency. Therefore, LDOs are mostly used in applications that convert lithium-ion battery voltages to 3V. Although 10% of the final discharge energy of the battery is not used, LDOs still provide longer battery life in low-noise structures
Whether the portable electronic equipment is powered by the AC mains after rectification (or AC adapter) or by the battery pack, the power supply voltage will vary within a wide range during operation. For example, when the single-cell lithium-ion battery is fully charged, the voltage is 4.2V, and the voltage after discharge is 2.3V, which varies greatly. The output voltage of various rectifiers is not only affected by changes in the mains’ voltage, but also by load changes. In order to ensure that the supply voltage is stable, almost all electronic devices are powered by a voltage regulator. Small precision electronic equipment also requires a very clean power supply (no ripple, no noise), so as not to affect the normal operation of electronic equipment. In order to meet the requirements of precision electronic equipment, a linear regulator should be added to the input of the power supply to ensure constant supply voltage and active noise filtering
I. The basic principle of LDO.
The basic circuit of the low dropout linear regulator (LDO) is shown in Figure 1-1. This circuit consists of a series adjustment tube VT (PNP transistor, Note: In practical applications, the P channel field effect transistor is commonly used here. ), sampling resistors R1 and R2, and comparison amplifier A.
Figure 1-1 Basic circuit of low dropout linear regulator.
The sampling voltage Uin is applied to the non-inverting input of comparator A, compared with the reference voltage Uref (Uout*R2/(R1+R2)) applied to the inverting input, and the difference between the two is amplified by amplifier A.Uout=(U+-U-)*A Note A is a multiple of the comparison amplifier.) Control the voltage drop across the series regulator to stabilize 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 of the comparison amplifier output increases, and the series regulator tube voltage drop decreases, thereby increasing the output voltage
Conversely, if the output voltage Uout exceeds the required set value, the pre-drive current of the comparator output is reduced, thereby reducing the output voltage. During the power supply process, the output voltage correction is continuously performed, and the adjustment time is limited only by the reaction speed of the comparison 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, over voltage shutdown, thermal shutdown, reverse connection protection, etc., and the series regulator can also use MOSFET
II. Main parameters of low dropout linear regulator.
1. Output voltage (OutputVoltage).
The output voltage is the most important parameter for low-dropout linear regulators, and is the first parameter that electronics designers should consider when choosing a regulator. Low dropout linear regulators are available in fixed output voltages and adjustable output voltages. The fixed output voltage regulator is convenient to use, and since the output voltage is precisely adjusted by the manufacturer, the regulator has high precision. However, the set output voltage values are common voltage values, and it is impossible to meet all application requirements, but the change of external component values will affect the stability accuracy
2. Maximum output current (MaximumOutputCurrent).
The power of the powered device is different, and the maximum current required by the regulator is also different. In general, the higher the output current, the higher the cost of the regulator. In order to reduce the cost, in a power supply system composed of a plurality of voltage regulators, an appropriate voltage regulator should be selected according to the current value required for each part
3. Input and output voltage difference (DropoutVoltage).
Input and output voltage difference is the most important parameter of low dropout linear regulator. Under the condition that the output voltage is stable, 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 by inputting 5.5V
4. Ground current (GroundPinCurrent).
Grounding circuit IGND refers to the operating current of the regulator supplied by the input power supply when the output current of the series adjustment tube is zero. This current is sometimes referred to as quiescent current, but this convention is incorrect when using PNP transistors as series regulator tube components. Generally, the ideal low-dropout regulator has a small ground current
5. LoadRegulation (LoadRegulation).
The load adjustment rate can be defined by Figure 2-1 and Equation 2-1. The smaller the load regulation of the LDO, the stronger the LDO’s ability to suppress load interference
Figure 2-1 OutputVoltage&OutputCurrent.
(2-1)
in the formula.
△Vload—Load adjustment rate.
Imax—LDO maximum output current.
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 between the output voltages when the load current is 0.1mA and Imax, respectively.
6. Linear Regulation (LineRegulation).
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
Figure 2-2OutputVoltage&InputVoltage.
(2-2)
in the formula.
△Vline—LDO linear adjustment rate.
Vo—LDO nominal output voltage.
Vmax—LDO maximum input voltage.
△V—LDO Inputs the difference between the maximum and minimum output voltages of Vo to Vmax’.
7. Power Supply Rejection Ratio (PSSR).
LDO’s input source often has many interfering signals present. The PSRR reflects the LDO’s ability to reject these interfering signals
III. Typical application of LDO.
The typical application of a low dropout linear regulator is shown in Figure 3-1. The circuit shown in Figure 3-1(a) is the most common AC/DC power supply. After the AC power supply voltage is passed through the transformer, it is converted into the required voltage, which is rectified to become 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 the ripple voltage, and eliminate the AC noise generated by the power supply
The operating voltages of various batteries vary within a certain range. In order to ensure a constant voltage output from the battery pack, a low-dropout linear regulator should normally be connected to the output of the battery pack, as shown in Figure 3-1(b). Low-dropout linear regulators have lower power, which extends battery life. At the same time, since the output voltage of the low-dropout linear regulator is close to the input voltage, the output voltage can be stabilized when the battery is nearly discharged
As we all know, the efficiency of the switching power supply is very high, but the output ripple voltage is high, the noise is large, and the voltage regulation rate is also poor. Especially when the analog circuit is powered, it will have a greater impact.Connect the low-dropout linear regulator to the output of the switching regulator, as shown in Figure 2-3(c), to achieve active filtering, and also greatly improve the regulation accuracy of the output voltage, while the power system The efficiency will not be significantly reduced
In some applications, such as radio communication equipment, which is usually powered by only one battery, each part of the circuit is often isolated from different voltages, so it must be powered by multiple regulators. In order to save the power of the common battery, it is generally desirable that the low dropout linear regulator operates in a sleep state when the device is not operating. To this end, linear regulators are required to have an enable control terminal. A power supply system with a single battery-powered multi-output and on-off control function is shown in Figure 3-1(d)
Figure 3-1 Typical Application of Low Dropout Linear Regulator (LDO).
DC-DC
It should be understood that DCDC means DC (to) DC (conversion of different DC power values), as long as it meets this definition, it can be called DCDC converter, including LDO. However, the general statement is that the device that converts DC to DC by switching is called DCDC
The DC-DC converter includes circuits such as boost, buck, boost/buck, and invert. The advantages of the DC-DC converter are high efficiency, high current output, and low quiescent current. As integration increases, many new DC-DC converters require only a few external inductors and filter capacitors. However, the output pulsation and switching noise of such power controllers is relatively large and relatively high in cost. In recent years, with the development of semiconductor technology, the cost of surface mount inductors, capacitors, and highly integrated power control chips has been decreasing and the volume is getting smaller and smaller. Since a MOSFET having a small on-resistance can output a large amount of 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 an NFET on the chip. Secondly, for small and medium power applications, low cost and small packages can be used. In addition, if the switching frequency is increased to 1 MHz, the cost can be reduced, and a smaller-sized inductor and capacitor can be used. Some new devices also add many new features, such as soft-start, current limit, PFM or PWM mode selection
In general, boost is a must choose DCDC, buck, is to choose DCDC or LDO, to compare in cost, efficiency, noise and performance
LDO compared to DC/DC:
First of all, in terms of efficiency, DC/DC efficiency is generally much higher than LDO, which is determined by its working principle. Secondly, DC/DC has Boost, Buck, Boost/Buck, (someone also classified ChargePump as This class). The LDO is only a buck type
Again, it is also very important that DC/DC has a large power supply noise due to its switching frequency, which is much larger than LDO. You can pay attention to the PSRR parameter. So when considering the more sensitive analog circuit At that time, it is possible to sacrifice efficiency to ensure the purity of the power supply and choose LDO
Also, the peripheral devices required by LDOs are usually simple and occupy a small area. DC/DC generally requires inductors, diodes, large capacitors, and some MOSFETs, especially Boost circuits. The operating current, the reverse recovery time of the diode, the ESR of the large capacitor, etc., are more complex than the LDO from the choice of peripheral devices, and the area is correspondingly much larger