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I want to build a circuit that will output multiple voltages (one at a time) to supply an FPGA. The core volt will be constant voltage. I want to be able to supply different voltages for the FPGA output and input pins. Virtex4,5,6 and 7 all can support 1.2v, 1.5, 1.8, 2.5 to name a few. I would like to have a MCU or another FPGA select different output voltages with SPI, I2C or GPIO. I was looking for a TI regulator module that supported I2C or SPI and found none. I also was thinking of using a LM317 adj regulator ( open to your input) and switching in and out resistors. I could switch the resistors in and out with a FET controlled by GPIO with a decoder.

I know it can be done, because I used a board in College that did just that. It was controlled through a FPGA and usb software. I did not have a schematic of this board and the vendor who sold it wouldn't give me any info.

I want to be able to supply the above voltages with 1A, 2A, 4A and 10Amp - depends on the FPGA used.

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  • \$\begingroup\$ Or a DAC used bass-ackwards? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 24 '13 at 5:32
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    \$\begingroup\$ There is no question in this post. \$\endgroup\$ – travisbartley Sep 24 '13 at 5:35
  • \$\begingroup\$ I really don't understand what you want to achieve here. You want a universal power supply for supplying power to FPGAs? Up to 10A and using a serial regulator? You know that you are going to create a space heater with that one instead of a power supply? \$\endgroup\$ – FarhadA Sep 24 '13 at 10:52
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    \$\begingroup\$ Anything can be done - what input power voltage are you considering feeding the programmable regulator with? Do you think a switching converter alone is good enough or do you consider that a purely linear regulator is the only option you have? Are you requiring programmable current limit too? Is there a default voltage that should be produced should there be no serial comms to the device? Would it need to have crowbar protection for extra security? \$\endgroup\$ – Andy aka Sep 24 '13 at 11:00
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You could use a digital potentiometer in the feedback path of a voltage regulator.

From Analog Devices MT-091:

Because they are digitally controlled, digital pots can be used in active control applications, in addition to basic trimming or calibration applications. For example, digital pots can be used in programmable power supplies as shown in Figure 8A. Typical adjustable low dropout voltage regulators (such as the anyCAP series) have a FB pin, where applying a resistor divider yields a variable output voltage. As shown, R1 and R2 are the feedback and input resistors, respectively. The FB circuit has an internal non-inverting amplifier which gains up a 1.2-V bandgap reference to the desired output voltage.
digital pot

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  • \$\begingroup\$ You slick card you. Same thing I found and was about to put. \$\endgroup\$ – Passerby Sep 24 '13 at 5:46
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    \$\begingroup\$ Only thing I would add is that OP might want to get a digital pot with non-volitial memory. If the pot defaults to a very high or low value which would then cause the regulator to adjust to an unsafe voltage, it can fry the circuit being powered. \$\endgroup\$ – Passerby Sep 24 '13 at 5:53
  • \$\begingroup\$ Good point about startup condition. \$\endgroup\$ – dext0rb Sep 24 '13 at 5:58
  • \$\begingroup\$ I thought of digitally controlled pots, but an FAE from OnSemi, told me that they don't have the accuracy needed. \$\endgroup\$ – user29548 Sep 24 '13 at 6:29
  • \$\begingroup\$ I will read more about the AD5227 device - thank you \$\endgroup\$ – user29548 Sep 24 '13 at 6:31
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Also Maxim has the same principal described in the applications note:

Add Margining Capability to a DC/DC Converter

You can easily add margining capability (digital adjustment of the output voltage) to a DC/DC converter by adding a 2- or 4-channel, I²C-adjustable current DAC (DS4402 or DS4404) at the converter’s feedback input. Because each DAC output is 0mA at power up, the extra circuitry is transparent to the system until a command is written via the I²C bus. enter image description here

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Considering the practical scenario of using FPGA's your core voltage to FPGA of 1.1V or 1.2V must always be supplied to FPGA. You can use LM2743 in WEBENCH, to get your circuit of required voltage and current. Next comes your voltage for FPLL of FPGA that might be 1.5V or 1.8V based on FPGA used, and this voltage must always be supplied to FPGA. I suggest to use LMZ10503, and the voltage output values can be adjusted based on the selection of Rfbb and Rfbt. Then comes the voltage of your GPIO lines that can be 1.8V or 3.3V and if you are looking for a high current say 5A, I suggest to use LMZ10505, and again the output voltages can be changed based on the selection of Rfbb and Rfbt resistors. For Rfbb and Rfbt you can keep digital pots to change the resistance values for your required voltage. All the regulators what I have suggested here comes with enable pin, that can be digitally controlled using UCD9081, if you are looking for power sequencing and can enable or disable your supplies as per the requirement.


Apart from all above here is ISL65426, from Intersil, which will surely fit into your requirement of enabling different voltages digitally, provided that all the pull-up and pull-down resistors required to control the voltage and current must be enabled with the help of MOSFET.

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