It should be understood that DC-DC means DC (to) DC (conversion of different DC power supply values), as long as it meets this definition, it can be called DC-DC converter, including LDO. But the general saying is that the DC change (to) DC by the switching mode of the device is called DC-DC.

1 What is LDO

    LDO is the meaning of low voltage drop, which has a description: Low voltage drop (LDO) linear regulator low cost, low noise, static current is small, these are its outstanding advantages. It also requires very few external components, usually only one or two bypass capacitors. The new LDO linear regulator achieves the following: output noise of 30μV, PSRR of 60dB, static current of 6μA, and voltage drop of only 100mV. The main reason why the performance of the LDO linear regulator can reach this level is that the regulator is a P-channel MOSFET, while the ordinary linear regulator is a PNP transistor. P-channel MOSFETs are voltage-driven and do not require current, so the current consumed by the device itself is greatly reduced. On the other hand, in a circuit using PNP transistors, in order to prevent the PNP transistor from entering a saturated state and reducing the output capacity, the voltage drop between the input and output can not be too low; The voltage drop on a p-channel MOSFET is roughly equal to the product of the output current and the on-resistance. Because the on-resistance of the MOSFET is small, the voltage drop on it is very low.

    If the input voltage and output voltage are very close, it is best to use the LDO regulator, which can achieve high efficiency. Therefore, LDO regulators are mostly used in applications that convert lithium-ion battery voltage to 3V output voltage. Although 10% of the battery's energy is not used at the end of the day, the LDO regulator can still ensure that the battery has a long working time and low noise.

    If the input voltage and the output voltage are not very close, it is necessary to consider the switch type DC-DC, because it can be known from the above principle that the input current of the LDO is basically equal to the output current, if the voltage drop is too large, the energy consumption on the LDO is too large, the efficiency is not high.

2 What is DC-DC

    DC-DC converters include boost, buck, boost/buck, and anti-equality circuits. The advantages of DC-DC converters are high efficiency, high output current and low static current. With improved integration, many new DC-DC converters require only a few external inductors and filter capacitors. However, the output pulsation and switching noise of this type of power controller are relatively high 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 control chips has been decreasing, and the size is getting smaller and smaller. Because the MOSFETs with small on-resistance can output a lot of power, there is no need for external high-power FETs. For example, for an input voltage of 3V, an output of 5V/2A can be obtained by using NFET on the chip. Secondly, for small and medium power applications, low cost small 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 limiting, PFM or PWM mode selection.

    In general, the boost must be selected DC-DC, buck, whether to choose DC-DC or LDO, to compare in cost, efficiency, noise and performance.

3 LDO compared with 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.

    Second, DC-DC has Boost,Buck,Boost/Buck,(some people also put the Charge Pump in this category). LDO only has a depressor type.

    Thirdly, it is also very important to point out that DC-DC power supply noise is very large because of its switching frequency, which is much larger than LDO. We can pay attention to the parameter PSRR. Therefore, when considering more sensitive analog circuits, it may be necessary to sacrifice efficiency to ensure the purity of the power supply and choose LDO.

    In addition, usually the peripheral devices required by LDO are simple and take up a small area, while DC-DC generally requires inductors, diodes, large capacitors, and some will also require MOSFETs, especially Boost circuits, which need to consider the maximum operating current of inductors, the reverse recovery time of diodes, the ESR of large capacitors, and so on. Therefore, the selection of peripheral devices is more complex than LDO, and the area will be correspondingly much larger