Doubt regarding FPGA Power Supplies and voltage levels

[hallovipin]

Hi ,

I am designing my first FPGA board and have few doubts regarding FPGA power supply selection. Please help in clarifying the following:

1.

Is it proper to use switched mode regulators directly as FPGA supplies without using LDOs. I used National Semiconductor's WEBENCH tool (**broken link removed**) and they are suggesting three different switched mode regulators for three supplies of FPGA (1.2V, 2.5V and 3.3V). All these regulators are down converting the 12 volt input supply to 1.2V, 2.5V and 3.3V. Is it proper to have 12V to 1.2V conversion without having an intermediate converter?

Please have a look on the following power supply solutions from TI. they are SMD power module by TI. Do you think they can be used in the design...
**broken link removed**

Plug-in Power Module - Non-Isolated POL - PTH08T230W - TI.com


2.

I am thoroughly confused in choosing the voltage level for VCCAUX supply. We have all the I/Os with 3.3Volts. What do you think VCCAUX should be; 3.3V or 2.5V? As per the datasheet both the levels are acceptable.

3.

If we are interfacing an SPI based device (in our case RTC) to FPGA.. is it necessary to use dedicated FPGA I/Os (MOSI, MISO etc) or any of the I/Os.

Thanx a lot.

- Vipin

 

[mrflibble]

I am thoroughly confused in choosing the voltage level for VCCAUX supply. We have all the I/Os with 3.3Volts. What do you think VCCAUX should be; 3.3V or 2.5V? As per the datasheet both the levels are acceptable.

Depends on your specific IO's and switching. Maybe this thread clarifies things somewhat?

If we are interfacing an SPI based device (in our case RTC) to FPGA.. is it necessary to use dedicated FPGA I/Os (MOSI, MISO etc) or any of the I/Os.

SPI doesn't really require any "dedicated" fpga pins. In one design I have 2 seperate SPI busses, all using regular boring IOs on a spartan-3e. You just have to provide the SPI master/slave HDL.

 

[emresel]

hi;

Is it proper to have 12V to 1.2V conversion without having an intermediate converter?

Yes it can be, there should be no problem. Probably it is your logic core supply, then it will draw important amount of current. So i advice a stand alone step down regulator directly from the 12v to 1.2v

I am thoroughly confused in choosing the voltage level for VCCAUX supply. We have all the I/Os with 3.3Volts. What do you think VCCAUX should be; 3.3V or 2.5V? As per the datasheet both the levels are acceptable.

Why you don't put an hw jumper switch option so that you can select and try different voltage depending on your applicaiton, requirements etc...
Good luck

 

[hallovipin]

@ mrflibble

Depends on your specific IO's and switching. Maybe this thread clarifies things somewhat?

That is my thread only :sad:

@emersal

putting hardware jumper will only increase my chip counts.

Can somebody suggest any LDOs

 

[emresel]

Hi;

putting hardware jumper will only increase my chip counts.

I mean if you already usinng both 2V5 and 3V3 on your board. Then it is suitable. On the orher hand you can use a simple LDO from 3v3 to 2v5 which doesn't occupy wide PCB space. If you don't need just do not stuff 2V5 LDO. See attached connection for better understanding.
**broken link removed**
You can use LM1117 adjustable DPAK or D2PAK case for 3v3->2v5. It depends on you current requirements. It may get hot.
So i advise stepdowns if your current ratings above or around 1A.