What are the design specifications for FPGA power supply design?

As a complex integrated circuit, the design of the power supply for the FPGA system has higher requirements than that of general electronic systems. It needs to have the characteristics of high precision, high density, controllability, high efficiency and miniaturization. This article systematically introduces the different characteristics of FPGA power supplies. At the same time, through examples, engineers will have a deeper understanding of the meaning of each feature, as well as FPGA specification constraints and their impact on power supply design, so as to quickly complete the power supply design of FPGA systems.

Preface

FPGA (Field Programmable Gate Arrays) is one of the most complex integrated circuits today. They use advanced transistor technology and chip architecture to achieve high-performance, small-volume high-end products, and the power supply for FPGA systems has higher requirements than general electronic systems.

With the popularization of FPGA system applications in the market, the demand for its power supply solutions is becoming increasingly strong. In addition to meeting the basic requirements, FPGA power systems also need to have high precision, high density, controllability, high efficiency and small size. characteristics.

In reality, engineers want to spend most of their time programming and not spend too much time and energy thinking about how to design appropriate power supplies. Therefore, in this article, we will introduce the different characteristics of FPGA power supplies, and at the same time, through examples, engineers will have a deeper understanding of the meaning of each feature, as well as FPGA specification constraints and their impact on power supply design.

Voltage accuracy

Core Power Supply is one of the most important key factors in balancing FPGA power consumption and performance. The acceptable voltage range is generally listed in the specification book, but this range is not a complete description. For FPGA, the power supply voltage also needs to be weighed and optimized while meeting the line operation requirements. Figure 1 below takes the core voltage requirements of Intel's Arria 10 FPGA as an example, and it also represents the voltage requirements of other FPGA cores. Generally, the rated voltage of the tolerance range is displayed. For example, the Arria 10 FPGA is ±0.03V. The FPGA will run very well within this voltage window, but the actual situation is much more complicated than what the picture shows.

In fact, FPGAs can operate at different voltages, depending on their special manufacturing tolerances and the specific logic design used. Even with the same voltage requirement, the static voltage required by one FPGA may be different from that of another FPGA. Therefore, when designing the power supply, the change between the dynamic and static voltage of the FPGA must be considered and adjusted accordingly.

Dynamic power and static power

The goal of designing an appropriate FPGA power supply scheme is to produce the appropriate performance levels to operate the programmed functions and reduce unnecessary power consumption. From a semiconductor physics perspective, both dynamic and static power increases significantly as core VDD increases, so our goal is to have enough voltage for the FPGA to operate properly to meet its timing requirements - because excess power consumption not only It does nothing to improve performance. On the contrary, it will cause the transistor leakage current to increase as the temperature rises, consuming more unnecessary power. For these reasons, it is imperative to optimize the design and operating point voltage.

This optimization process requires a very precise power supply to be successful. If the core voltage is lower than required, the FPGA may fail due to timing errors. If the core voltage drifts beyond the maximum specification, it can damage the FPGA, or it can create hold-time faults in the logic. Therefore, the power supply tolerance range must be considered to prevent all these situations and only the command voltage is guaranteed to remain within the specification limits.

The problem is that most power regulators are not accurate enough. The regulated voltage can be anywhere within a tolerance around the commanded voltage, and it can drift with load conditions, temperature, and aging. A power supply with a ±2% tolerance means that it can output any value within a 4% voltage range. To compensate for the possibility of the voltage being 2% too low, the commanded voltage must be 2% higher than the voltage required to meet the timing. If the voltage drifts 2% above the commanded voltage after going through the regulator, it will run 4% higher than the minimum voltage required for that operating point. This still meets the specified voltage requirements required by the FPGA, but wastes a lot of power, as shown in Figure 2 below.

The solution to this problem is to choose a power regulator that can operate with tighter voltage tolerances. Using a regulator with ±0.5% tolerance allows operation at the desired operating frequency, closer to the required minimum specifications, and is guaranteed to be within 1% of the desired voltage. In this way, the FPGA can work normally with minimum power consumption.

High power requirements

Devices in FPGA systems usually require different adjustment voltages. For example, the voltage required by the core voltage processor can be 0.8V, 1.0V, 1.2V, 1.5V or 1.8V, etc. Although it is a low-voltage supply, its dense transistor structure and long-term high-speed operation may require a power supply solution of 10A or more. Specific processor requirements usually determine other power requirements, such as load transient recovery and standby mode. etc. This requires a Point-of-Load (or PoL) regulated power supply designed specifically for the core voltage. The PoL regulated power supply is a high-performance voltage regulator with each Vout voltage rail independent of its respective load setting. This helps address the high transient current requirements and low noise requirements of high-performance semiconductor devices such as FPGAs. For example, ADI's LTM4678 series includes two sets of power supply outputs that can simultaneously provide high-density power supply outputs, respectively 1V@25A and 1.8V@25V.

Controllability requirements

FPGA contains a large number of complexly arranged transistors. A chip contains hundreds of millions of transistors, which are divided into core segments, module segments and partitions that can be designed and managed independently. These specific orchestrations have many different power domains, including voltage, current, ripple, and noise, as well as sequences during startup, shutdown, and fault conditions, so a controllable FPGA power supply needs to properly manage the output. sequence and its power.

Newer FPGAs on the market will provide specific requirements for the sequence sequence when powering up and shutting down in the specifications to ensure that the FPGA turns on and reset normally, maintains minimum current consumption, and keeps the I/O at during power transitions. Under the correct tri-state configuration. Taking Arria 10 again as an example, the technical specifications divide the power supplies into three sequence groups (1, 2, 3) and require them to be arranged in ascending order as 1, 2, 3, and then in reverse order in descending order: 3, 2 ,1.

Analog Devices' LTC2936, for example, provides six programmable threshold analog comparators for detecting fast events and sending digital status to logic. The device also has three programmable GPIO pins to provide additional functionality. The programmable IC includes an EEPROM that operates nearly instantaneously at boot and stores fault telemetry data through its I2C/SMBus interface for debugging purposes.

FPGA development kit support

Engineers can use FPGA development kits to assist with development. For example, the Arria 10 SoC Development Kit (DK-SOC-10AS066S-A) demonstrates ADI's LTM4677µmodule power solution for the Arria 10 SoC power requirements.

In the kit, the core power supply has an operating voltage of 0.95V and an operating current of 30A. Because these power requirements are relatively relaxed, a single LTM4677 module can easily provide the required current (up to 36A). For applications requiring more current and under more severe conditions, up to four LTM4677 modules can be run in parallel to deliver up to 144A, as shown in Figure 7.

Select material using parameter list

After understanding the application requirements, engineers can select the "DC Converter" subcategory in the "Power Supply - Board Mounting" category on the Digi-Key official website. In the "Application Filter", engineers can find "POL" in "Type", or directly enter "POL" in "Search in Results" to filter PoL regulated power supplies.