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In automotive applications, the layout of the DC/DC converter is very important. Even if you design according to the reference circuit diagram in the datasheet, the device may not work or become noisy. Unfortunately, the time to notice these phenomena is after the board has already been made, and there are many cases where there is not much time to correct them. Many of these problems are often due to the layout of the board you created.
For example, regarding the layout of the DC/DC converter in Fig. 1, although it seems that there are no problems, problems may occur during mass production, so caution is required.
Verification with a demo board is recommended before starting the design
When using a DC/DC converter, we recommend that you first experiment with a demo board, etc. As a result, a normal waveform can be obtained, and the layout information of the demo board can be used as a reference to actually create a board for the application. It would be nice if the layout of the demo board could be used as is, but in reality it is often difficult to implement an ideal layout due to various restrictions (area, board rules of each company, etc.). Therefore, I will explain in advance what kind of layout is important in the operation of the DC/DC converter, starting with "Points of Layout."
Most important hot loop in layout
Automotive applications often require EMI such as CISPR25 Class5, and the hot loop is said to be the most important point in terms of layout.
Let's talk about hot loops. HI Side of the side FETs But ON and Low side of the side FETs But OFF During the on-cycle when AC Current flows through the red loop. Hi Side of the side FETs But OFF and Lowside of the side FETs But ON During the off-cycle when AC Current flows through the blue loop. Both currents will be trapezoidal waves. from scratch Ipeak switching back to zero AC Current flows only in the green loop, which is the most AC current and EMI energy increases. This green loop is called a hot loop.
For low EMI, the effect in the green loop should be as small as possible. The magnetic field strength in this loop is proportional to the switching current and the area of the loop.
Therefore, the first step in designing a layout for noise is to keep hot loops as small as possible. Copper spacing should be as small as design rules allow.
Use the shortest, flattest ceramic decoupling capacitors closest to the hot loop. Using several blocking or decoupling capacitors in parallel in the hot loop is fine.
layout point
When designing a DC/DC converter, be careful of hot loops.
Please also refer to the article "Three points of low noise power supply circuit design", which explains the LT8640, one of the LT86xx series, which is often used in automotive applications.
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