In electronic products, we often see DC/DC and LDO. In PCB, the major categories are relatively clear, mainly linear power supply and switching power supply. Linear power supply mainly uses LDO power supply, and switching power supply is what we usually say DC-DC power chip. But strictly speaking, the linear power supply cannot be equal to the LDO power supply. The LDO power supply chip is only one of the linear power supplies, but it is named after it has a relatively low adjustment tube voltage difference.

DC/DC converts a DC input voltage into another DC output voltage. Commonly used are boost, buck, boost and inverting structures. If it is used in voltage drop In a relatively large case, choose DC/DC because of its high efficiency, while LDO will lose a large part of its efficiency due to the large voltage drop; the energy loss is relatively small during the step-down process, and the chip heating is not obvious. The chip package is relatively small and can realize PWM digital control.

LDO is the abbreviation of low dropout voltage regulator, which is a low dropout linear regulator. They all stabilize an input voltage to a certain voltage. LDO can only be used as a step-down output. If the voltage drop is relatively small, choose LDO, because The noise is low, the power supply is clean, the peripheral circuit is simple, and the cost is low.

The difference between the power chip DC/DC and LDO:

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, Boost/Buck, (some people also classify Charge Pump into this category). And LDO only has 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.

The following parameters should be paid attention to when selecting the power supply IC:

1. Output voltage: DC/DC output voltage can be adjusted through feedback resistance, LDO has two types of fixed output and adjustable output;

2. Input and output voltage difference: The input and output voltage difference is an important parameter of the LDO. The output current of the LDO is equal to the input current. The smaller the voltage difference, the smaller the internal power consumption of the chip and the higher the efficiency.​​

3. Maximum output current: The maximum output current of LDO is generally several hundred mA, while the maximum output current of DCDC is several A or even greater.

4. Input voltage: Different chips have different requirements for input.

5. Ripple/Noise: The ripple/noise of DC/DC is worse than that of LDO due to DC/DC working in switching state. Therefore, LDO should be selected as power supply for sensitive circuits during design.​​

6. Efficiency: If the input and output voltages are close, the relative efficiency of LDO is higher than DC/DC. If the voltage difference is large, DC/DC is selected. Because the output current of LDO is basically equal to the input current, the voltage drop is too large, and the LDO is consumed. The energy is too large, the efficiency is not high.​​

7. Cost, peripheral circuit: LDO has lower cost than DCDC, and the peripheral circuit should be simple.

When choosing a DC-DC chip, avoid close to sensitive weak signals and avoid directly supplying power to such circuits. The switching frequency of DC-DC operation should also be considered in the design to avoid the switching frequency directly or indirectly interfering with the signal through frequency mixing. Under uncertainty, it is best to connect the synchronization signal SYNC to a controllable PWM to adjust the work in different at the switching frequency.

In recent years, with the development of semiconductor technology, the cost of surface-mounted inductors, capacitors, and highly integrated power control chips has been continuously reduced, and the volume has become smaller and smaller. Due to the emergence of MOSFETs with small on-resistance, The output is very large, so 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 be reduced, smaller inductors and capacitors can be used, and some new devices have added many new functions, such as soft-start, current limiting, 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.