Electronic Technology Decrypted: Simplifying FPGA Power Supply Design

　　FPGA is a chip with multiple power requirements, mainly three types of power requirements:

　　1.Vccint core operating voltage

　　Generally, the voltage is very low, and the commonly used FPGAs are around 1.2V. To power various internal logics of the FPGA, the current ranges from several hundred milliamperes to several amperes, depending on the working clock rate of the internal logic and the occupied logic resources. For this power supply, the load is a highly capacitive impedance, which has high requirements for the transient response of the power supply. In addition, due to the low driving voltage and high working current, it is very sensitive to the wiring resistance of the PCB. Special attention should be paid to the trace width to minimize the loss caused by the wiring resistance.

　　2.Vcco I/O driver power supply

　　FPGAs often need to communicate with multiple chips with different level interfaces, so they usually support a lot of level standards. Vcco is used to power the I/O driver logic of the FPGA. Different level standards require different I/O voltages to correspond. For details, please refer to the relevant chip manuals of the FPGA. For example, for the Xilinx Spartan6 series, you should check the Xilinx official document, number ds162.pdf, which has detailed instructions in 7 table7 Recommended Operating Conditions for User I/Os Using Single-Ended Standards. In addition, in order for FPGAs to communicate with multiple different level standard interface chips at the same time, Vcco is usually bounded by BANK and is independent of each other. In other words, there are several different I/O voltages on an FPGA chip at the same time. Of course, only one I/O voltage can exist in the same BANK.

　　3.Vccaux auxiliary power supply

　　FPGA is not just a digital logic chip. It also has some analog components inside, such as Xilinx's DCM digital clock management component. These analog components are very sensitive to the power supply rejection ratio (PSRR), that is, power supply noise, or power supply ripple, so usually an independent power supply is used. The current demand of this power supply is generally not large, but the tolerance to power supply noise is very low. Therefore, the purity of its power supply should be improved as much as possible. For example, instead of directly using a switching power supply, use LDO voltage regulation before supplying Vccaux. In addition, Vccaux also supplies power to some I/Os, such as function selection pins, JTAG, etc. For details, please refer to the power supply instructions in the chip manual.

　　Since there are so many power requirements, some power design issues have to be considered when designing the power supply.

　　There are several power supply design aspects to consider when powering an FPGA:

　　1. Increased number of output voltage rails

　　2. Need to set the set point accuracy for the rail

　　3. The passive board layout needs to be optimized in the design to achieve extremely low ripple noise

　　4. AC transient response and compensation loop are required

　　Also, don't forget sorting and more.

　　Figure 1 shows a typical FPGA power solution in an FPGA development kit. Designing this solution requires some additional expertise in addition to selecting the right components and inductors. For example, details of component placement and board layout need to be considered.

Figure 1. Typical FPGA power solution

　　So, how can we simplify power supply design?

　　Fortunately, there are a variety of solutions that can help simplify things. We highlight two innovative techniques that can help you achieve your design goals quickly and easily.

　　Simplified power supply design method 1:

　　The technology that needs to be highlighted is the WEBENCH FPGA Architect design tool (schematic design plus simulation results) used to create FPGA power solutions

　　WEBENCH FPGA Architect design tool helps you:

　　Select the FPGA vendor you want, select the FPGA product family you will use, and meet the power requirements defined by the FPGA Power Estimator tool provided by the FPGA vendor.

　　Following these few simple steps will create an FPGA power solution that optimizes product size, efficiency, and cost. This easy-to-use tool helps you overcome initial hurdles and reduce design time from days to minutes, giving you a leading edge.

　　Simplifying power supply design method 2: integrated inductor power module, which can effectively simplify the design solution

　　Power modules primarily integrate inductors in a single IC package, but can also integrate other external components such as FETs, compensation loops (if required), and feedback resistors. Power modules have rich features such as enable and power good pins for sequencing, and require minimal external components such as input capacitors, output capacitors, and possibly a feedback resistor. This module helps simplify the power solution of Figure 1 to that of Figure 2. Integrated inductor power modules can be used to power FPGAs and are considered a suitable power solution by FPGA vendors. In addition, these modules help simplify board layout, helping you complete your design faster than ever before.

　Figure 2. Integrated inductor power module using LMZ30602