In this 7th issue, we will explain what to consider when selecting a power supply for FPGAs from the viewpoint of power supply specifications.
This time again, we will use the example of power supply selection for Cyclone® V (5CSXFC5D) as an example.
The following four items must be checked to ensure that the selected power supply device is compatible.
1. output voltage accuracy
2. output voltage ripple
3. FPGA startup sequence
4. existence of external soft-start control
1. output voltage accuracy
The power supply device must be selected to meet the voltage accuracy described in the previous column.
For the Cyclone🄬 V SX, the required voltage for VCC is 1.1V with a range of ±30mV.
VCC voltage range is 1.07V-1.13V, ±2.72% variation range with 1.1V as the center.
・VCCA_FPLL voltage range is 2.375V-2.625V, ±5% variation range with 2.5V as the center.
・IO voltage range is 1.71V-1.89V, ±5% variation range with 1.8V as the center. The IO voltage range is 1.71V-1.89V, with a ±5% variation range when 1.8V is used as the center.
The output voltage accuracy of the LTM4664, a product with a PMBus interface that supports the Intel Agilex🄬 series Smart VID, is shown in the data sheet excerpt in Figure 1. The LTM4664 has a servo mode that provides better output voltage accuracy than normal DC/DC converters: accuracy within ±1.5% with servo mode off under 1.0V output conditions and within ±0.5% with servo mode on.
Therefore, the Cyclone🄬 V SX's VCC voltage of 1.1V ±30mV is a power supply device that can handle this voltage without problems.
Figure 1: LTM4683 Output Voltage Accuracy
2. output voltage ripple
It is necessary to select a power supply product that meets the Allowable Ripple of the FPGA specification.
As described in the previous column, the Allowable Ripple for each device on the Introduction sheet of the PDN Tool (in this case, 5%)
Figure 2: Checking the PDN Tool screen Allowable Ripple
Since the Allowable Ripple of 5% is specified as the required voltage accuracy of FPGA, it is necessary to check whether the ripple voltage and load response characteristics of the DC/DC converter are satisfactory. The waveforms in Figure 3 show the ripple voltage and load response characteristics at 1 V output. Both are within 5%, so there is no problem.
Figure 3: Ripple voltage and load response characteristics
3. Checking the FPGA startup sequence
Configure the power supply so that VCC and VCCA_PLL are activated in this order (see the device handbook in the previous column).
For general DC/DC converters, select a power supply IC or module with PowerGood and Enable functions, or use an external sequencer to design a power supply that satisfies the sequence conditions required by the FPGA.
For the LTM4683, the sequence time can be set in a register inside the module. It also has PowerGood and Enable (RUN Pin), which are sufficient to meet the requirements.
4. soft start
FPGA startup has a tRamp specification (startup time specification), and it is necessary to select a power supply with specifications that fall within this range.
Some products have a fixed start-up time on the device side, while others have a start-up time that can be adjusted with an external capacitor (CSS).
For the LTM4683, the rise time can be set in a register inside the module.
The above is an introduction to selecting a power supply product that meets the power requirements of FPGAs.
Please refer to the previous column for more information on selecting the right power supply for your FPGA.
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