Board layout and component placement

The LT8640 is a silent switcher series of monolithic DC/DC converters with built-in power elements. It is a power element built-in type, and the layout design and parts arrangement are easy. It is recommended to mount the input and output capacitors and inductors on the same side as the LT8640 for low noise power supply design.

The most important thing is the placement of the input capacitor. Figure 1 is a reference layout for the LT8640. The part circled in red is the input capacitor, and you can see that it is placed as close as possible to the LT8640.

The reason for arranging the input capacitor as close as possible to the IC is to reduce the loop in which high di/dt current flows, drawn in red and called a hot loop by Analog Devices in Figure 2. . To reduce EMI noise, it is important to keep this hot loop small. To make the hot loop smaller, the input capacitor should be placed closer to the IC.

If the hot loop becomes large, even if a device such as the LT8640 designed to reduce EMI noise is used, the noise will increase due to the increase in the parasitic inductance component in the hot loop. Therefore, it is important to place the input capacitor physically close to the LT8640, but it is important to lay it out so that there is no parasitic inductance.

parts used

◎Leadless

A leadless type package is desirable for the components used. Packages with leads always have parasitic inductance, which causes noise. It is recommended to use leadless surface mount type components for passive components used around power supply ICs and modules.

 

In addition, noise can be reduced by selecting leadless type packages such as QFN and LGA/BGA types for power supply ICs and modules.

 

◎ flip chip

If a bonding wire is used inside the IC, the inductance component of the wire will remain. Furthermore, in order to reduce the inductance component, it is desirable to have a flip-chip structure inside the IC package.

 

Silent switcher series products from Analog Devices use a leadless package with a flip-chip structure instead of a bonding wire structure.

Power supply IC performance

Even if you pay attention to the structure of the parts used and pay attention to the layout of the designed circuit diagram, if the design concept of the power supply IC or module you initially selected does not aim for low noise, the power supply circuit will result in louder noise.

 

Figure 4 shows the switching voltage waveforms of a product with EMI countermeasures (light blue) and a product without countermeasures (purple). The voltage waveform without EMI countermeasures shows a large overshoot. This is due to the fact that the rise of the switch voltage is becoming faster due to the higher efficiency. Silent switcher series products have countermeasures inside the IC to prevent overshoot while achieving high efficiency (see EMI countermeasure product waveform in Figure 3).

 

In addition, two power supply input pins (VIN) are provided and a dual hot loop structure suppresses radiation noise. We will explain in detail in the next demo video how to suppress radiation noise.

 

Silent Switcher product description and demo videos

Summary

In order to design a low-noise power supply circuit, please pay attention to three points. It is important to pay attention to the selection of components used in power supply ICs and peripheral circuits, and to be very careful with layout design.

Recently, a new type of DC/DC converter IC with high efficiency and low noise countermeasures has been developed. If you are designing with a familiar IC and are worried about noise countermeasures every time, why not consider a new type of IC such as the Silent Switcher series?

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