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Valued Contributor III
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Power available on 3.3 V pin of 14-pin header (DE2-115)

I'm using an Altera DE2-115 board together with a Terasic LTM (connected through GPIO header) and an ADC EVM from Texas Instruments (connected through HSMC header) for my prototype. 

 

Actually, LTM is powered through 3.3 V pin on GPIO; TI's EVM requires instead an external power supply of 3.3 V through a couple of banana jacks. I already have to realize a custom small pcb for my prototype that requires to be connected to the 14-pin header from DE2-115. 

 

For the sake of practicalness, I'm planning to add to a couple of banana socket to take the 3.3 V power supply I need for TI's EVM directly from DE2-115 board through 14-pin header. 

 

My problem is assuring myself I can do this, so I need to know how much current DE2-115 can source on VCC3P3 node (so called on its schematics) and how much current is already sinked by the board and by the LTM through GPIO. How can I get these informations? 

 

Regards, 

Lorenzo
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Valued Contributor III
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Is my question confused? Is it a totally wrong idea? 

Any help would be greatly appreciated. 

 

Hopeful regards, 

Lorenzo
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Valued Contributor III
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Hi Lorenzo, 

 

--- Quote Start ---  

 

My problem is assuring myself I can do this, so I need to know how much current DE2-115 can source on VCC3P3 node (so called on its schematics) and how much current is already sinked by the board and by the LTM through GPIO. How can I get these informations? 

 

--- Quote End ---  

The schematic has the part numbers and the data sheets have the maximum currents for each device. However, you've got to consider; 

 

1) A 100-mil header can support at most 1A. How many of the 100-mil header pins are dedicated to 3.3V and ground? How much current does your external circuit need? 

 

2) Find the 3.3V regulator and touch it. Is it hot/warm? If you draw current through the GPIO 3.3V pin, then the regulator will get warmer. Is it a linear regulator (it'll get a lot warmer) or switching regulator (its temperature won't change much)? 

 

3) What is worth more to you; your DE115 board, or an external 3.3V supply?  

 

If the voltage regulator on the board supports say up to a 1A load, and you can estimate the current draw from the other components on the board, then you can determine what a safe level of current draw is. 

 

However, I would not recommend drawing anything over a few hundred mA, even if your analysis indicates you can draw more. Being conservative lets you keep your evaluation boards for much longer :) 

 

Cheers, 

Dave
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Valued Contributor III
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--- Quote Start ---  

 

 

3) What is worth more to you; your DE115 board, or an external 3.3V supply?  

 

 

--- Quote End ---  

 

 

Always putting things into perspective :) 

 

great help as always Dave!
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Valued Contributor III
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Hi Dave, 

 

I already supposed I had to do the analysis you talked about, I was just wondering if the information about the total current consumption of the board itself from 3.3 V reference line was already known and available somewhere. 

 

Meanwhile, in these days, a few other system considerations persuaded me I have to use some other voltage regulators on my custom pcb, so I'll need an external power supply anyway. 

 

Again, I have to thank you for your always useful, clear and precise support. 

 

Cheers, 

Lorenzo
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Valued Contributor III
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Hi Lorenzo, 

 

--- Quote Start ---  

 

I already supposed I had to do the analysis you talked about, I was just wondering if the information about the total current consumption of the board itself from 3.3 V reference line was already known and available somewhere. 

 

--- Quote End ---  

Even if it was, you're always better off double-checking with your own analysis. 

 

 

--- Quote Start ---  

 

Meanwhile, in these days, a few other system considerations persuaded me I have to use some other voltage regulators on my custom pcb, so I'll need an external power supply anyway. 

 

--- Quote End ---  

When designing systems with multiple power rails, its always worth considering the power-on sequence. In the case of your design, since it will be driving FPGA I/Os, and those I/Os are hot-swappable, there is no power-sequencing issue. However, I would recommend using an FPGA GPIO to enable the power to your custom board. That way, even though you have multiple power sources (the DE115 and the external wall-wart), when you turn on the power to the DE115, it will in turn power-up your custom I/O board. 

 

 

--- Quote Start ---  

 

Again, I have to thank you for your always useful, clear and precise support. 

 

--- Quote End ---  

You're welcome. 

 

Cheers, 

Dave
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