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Vampire 500: Voltage regulation
Posted on Friday, April 10 2015 @ 11:39:02 UTC by majsta

My only concern about this complete project was how to provide enough current for this type of accelerator to work. In worst case scenario I had to think about someone who have 2.5A Amiga power supply. What if our FPGA core at some point become so complex and FPGA needs to sink 3A to work properly, stable... External power supply was out of the question so I had to find another way. To solve this problem I had to enter the territory who is not known to me, understanding how switching regulators works. All the time in my previous designs I have used linear regulators with fix output values and that was easy to do. Let's try to compare linear and switching regulators first.

Linear Switching
Function Only steps down (buck) so input voltage must be greater than output voltage Step up (boost), step down (buck), inverts
Efficiency Low, about 25% regarding efficiency VS load current High, we are about 85% regarding efficiency VS load current
Waste heat High Low
Complexity Low, few capacitors needs to be used High, requiring inductor, filter caps, resistors
Size Small Large, part itself is not big, but together with other components needed takes lot of space on PCB
Total cost Low High, mostly because external components used
Ripple/Noise Low Medium, needs to be fixed in PCB design itself by creating AGND planes...
*Table used from article "Understanding the Advantages and Disadvantages of Linear Regulators" By Steven Keeping, simplified for our needs.

As you can see from the table above there are lot of advantages we got this way, but to simplify all let's say that we don't have heat problems anymore and that our current will be more stable and in values we need, so in short from 1.5A we have left after Amiga motheboard takes what's needed we create 3A needed for the accelerator to work properly.

Vampire 500 voltage regulation simplified
On this specific accelerator few voltage regulations are needed and 5V we get from Amiga motherboard we can use directly only for HDMI, rest of the included devices works on different voltages. To suit their needs we need to create lower voltages and enough current. First switching regulator who is connected to 5V will create 3.3V for voltage translators and dedicated FPGA I/O banks. From there we use linear regulator to create 2.5V for internal FPGA PLL. We use linear regulator there since PLL don't consumes much current and we are using it from 3.3V since voltage difference is smallest there so we will not create much heat. Last two switching regulators are for creating 1.2V needed for powering FPGA core and 1.8V for mobile DDR memory used, dedicated FPGA I/O banks.

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