Comparison between LDO and BUCK step-down regulators

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Comparison between LDO and BUCK step-down regulators [Copy link]

In the control system designed with MCU/DSP/FPGA, the commonly used power chips are BUCK (step-down) switching regulator and LDO (low dropout) linear regulator in the circuit with low voltage input stage (generally below 12V) and output of 5V/3.3V/1.8V/1.5V/1.2V. These two power chips have their own advantages and disadvantages in application, and they need to be used selectively according to the actual situation when designing the circuit. 1. Comparison between LDO and BUCK step-down regulators 1. When the input voltage is high (generally >5V) and the input-output voltage difference is large, a BUCK switching regulator is required. In this case, a switching power supply chip is used, which has high efficiency and low heat generation. If a linear regulator is used, the input-output voltage difference is too large, and this part of the power is consumed, resulting in low efficiency and huge heat generation, and an additional large heat sink is required. When the input voltage is below 5V, LDO linear regulators are preferred. The characteristics of this type of chip are low cost. If cost and high requirements are not considered, switching regulator chips can also be used. 2. When the output current of the board-level output power supply is >1A, it is advisable to use a BUCK switching regulator. There are many types of such chips, so I will not list them one by one here; when the output power supply is below 1A, it is best to choose an LDO chip. Using a switching regulator is a waste of resources, haha. 3. The output ripple and voltage stability of the BUCK switching regulator are not as good as those of the LDO, so core power supplies such as MCU/DSP/FPGA (1.2V, 1.5V, 2.5V, etc.) generally choose LDO. You can read more about TI's power management chip manual, which contains many recommended chip models for different processors. In addition, when the input voltage is very high or the input/output voltage difference is large, and the output current is relatively large, the "BUCK+LDO" solution can be used. This solution is relatively easy to implement on a general control board and the cost is not high. 4. In terms of the complexity of circuit design: BUCK switching regulator circuits require external inductors, which are relatively large in size. Some also require external MOS tubes. Circuit design and debugging take a certain amount of time, unless it is a mature design accumulated in the early stage; while LDO circuits are very simple, and their peripheral circuits only require a few filter capacitors. 5. The conversion efficiency of BUCK switching regulators is higher than that of LDO, and its thermal characteristics are also better than LDO; when designing circuits, when the output voltage accuracy is required to be very high, LDO must be used to achieve it. Summary: Only by combining the two can a stable power supply circuit that is considered perfect be obtained. II. Precautions for the application of LDO and BUCK buck regulators 1. What is the most common cause of LDO oscillation? It is the output capacitor! A. ESR is too high. Poor quality tantalum capacitors will have high ESR, so imported devices are generally used. Aluminum electrolytic capacitors will have high ESR under low temperature conditions, so tantalum capacitor devices are generally used. B. ESR is too low. In the circuit, it is best to use well-known brand SMD devices. 2. One of the reasons for the serious heating of the BUCK switching regulator chip is the inductor. We know that the choice of inductor is determined by the load resistance, operating frequency, output voltage (duty cycle) and working efficiency, not the larger or smaller the better. At present, some purchased SMD inductors are prone to the situation where the capacity does not match the label, causing serious heating of the chip. The solution is to use a direct-insert columnar inductor. Three, power supply design sharing The first figure is a LP3965-ADJ LDO power supply design. You can see that a large ground plane is designed in the circuit as a heat sink. The second figure is a LM2596 step-down switching power supply design. You can see that the chip area of the switching power supply is only 1/2, and the loss is greatly reduced, which means that the switching power supply can withstand higher thermal resistance and reduce the area of heat dissipation.