The LDO is a Low Dropout Regulator, which means a low dropout linear regulator, compared to a conventional linear regulator. Traditional linear regulators, such as the 78xx series, require input voltages that are 2v~3V higher than the output voltage, otherwise they will not work properly. However, in some cases, such conditions are obviously too harsh, such as 5v to 3.3v, and the pressure difference between input and output is only 1.7v, which is obviously not satisfied. For this situation, LDO-like power conversion chips are available.
LDO is a linear regulator. Linear regulators use transistors or FETs that operate in their linear region and subtract the excess voltage from the applied input voltage to produce a regulated output voltage. The voltage drop voltage is the minimum value of the difference between the input voltage and the output voltage required by the regulator to maintain the output voltage within 100 mV of its rated value. Positive output voltage LDO (Low Dropout) regulators typically use power transistors (also called transfer devices) as PNPs. This transistor allows saturation, so the regulator can have a very low dropout voltage, typically around 200mV; compared to a conventional linear regulator using NPN composite power transistors, the voltage drop is around 2V. The negative output LDO uses NPN as its pass device and its operation mode is similar to the PNP device that is outputting LDO. The newer development uses CMOS power transistors, which provide the lowest voltage drop voltage. With CMOS, the only voltage drop across the regulator is due to the ON resistance of the power device's load current. If the load is small, the pressure drop produced by this method is only tens of millivolts.
DCDC means DC change (to) DC (conversion of different DC power values), so long as it meets this definition it can be called a DCDC converter, including an LDO. But the general statement is that the device that converts DC to DC by switching is called DCDC.
LDO is the meaning of low voltage drop, which has a description: Low-dropout (LDO) linear regulator has the advantages of low cost, low noise, and low quiescent current, which are its outstanding advantages. It requires few external components, usually only one or two bypass capacitors. The new LDO linear regulator achieves the following specifications: output noise 30μV, PSRR 60dB, quiescent current 6μA, voltage drop only 100mV. The reason why the performance of LDO linear regulator can reach this level is mainly because the adjustment tube is P-channel MOSFET, and the common linear regulator is PNP transistor. P-channel MOSFETs are voltage-driven, do not need current, so greatly reduce the current consumed by the device itself; on the other hand, in the circuit using PNP transistors, in order to prevent the PNP transistor from entering the saturation state and reduce the output capability, the input and output The voltage drop between them must not be too low; the voltage drop across the P-channel MOSFET is approximately equal to the product of the output current and the on-resistance. Since the MOSFET's on-resistance is small, the voltage drop across it is very low.
If the input voltage and output voltage are close, it is best to use an LDO regulator to achieve high efficiency. Therefore, LDO regulators are mostly used in applications that convert lithium-ion battery voltages to 3V output voltages. Although the last 10% of the battery's energy is not used, the LDO regulator can still ensure that the battery's working time is longer and the noise is lower. If the input voltage and output voltage are not very close, we should consider switching DCDC, it should be able to know from the above principle, LDO input current is basically equal to the output current, if the voltage drop is too large, consumed in the LDO The energy is too high and the efficiency is not high.
The DC-DC converter includes boost, buck, boost/buck and invert circuits. The DC-DC converter has the advantages of high efficiency, high output current, and low quiescent current. With the increase in integration, many new DC-DC converters require only a few external inductors and filter capacitors. However, the output ripple and switching noise of such power controllers are relatively large and the cost is relatively high.
In recent years, with the development of semiconductor technology, the cost of surface mount inductors, capacitors, and highly integrated power supply control chips has continuously decreased and the volume has become smaller and smaller. Since a MOSFET with a very small on-resistance can output a large amount of power, an external high-power FET is not required. For example, for a 3V input voltage, an output of 5V/2A can be obtained using an on-chip NFET. Second, for low- and medium-power applications, low-cost, small-sized packages can be used. In addition, if the switching frequency is increased to 1MHz, the cost can be reduced, and smaller inductors and capacitors can be used. Some new devices also add many new features, such as soft start, current limit, PFM or PWM mode selection.
In general, the booster must choose DCDC, step down, choose DCDC or LDO, compare in cost, efficiency, noise and performance.
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