In hardware design, we often need to convert 12V power supply to 5V power supply for use, or convert 5V power supply to 3.3V for use. At this time, we need a voltage conversion chip. In the previous article, I also briefly introduced a voltage conversion chip, that is, low dropout linear regulator, which is called LDO for short. It is widely used. As long as you have touched the hardware circuit, you must have used it directly or indirectly. Its biggest problem is that it is easy to get hot under the condition of high voltage difference and large current.

In fact, in addition to LDO, there is also a more efficient voltage conversion mode with lower heating. This is the switching regulator, which is called DCDC for short in English. It is generally like this. Relatively speaking, the circuit will be more complicated. The most obvious feature is that there is an inductance in the circuit. Today, let’s talk in detail about the advantages and disadvantages of LDO and DCDC, and which one you should choose as your own voltage conversion scheme in the design.

First, let’s briefly introduce the principles of the two chips. For LDO, its interior can be seen as a small person, an adjustable resistor and a voltmeter. The little man is always observing the voltage value of the output. When the output voltage is higher than 5V, the villain will increase the value of the adjustable resistance. When the output voltage is lower than 5V, the villain will reduce the value of the adjustable resistance. By constantly adjusting the value of the adjustable resistance, the output voltage can be guaranteed to be constant. The biggest disadvantage of this method is that the excess 7V voltage is actually consumed by the adjustable resistor, so the heating of the adjustable resistor may be very large, and the overall conversion efficiency will be very low. Inside the DCDC, we can also regard it as a villain, a voltmeter. It just says that the adjustable resistor is replaced by a switch. At this time, the villain will do two things. The first thing is that he will switch periodically. This switch will generate a PWM wave. The second thing is to keep looking at the voltmeter. When he finds that the output is higher than 5V, he will turn off the switch for a little longer, that is, the low-level PWM wave will last a little longer. When he finds that the output voltage is lower than 5V, he will make the switch close for a longer time. That is to say, the high level of PWM wave lasts a little longer, so that the output voltage can be stabilized at 5V. In this case, the excess 7V voltage is actually blocked by the switch, so the chip itself does not need to bear the excess voltage, so the heat will be much smaller. Here we can see the advantages and disadvantages of DCDC and LDO.

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We can evaluate it from the following dimensions: the first is the output power linear regulator, which is not suitable for scenarios with high input and output voltage difference and large current. If so, the chip itself will be seriously heated, but DCDC has no such limitation. Therefore, LDO can be considered if DCDC low-voltage differential current is selected for high-voltage differential current.

The second dimension considered is the smoothness of the output voltage, that is, the ripple of the power supply. For the output of a power supply, we certainly want it to be as stable as possible. Such a choppy output is obviously not appropriate.

Because of the periodic switching of DCDC, the output stability will be affected more or less, and the ripple will be slightly larger. The output of LDO is very stable, and the ripple will be very small, so LDO will be better at this point.

The third is efficiency. At present, the efficiency of DCDC is basically more than 80%. Well, it can reach 95%. How can we understand that? For example, you need a 5v2a output, that is, the output power is 10W. If you use the DCDC scheme, you only need to use a 12V1A input power supply. In this way, the input current will be much smaller than the output current.

For LDO, the efficiency is basically equal to the output voltage divided by the input voltage multiplied by 100%, so the greater the voltage difference, the lower the efficiency. Take 12V to 5V as an example, its efficiency is basically less than 42%. Therefore, if you need to make the LDO output 2a, you need to select at least one 12v2a input power supply, which is obviously far less cost-effective than DCDC.

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The next dimension to consider is temperature, which can be seen from the efficiency. All the parts except the efficiency are used for heating, so the heating of LDO is much larger than that of DCDC.

The fifth dimension is the consideration of cost. The overall cost of DCDC is higher than that of LDO because the process of DCDC is relatively complex and more peripheral components are required to cooperate with it.

The sixth dimension is the perspective of usability. LDO’s words have been popularized for a long time, and even textbooks have explanations about them. Moreover, there are few peripheral components of LDO.

In the simplest case, you only need one input capacitor and one output capacitor. There are many peripheral components of switching power supply, and because PWM wave will be generated, more factors should be considered for PCB wiring. At this point, LDO has the advantage.

The last point is static power consumption. Even without any load, these power chips themselves will consume some power, which is called static power consumption. Of course, the static power consumption is generally very small, which can be ignored in most scenarios. However, some scenarios require low power consumption. For example, smart watches require a long standby time, so static power consumption needs to be considered. Generally speaking, LDO is easy to make the static power consumption very low, which can be less than 10uA or even lower. What is the concept of 10uA? Even if it is powered by a small 500mah battery, it can be used for 5.7 years. So we use LDO in many low-power scenarios. These are the differences between DCDC and LDO that I have summarized. We can see that they are almost the DC complement relationship. The advantages of LDO are the disadvantages of DCDC, and the disadvantages of LDO are the advantages of DCDC.

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Of course, in most cases, these factors do not need to be considered, because not all projects need to consider these factors. Seeing this, I believe you should have some concepts when you choose the power supply scheme in the future.

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