Power consumption

[Guest helltone]

Hi,

Do you have any estimates on the current available to the wings on the 5V and 3.3V pins? i.e. 500/800mA minus the fpga and board consumption. Would be nice to add this info to the specification page.

Thanks in advance.

[Jack Gassett]

Hello,

The voltage regulators are capable of 800mA so if the power jack is used then 800mA can be made available on the 5V power rail. If using the USB connection for power then only 500mA is available on the 5V power rail.

The 3.3V and 2.5V voltage regulators get their power from the 5V power rail and the 1.2V voltage regulator is powered from the 2.5V voltage regulator in order to keep the dropout voltage low (the lower the dropout voltage the less energy is wasted as heat).

According to the datasheet for the FT2232 it uses 30mA in normal operation and it is supplied by the 5V rail.

The current used by the Xilinx FPGA is completely dependent on what is running on it. You can determine how much power your ISE project is going to use by running the XPower Estimator (XPE) spreadsheet. For the PACMAN project the XPE reports that the project uses 31mA @ 1.2V  and 18mA @ 2.5V.

The other thing to take into consideration is the RPAR resistor which protects the 2.5 voltage regulator from reverse current caused by connecting the 3.3V FT2232 JTAG lines to the 2.5V Spartan 3E JTAG lines The price for this protection is that energy is wasted. If RPAR is populated then 25mA is dissipated.

Power is lost in the voltage regulators and we are not accounting for voltage sourced for the I/O pins so this is just a rough estimate but:

If USB power is used: 500ma - (30mA + 31mA + 18mA +25mA) = 396mA free.

If power jack is used: 800ma - (30mA + 31mA + 18mA +25mA) = 696mA free.

This free current should be available on both the 5V and 3.3V power rails.

[Guest helltone]

Hey,

Thanks for the quick answer.

Say for instance I have a total power consumption of ~1.5A on the 5V net (powering an 8" LCD backlight :-P). Can I plug somehow a regulated 5V power source (without going through the onboard 5V regulator)?

I saw that there is a jumper for switching the power source between USB and unregulated >6.5V power jack.

[Jack Gassett]

Yes, if you take the jumper off the PWRSELECT pin then you can supply your own 5V power source to either the middle pin of PWRSELECT or any of the 5V pins of the Wings.

My biggest concern was if the traces are wide enough to support 1.5A. The traces from the 5V pins of the Wing connectors are 16 mils wide and according to this trace width calculator they should be able to handle your 1.5A.

The only last thing to consider is if a single pin can handle 1.5A, the rule of thumb that I have heard was that each pin can handle 1A but that is just a rule of thumb. I'm not sure how accurate it is, you might be fine with 1.5A on one pin.

Jack.