Many years ago, the majority of the board designs that I was involved with were simple digital designs. Placing and routing bypass capacitors in those designs was very easy, and often the circuitry was slow enough that ceramic capacitors were not needed. Most of the time we considered bypass capacitor placement and routing as more of an afterthought than as a primary design consideration. I remember how some of the engineers even instructed me to “sprinkle” bypass capacitors here and there as if they were little more than candied decorations on a cake. There didn’t seem to be much concern about how many to use, where they should be placed, or how they should be connected.
Later on, as designs grew in complexity and speed, the need for designing a more refined power distribution network (PDN) on the circuit board increased as well. Now with the signal integrity requirements of a high speed design, a precisely designed PDN has become imperative for the best performance of the circuit board.
Let’s look at what some of the bypass capacitor placement requirements are in the PDN of a high speed design, and what kind of placement and routing strategies will best help you.
What does this capacitor do?
What are the bypass capacitors doing?
The processors and other IC’s on a high speed PCB will demand sharp spikes of current that the power supply cannot deliver. Power supplies are designed to supply a level amount of power across the entire board, rather than short bursts. To solve this issue, a bypass capacitor is can be placed near the IC to supply the required current for these quick spikes. A bypass capacitor does this by storing power and then discharging it to the IC when it requires excess current. This gives the power supply time to respond. Following the spike, the bypass capacitor is recharged and ready for the next cycle.
Bypass capacitors are also important for reducing the ground bounce that can come from digital devices that have faster switching times. Bypass capacitors also are used to filter the low-frequency noise caused by the power supply and are helpful with other signal integrity and EMI issues as well.
How many bypass capacitors should be used?
The amount of bypass capacitors needed for a design will depend in relation to the parts that they are assigned to, and how many of those parts are being used. Bulk capacitors in the 10uF range are usually used for each voltage drop on the board. They should be positioned where the voltage is developed or where it enters the board. On some devices, they are used in conjunction with high speed bypass capacitors.
In general, at least one high speed bypass capacitors in the 0.1uF range should be placed by each IC. They should be placed as close as possible to their respective IC to supply current immediately. I recommend that devices with multiple power pins should have at least one bypass capacitor for each power pin. Although this will use up more board space, it can significantly help to reduce ground bounce.
Capacitors and other parts at work on a PCB
Bypass capacitor placement and routing best practices
As I mentioned earlier, you should place your bypass capacitors as close as possible to the device that they are assigned to. This can be underneath the device on the opposite side of the board, or just off the pins that the bypass capacitor is connected to on the same side of the board.
For circuits requiring multiple bypass capacitors placed near the power pin of a specific device, the capacitors should be placed next to that pin in ascending order of value. For instance, if both a .01uF and a 10uF capacitor are specified for a specific device, place the .01uF closest to the device with the 10uF outboard of that. In this way, the larger bulk capacitor will recharge the high-frequency capacitor that is closest to the device pin.
When routing a bypass capacitor, you should start from the power or ground pin of the device and go directly to the capacitor pin. From there the route can continue on to a via connecting it to the power or ground plane. You should also use as short and wide traces as possible to connect the bypass capacitor, and use as many vias to connect to the power or ground plane as possible.
The placement and routing of your bypass capacitors for the best possible power distribution network is an important part of your high speed PCB design. The tips that have been discussed here will give you a good start, and there are other resources available to you as well. PCB software, like Altium Designer, has DC analysis tools, like their PDN Analyzer. These let you analyze and resolve power distribution network issues during the layout of the PCB design before you build an actual board.
Would you like to find out more about how Altium can help with the current loop layout of your high speed PCB design? Talk to an expert at Altium.
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