Comparative analysis of LDO linear regulator and DC/DC device
LDO is a low dropout linear regulator, compared with the traditional linear regulator. Traditional linear regulators, such as 78xx series chips, require input voltage to be more than 2v~3V higher than output voltage, otherwise they will not work properly. However, in some cases, such conditions are obviously too harsh, such as 5V to 3.3v, the pressure difference between input and output is only 1.7v, which is obviously not satisfied with the conditions. In view of this situation, there is LDO power conversion chip.
LDO is a linear regulator. Linear regulators use transistors or FETs operating in their linear regions to subtract excess voltage from the input voltage of the application and generate regulated output voltage. The so-called voltage drop voltage refers to the minimum difference between the input voltage and the output voltage required by the regulator to maintain the output voltage within 100 mV above and below its rated value. LDO regulators with positive output voltage usually use power transistors (also known as transfer devices) as PNPs. This transistor allows saturation, so the regulator can have a very low voltage drop, usually about 200 mV; compared with the traditional linear regulator using NPN compound power transistor, the voltage drop is about 2 V. Negative output LDO uses NPN as its transfer device, and its operation mode is similar to that of positive output LDO.
The updated development uses the CMOS power transistor, which provides the lowest voltage drop voltage. With CMOS, the only voltage drop through the regulator is caused by the ON resistance of the load current of the power supply equipment. If the load is small, the pressure drop produced in this way is only tens of milli.
DCDC means DC to DC (conversion of different DC power supply values). DCDC converters, including LDOs, can be called as long as they meet this definition. But the general saying is that the DC conversion (DC) device is called DCDC by switching mode.
LDO is the meaning of low voltage drop, which has a paragraph to explain: low cost, low noise, low static current of low voltage drop (LDO) linear regulator, these are its outstanding advantages. It requires very few external components, and usually only needs one or two bypass capacitors. The new LDO linear regulator can achieve the following targets: output noise of 30 mV, PSRR of 60 dB, static current of 6 mA, voltage drop of only 100 mV. The main reason why the performance of LDO linear regulator can reach this level is that the regulator is P-channel MOSFET and the common linear regulator is PNP transistor. P-channel MOSFET is voltage-driven and does not need current, so it greatly reduces the current consumed by the device itself; on the other hand, in order to prevent the PNP transistor from entering saturation state and reduce the output capacity, the voltage drop between input and output can not be too low; on the other hand, the voltage drop on P-channel MOSFET is too large. The output is equal to the product of output current and on resistance. Because the MOSFET on resistance is very small, the voltage drop on it is very low.
If the input voltage and output voltage are very close, it is better to choose LDO regulator, which can achieve high efficiency. Therefore, in the application of lithium ion battery voltage to 3V output voltage, LDO regulator is used mostly. Although 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 is not very close to the output voltage, we should consider using the switch-type DCDC. It should be known from the above principle that the input current of LDO is basically equal to the output current. If the voltage drop is too large, the energy consumption on LDO is too large and the efficiency is not high.
The DC-DC converter includes boost, buck, up / down and reverse circuits. The advantages of the DC-DC converter are high efficiency, high output current and small quiescent current. With the improvement of integration, many new DC-DC converters only need a few external inductors and filter capacitors. However, the output pulse and switching noise of this type of power controller are large and the cost is relatively high.
In recent years, with the development of semiconductor technology, the cost of surface mounted inductors, capacitors and power control chips with high integration has been decreasing and the volume has become smaller and smaller. Because MOSFETs with small on-resistance can output large power, they do not need external high-power FETs. For example, for the input voltage of 3V, the output of 5V/2A can be obtained by using NFET on chip. Secondly, for small and medium power applications, low cost and small encapsulation can be used. In addition, if the switching frequency is increased to 1MHz, the cost can be reduced and smaller size inductors and capacitors can be used. Some new devices also add many new functions, such as soft start, current limiter, PFM or PWM mode selection.