Exar – A ready-made circuit for powering a SmartFusion2 reference design from Exar

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Field Programmable Gate Arrays (FPGAs) continue to grow in popularity because they enable OEMs to get to market quickly while providing great design flexibility and enabling the re-use of Intellectual Property (IP) from one design in another. FPGAs allow system designers to react quickly to last-minute changes to product definitions and to reduce the time spent on re-development.

Users of FPGAs do, however, have to take care to design a power circuit which provides multiple rails dedicated to the supply of the core, I/Os, memory and other peripherals at precisely regulated voltages. To function properly, an FPGA requires a carefully designed power- management solution. Improper start-up/shut-down sequencing, uncontrolled rise times or non-monotonic ramp-up of sensitive multi-rail systems can impair the system’s reliability and lead to faults. An ideal power-management solution will provide for close control of the many power rails, while also being compact, flexible and easy to implement.

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Fig. 1: Block diagram of Exar’s power-system reference design for FPGAs based on the XRP7714 PMIC

A power solution for users of the SmartFusion2 FPGA
Every series of FPGAs has different power requirements, and the power-management circuit should be as closely matched to the precise needs of each FPGA as possible. For users of the SmartFusion2 FPGA system-on-chip from Microsemi, a ready-made, dedicated power solution is available today as a reference design from Exar (see Figure 1). Based on the XRP7714, a four-channel universal Power Management IC (PMIC) which offers programmable power outputs, the reference design supplies four power rails:
• 1.2V input to the main supply rail
• 1.5V input to general-purpose I/Os
• 2.5V auxiliary voltage
• 1.8V supply to the internal DRAM

This programmable power solution is flexible and allows for dynamic changes to output-voltage levels, sequencing, timing, switching frequency and other functions. Modifications to power rails may be implemented at any time without the need for hardware changes, even after a product is in the field.

The circuit also provides for the monitoring of faults, output voltages and currents. Four GPIO signals are available: they can be programmed to provide the status of Power Good, Enable and Fault signals.

Fig. 2: Exar’s FPGA power reference design, providing 1.2V, 1.5V, 1.8V and 2.5V power rails, has a compact 38mm x 38mm board footprint

Fig. 2: Exar’s FPGA power reference design, providing 1.2V, 1.5V, 1.8V and 2.5V power rails, has a compact 38mm x 38mm board footprint

Comparison to traditional analogue power circuits for FPGAs
Traditional analogue power solutions are static: they are designed for one particular application.

This means that, if modifications are required to a power rail’s voltage, ramp rate or sequencing, the power-system design must change: the values of components must be recalculated and the previous components substituted for new ones. If this modification comes late in the design cycle, this change to the board design could delay the product’s release to the market.

As well as being fixed and inflexible, analogue power solutions also typically require more board real-estate than a programmable power solution. A typical four-channel analogue power solution uses nearly 150 components, and more than this if soft-start, margining or sequencing functions are implemented discretely.

The XRP7714 SmartFusion2 power reference design from Exar, on the other hand, is programmable. It supplies four power rails with fewer than 35 components, and includes many features not available in a basic analogue circuit:

• Soft start
• Margining
• Sequencing and tracking
• Dynamic voltage control
• Conditional fault management

The SmartFusion2 power solution is easily customisable using Exar’s design and configuration software tool, PowerArchitectTM, which enables fast prototyping and reduces time to market while helping developers to implement last-minute changes to production designs without long delays. The parameters of output voltage, switching frequency, sequencing and fault management are programmable via an I2C interface.

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Fig. 3: The PowerArchitect tool provides for easy configuration of power-system parameters

In the reference design, each channel of the XRP7714 is independently configured to meet the sequencing requirements of the SmartFusion2. Figure 4 illustrates the power-on and power-down supply sequencing.

Fig. 4: Power-on and power-down sequencing

Fig. 4: Power-on and power-down sequencing

The reference design includes configuration files, and connects to the Exar communications module XRP77XXEVB-XCM-V80. This module provides an interface to the PowerArchitect tool and enables the board to be programmed from it.

Supply (V)Power-on Ramp Rate (V/ms)Power-down Ramp Rate (V/ms)
1.20.60.6
1.50.750.075
2.50.670.25
1.80.90.09

Conclusion
The SmartFusion2 power reference design is a complete four-output system designed to power a SmartFusion2 SoC FPGA. The order and ramp rates for each supply are programmed to accommodate the requirements of SmartFusion2 sequencing. All power-supply operations can be controlled over an I2C interface. The reference design is thus an ideal power-management solution for powering the SmartFusion2, but the XRP7714 can equally be used as the PMIC for other FPGAs. When used to power the SmartFusion2, its circuit is compact, flexible and easy to implement.

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Orderable Part Number: XRP7714EVB-DEMO-2P-KIT

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