SPARTAN-3 VCCAUX supply current

The expected current on the VCCAUX supply depends on the specific device used and the application. As described starting on page 15 of the following document, the VCCAUX voltage input supplies the Digital Clock Managers (DCMs), the dedicated configuration pins, the JTAG pins, the LVDS output drivers, and forms an input reference voltage for some of the I/O standards.

The best method to determine power consumption is the Web Power Tool, available online at the following link.

This web-based calculator allows you to enter your expected operating conditions. Based on the numbers of functions used, the device size, and the operating frequency, the Web Power Tool provides an expected typical power consumption for each supply.

An even more accurate caculator, called XPower, is built into the Xilinx ISE software. However, XPower requires a completed FPGA design.

At quiescent levels, VCCAUX and VCCINT are in the same general ballpark. However, these are likely different in an operating application.

Yes, the DCMs are powered by VCCAUX.

The answer depends on the type of application implemented in the FPGA. The Web Power Tool can help you decide.

Either works. However, if you are using DCMs, especially in high performance applications, be aware that the DCMs are powered by VCCAUX. Noise on the VCCAUX supply adversely affects jitter on the DCM clock outputs.

--------------------------------- Steven K. Knapp Applications Manager, Xilinx Inc. General Products Division Spartan-3/II/IIE FPGAs

--------------------------------- Spartan-3: Make it Your ASIC

Hello Steve,

thanks for the information. The power tool is a nice piece of SW, but fully unusable for my purposes. I want to design a robust evaluation system. Therefor i need a worst case maximum value for the currents for the X3S200TQ144 and X3S400TQ144 devices. It's not the exact value like 845,8765mA and 988,887mA i need for the given voltages, but the general range. I expect, there are special test patterns for your semiconductor end tests - like all FF's toggling or shifting which needs a certain amount of current on vccint or vccaux. What's the average current in these 'all chip resources tests' at the thermal limit ?

Ok, this question is rather weird because of the limit definitions, but i hope you understand what i mean. The Xilinx Spartan-2 Starter Kit (an excellent and low priced system) uses a

500mA regulator for the VCCAUX and a 1000mA regulator for VCCINT. Are these ranges recommended from Xilinx, or a rough estimation of an experienced designer ?

again thank you very much for your fast response and quality of information.

MIKE

Sorry, i mean the Spartan-3 Starter-Kit in the posting above.

MIKE

MIKE, It's like asking the question how big a gas tank do I need on my sports car to get 300 miles on a tank. It all depends on who's driving it and where. If you really want worst case, you've gotta plug in the max toggle rate on all the CLBs, FFs and BRAMs to the power consumption tool. But is anyone gonna drive with their pedal to the metal constantly for an hour? Not even Michael Schumacher does that. Cheers, Syms.

given

tests -

The Web Power Tool will provide this information. If you believe the end application will have all nodes switching, then you can model this in Web Power Tool. However, most applications have a much smaller portion of the device toggling.

However, you may be asking a different question. The performance of a device is guaranteed up to a specific junction temperature--85C for the commercial temperature range or 100C for the industrial.

Using the expected ambient temperature range, zero airflow, and the thermal characteristics of the package, you can calculate the maximum possible power that you can dissipate through the package and not violate the maximum junction temperature.

For example, the Theta-JA of the XC3S200 in the TQ144 package is 34.1 degrees C per Watt. Assume that the ambient operating temperature is 25C.

25C + 34.1C/W (P) = 85C

Solving for P ...

P = 1.76 Watts maximum

a

these

Although the Spartan-3 Starter Kit board has an XC3S200 in the FT256 ball grid array package, the regulator is sized larger than actually required. We used the Web Power Tool to generate a number of expected applications. The board as presently defined meets the requirements of every XC3S200 design we could contemplate. It also meets most every XC3S400 design and almost every XC3S1000 design, giving ourselves maximum flexibility with the PCB design. Yes, you can create pathological designs that will break one or more supply limits, but these require effort.

If you are interested, the schematics are available online.

Spartan-3 Starter Kit Board web site

Schematics (part of the Spartan-3 Starter Kit Board User Guide starting on page 53)

--------------------------------- Steven K. Knapp Applications Manager, Xilinx Inc. General Products Division Spartan-3/II/IIE FPGAs

--------------------------------- Spartan-3: Make it Your ASIC