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binarus
Observer
Observer
3,366 Views
Registered: ‎12-09-2009

Questions regarding Xilinx Spartan-3A starter kit voltage regulation

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Hello,

we are in the process of designing a custom application incorporating a Xilinx Spartan-3A FPGA (xc3s1400a). As as starting point, we have looked into the schematics of the Xilinx Spartan-3A Starter Kit (http://www.xilinx.com/support/documentation/boards_and_kits/s3astarter_schematic.pdf). Now we have a problem understanding the voltage regulation (page 5), especially the external components for the buck converters.

First, on the upper right of the respective page, there is a notice reading like this: "No Load R273, R274 and C49, C201, C202". Secondly, in the lower left of this page, there is the following notice: "Resistors R272, R275, R277, R279 must be 0 ohm if external feedback networks are removed". Thirdly, according to the schematics, if all components are equipped, R272 and R273 act as a voltage divider so that the voltage behind L1 is divided by 2 (very roughly) before being fed back to the LP3906. But as far as I have understood from the datasheet of the LP3906, the regulator is expecting the unaltered output voltage (as it exists after the inductor) at its feedback input to work correctly.

In the moment, we don't understand any of these things, but feel that they are relating to each other. Perhaps someone can answer the following questions:

1) What does "No Load" mean (notice 1)?
2) What are these "external feedback networks" (notice 2)?
3) What is the reason for dividing the output voltage of the buck converters (as existent after the inductor) before feeding it back to the regulator, and why doen't this disturb the regulation and produce an output voltage which is too high?

Thanks you very much!

Regards,

Peter


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austin
Scholar
Scholar
4,058 Views
Registered: ‎02-27-2008

b,

 

1. "no load" means do not put those parts down on the board (they are not used)

 

2. the external feedback networks were for changing the voltage

 

3. to change the voltage. The voltage is unchanged because the resistor feeding back is 0 ohms, and the resistor to ground is not present (no load)

 

Often we start design very early, and use the latest regulators, sometimes while these parts are being sampled, and are not fully characterized by their vendors.  We may also not have first silicon for the FPGA devce, and we may need to change its operating voltage early on.  The boards have to be ready before we have parts!  The use of "no load" and zero ohm resistors allows flexibility if the regulator circuit needs changing before we are done.  Now that everything is stable, the "no load" components can be removed completely from a new design, and zero ohm resistors replaced by wires.

 

Once designed, tested, and working, there is no reason to go back and change the board for our purposes, but you should simplify everything so that you only use want you need to use.

 

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose

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2 Replies
austin
Scholar
Scholar
4,059 Views
Registered: ‎02-27-2008

b,

 

1. "no load" means do not put those parts down on the board (they are not used)

 

2. the external feedback networks were for changing the voltage

 

3. to change the voltage. The voltage is unchanged because the resistor feeding back is 0 ohms, and the resistor to ground is not present (no load)

 

Often we start design very early, and use the latest regulators, sometimes while these parts are being sampled, and are not fully characterized by their vendors.  We may also not have first silicon for the FPGA devce, and we may need to change its operating voltage early on.  The boards have to be ready before we have parts!  The use of "no load" and zero ohm resistors allows flexibility if the regulator circuit needs changing before we are done.  Now that everything is stable, the "no load" components can be removed completely from a new design, and zero ohm resistors replaced by wires.

 

Once designed, tested, and working, there is no reason to go back and change the board for our purposes, but you should simplify everything so that you only use want you need to use.

 

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose

View solution in original post

binarus
Observer
Observer
3,328 Views
Registered: ‎12-09-2009

Hello!

 

Thank you very much for this comprehensive yet understandable answer. That's the way support should work :-)

 

Regards,

 

Peter

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