Narrow down by specifying conditions
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This is Sasaki in charge of power supply dock technology.
In this column, I would like to introduce some examples of problems related to power supplies that I encountered multiple times during various customer support sessions.
As the number of power supplies for FPGAs increases, the number of capacitors placed on power lines is also increasing.
This time, I would like to talk about questions I received regarding capacitor placement in power lines.
Are you careful about the placement of capacitors?
It is said that the capacitor should be placed close to the FPGA, but due to board constraints, the 100uF capacitor must be placed away from the FPGA.
Is there a problem?
Yes, large capacitors such as 100uF are called bulk capacitors, and they don't need to be close to the FPGA, as long as they are in the power line.
However, it is necessary to select one with a large capacity so that there is no problem even with some wiring inductance, so that a voltage drop does not occur when the current consumption value of the FPGA suddenly changes.
FPGA It doesn't have to be recent, right?
So, is it okay to put a 0.1uF or 10pF capacitor in the power line?
No, please place a small capacitor such as 0.1uF or 10pF close to the FPGA.
Also, when placing capacitors, place smaller capacitors closer to the FPGA, as shown in the following figure.
In particular, it is desirable to place a capacitor with low ESL/ESR near the FPGA.
Why would you put the smaller ones closer to the FPGA?
The further away you are from the FPGA, the greater the inductance component due to the wiring on the board.
This cancels the capacitance component of the capacitor, resulting in the same state as if no capacitor was placed.
That's right.
Now, place the small capacitor close to the FPGA.
Recently, DDR may malfunction due to the impedance of the "anti-resonance point" which changes due to differences in capacitor placement and constant selection.
We will explain this in detail at power integrity seminars, etc., so please join us.
The placement of the capacitor also requires some consideration.
I will also consider attending a seminar.
The point of this time
- Place small capacitors as close to the FPGA as possible.
・Capacitors with large capacitance and size such as 100uF can be placed away from the FPGA (as long as they are in the power line).
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