In the world of PCB design, engineers often have a wealth of knowledge to share—whether it's experience, tips, or even stories from past projects. These insights can be incredibly valuable, especially when they come from seasoned professionals who've seen it all. In this article, we'll dive into some key PCB layout rules that are essential for anyone working with high-speed converters.
One of the most common questions is whether AGND and DGND ground planes should be separated. The short answer is: it depends. However, in most cases, separating them isn't recommended. Why? Because it increases the inductance of the return current path, which can lead to more noise than benefit. As the formula V = L(di/dt) shows, higher inductance means higher voltage noise, especially as switching currents increase with faster sampling rates. Therefore, connecting the ground planes together is usually the better approach.
There are exceptions, though. In some applications where traditional design constraints or size limitations make it difficult to achieve good layout segmentation, separating the ground plane might be necessary. But even in those cases, it's important to connect the separate ground planes through bridges or connection points across the board. These points should be evenly distributed to ensure smooth return current paths without degrading performance. Usually, the best place for this connection is near or under the converter itself.
When designing power planes, it's crucial to use as much copper as possible on each layer. Avoid sharing traces between power layers, as this can fragment the power plane and reduce its effectiveness. A sparse power plane forces current to take a longer path, increasing resistance and causing a small voltage drop at the converter’s power pin. Additionally, the placement of power planes matters—don’t mix high-noise digital power supplies with analog power planes, as they can still couple despite being on different layers.
Another critical aspect of PCB design is the Power Delivery System (PDS). This is often overlooked but plays a vital role in ensuring stable power delivery to all components. A well-designed PDS minimizes voltage ripple by providing a low-impedance path for current. For example, if the switching current is 1A and the PDS impedance is 10mΩ, the maximum voltage ripple would be 10mV.
To optimize the PDS, consider using a six-layer PCB stack with a top signal layer, first ground layer, first power layer, second power layer, second ground layer, and bottom signal layer. The first ground and power layers should be close together, spaced about 2–3 mils apart to form a laminar capacitor. If the power plane must be split, use the largest possible area for each VDD rail and avoid leaving holes. Adding more ground planes between power layers can further enhance the inherent capacitance of the stack.
Decoupling capacitors placed at the power entry point and around the DUT (Device Under Test) help maintain a low PDS impedance across the frequency range. Using capacitors ranging from 0.001μF to 100μF covers most needs. Don’t overdo it—placing capacitors everywhere may violate manufacturing rules. If such measures are required, there may be deeper issues in the circuit.
The exposed pad, found under many modern high-speed ICs, is another critical feature. It connects the chip’s internal grounding to the PCB. Ensuring a solid electrical and thermal connection is essential for both performance and heat dissipation. Here are three steps to achieve the best connection:
1. Replicate the exposed pad on every PCB layer to create a thicker thermal path, especially for high-power devices. This also provides a good equipotential bond for all ground planes.
2. Divide the exposed pad into multiple sections, ideally in a checkerboard pattern. This ensures uniform connectivity during reflow, avoiding uneven or weak connections.
3. Ensure each section has a via connected to ground. Fill these vias with solder paste or epoxy before assembly to prevent the exposed pad solder from flowing back into the via hole.
Finally, cross-coupling between layers is often underestimated. Even with a 40-mil spacing, adjacent layers can act like a capacitor, allowing signals to couple between them. This can introduce unwanted noise, especially in high-resolution systems. Ignoring this coupling might not cause immediate failure, but it's something to be aware of, particularly when debugging sensitive designs.
In summary, PCB design requires careful attention to details like ground planes, power delivery, exposed pads, and layer coupling. By following these best practices, you can improve the performance and reliability of your high-speed converter circuits.
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