In this 10th issue, we will introduce the layout method when designing FPGA power supply circuits!
Basically, for any power supply, please refer to the Gerber data on the evaluation board. Please refer to the datasheet only for the explanation of layout notes. Note that the layout on the data sheet may be written in a simplified manner.
Step 1: Check the schematic diagram
Step 2: Design the power circuit including FPGA
Step 3: Layout Preparation and Checking Points to Note
Step 4: Board design
Step 5: Design verification
Step 6: Board fabrication and actual device verification
Step 1: Check the circuit diagram
First, check the schematic as a preparation. Prepare the schematic in the datasheet and the schematic of the demo board, and find a schematic that matches the desired specifications. If there is no schematic with the desired voltage setting, determine the peripheral constants based on the formula in the datasheet to complete the power circuit.
Step 2: Design the power circuit including FPGA
Next, modify the schematic to a circuit diagram suitable for your application. The entire power line connecting the power supply circuit to the FPGA is designed, including the power supply sequence of the FPGA.
We also design the power supply line for the FPGA, including output voltage settings, output voltage slope at startup, Enable operation, device-specific function settings, and more.
For FPGA power supply design, the capacitance calculated by the PDN tool is also reflected.
The output voltage setting is calculated and reflected by EPE.
The sequence of Enable, etc., is also taken into account.
Step 3: Layout Preparation and Precautions
Check the Gerber data on the evaluation board for the power circuit and prepare a layout for the power circuit alone. Carefully check the notes on the datasheet for precautions for the layout.
Figure 1 shows the µModule LTM4650 recommended layout by Analog Devices, Inc. Some of the layout notes in the datasheet are also shown next to it. Using this as a reference, design the layout of the power supply circuit using CAD.
Figure 1: LTM4650 Recommended Layout and Notes
**** Layout notes *****
- Place input and output high-frequency ceramic capacitors adjacent to the VIN, PGND, and VOUT pins to minimize high-frequency noise.
- Place a ground layer on the second layer.
- Use a number of vias for interconnections between the top layer and the other power layers to minimize via conduction losses and reduce thermal stress on the module.
- Do not place vias directly on top of pads unless they are filled or plated vias.
- Use a separate SGND ground copper area for components connected to signal pins; connect SGND and GND under the device.
Step 4: Board Design
Design the circuit and board with the FPGA and other peripheral devices in mind, as well as the power supply device.
When placing bead filters between the power supply circuitry and the FPGA, care should be taken to avoid placing the filters in the power supply feedback loop. Also, consider reviewing the constants of the voltage settings, since IR drop caused by the filter may cause the voltage accuracy of the power supply to deviate from that required by the FPGA.
If the voltage accuracy required by the FPGA is not met, consider a power system management device with a remote sense function for the power supply or a servo function to improve the accuracy of the power supply voltage.
Step 5: Design Verification
After this step, the board design should be verified.
If the SI/PI simulation identifies any problems with the quality of the power supply, repeat Step 2 through Step 5 to prevent any design problems. Steps 2 through 5 should be repeated to prevent problems in the design.
Step 6: Board fabrication and actual device verification
Once you have repeated Steps 2 through 5 and have come to the conclusion that the board design is optimal, you may proceed to board fabrication.
Finally, verify that there are no problems with the board by testing it on an actual device.
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