Power Management Design Guide for AlteraÂ® FPGAs and CPLDs ...

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Design considerations for powering FPGAs & CPLDs Bulk capacitance In any power supply, the output capacitors are a vital element for adequate performance. They are used to control the output voltage ripple (ΔV OUT ) and to supply load current during fast load transients. Various types of capacitors may be used. However, the ceramic type often do not have the large capacitance needed to supply current for load transients, and tantalums tend to be more expensive than aluminum electrolytic. Adding bulk capacitance to the output of high-current switching supplies is always recommended and is also very important for adequate FPGA performance. Bulk capacitance helps provide current during start-up transients, thus supporting an adequate monotonic voltage rise in highcurrent power supplies. More in-depth recommendations on the use and benefits of bulk capacitance in FPGA power supplies are available in Altera’s application note AN-355 (see page 23 for download information). Layout FPGA power design can involve very high currents (5A, 10A, and 15A are common) flowing in the traces of the PCB. When these larger currents are present and change over time in a switching pattern with sharp edges, it is easy to realize that noise, induced voltages, and EMI may be present and may cause undesirable behavior in the power supply if proper care is not taken. This is why proper layout is critical in every switching regulator design. Rapidly switching currents associated with wiring inductance can also generate voltage transients which may cause additional problems. For minimal inductance and ground loops, the PCB traces conducting high current and/or switching waveforms should be kept as short as possible. For best results, external components should be located as close to the switcher IC as possible, using ground-plane construction or single-point grounding. If open core inductors are used, special care must be taken as to the location and positioning of this type of inductor. Allowing the inductor flux to intersect sensitive feedback IC ground path and C OUT wiring can cause problems. When using a switching regulator or controller with an adjustable output, special care must be taken as to the location of the feedback resistors and the associated wiring. Physically locate both resistors near the IC and route the wiring away from the inductor, especially an open core type of inductor. Ferrite bobbin or stick inductors have magnetic lines of flux flowing through the air from one end of the bobbin to the other end. These magnetic lines of flux will induce a voltage into any wire or PC board copper trace that comes within the inductor’s magnetic field. The strength of the magnetic field, the orientation and location of the PC copper trace to the magnetic field, and the distance between the copper trace and the inductor determine the amount of voltage generated in the copper trace. For a deeper understanding of buck converters and PCB layout guidelines around them, see National’s application notes AN-1149 and AN-1229 (see page 23 for download information). Softstart As with most digital systems, FPGAs need their supply voltage to rise from zero to full voltage within a specified period of time. Altera FPGAs are very flexible and the requirements are not difficult to meet, but start-up timing still needs to be observed. For example, Stratix II FPGAs need V CCINT to rise in a window anywhere from 30 µs to 100 ms. To achieve that controlled rise time, softstart in the power supply must be used. Most switching regulators and controllers already have softstart incorporated, either internally fixed or externally programmable. When user programmable, a softstart pin (SS) is available and softstart timing is typically adjusted by placing a small capacitor between that pin and ground. Depending on the capacitor value, the softstart time will change. Information on which capacitor values to use and how to calculate the needed value to achieve a particular ramp-up time is described on each switching regulator’s datasheet. For linear regulators where softstart is not typically an internally implemented function, an external pass device is used along with a resistor and a capacitor to achieve this same effect. 12
Monotonic voltage rise Although not typically required in CPLDs, most FPGAs need the supply voltage to turn on steadily and gradually. This is called monotonic voltage rise and is needed for internal elements in the FPGA to turn on sequentially as the input voltage rises. As these elements are turning on during the ramp-up period, the “load” to the power supply will not be constant, so it is important that the power supply chosen regulates its output voltage not only during steady state but also during ramp up. As mentioned before, bulk capacitance is also a very important element for ensuring that the power supply has an adequate monotonic rise for powering FPGAs. The figure below shows the monotonic rise achieved in V OUT when using National’s LM2743 switching controller to power a Stratix II FPGA. The LM2743 power supply design used for this graph is included in the reference design section of this guide. LM2743 V OUT monotonic rise Selecting the best power solution for Altera devices Depending on the operating conditions and the specific Altera device used, all three of the power topologies previously mentioned (linear, buck, and synchronous buck) come into play. As seen throughout the selection tables, a choice of either an LDO or a switching regulator solution is provided for relatively small output currents in FPGAs (500 mA and 1A). The exception is when the input-tooutput voltage ratio is high and an LDO solution is no longer suitable because of high thermal dissipation (e.g., 12V to 3.3V @ 500 mA or 5V to 1.2V @ 1A). For output currents >5A, synchronous buck controllers are always recommended as they provide the best option in terms of efficiency, heat dissipation, performance, and cost. CPLDs are low-power devices. As seen in the MAX II solutions table, most of the power supplies recommended are LDOs and monolithic (integrated) switching regulators because typical current requirements are always below 1A. One alternative for high efficiency and simple, hasslefree design is to use inductorless switching regulators. These devices provide efficiencies in the 80% range, thus dissipating very small amounts of power as heat. As implied by their name, these devices do not require an inductor (only small external capacitors) and are as easy to use as linear regulators. Inductorless switching regulators are a good option for powering loads in the 10 mA to 250 mA range. For all digital supplies needed by the FPGA (such as V CCINT and V CCIO ), either a switching or a linear solution is sufficient. When the supply is powering an analog FPGA block (V CCPLL , for example) a linear solution is usually recommended as it has a cleaner low-noise output. 13
Page 1 and 2: Power Management Design Guide for A
Page 3 and 4: Contents Featured power products fo
Page 5 and 6: Selecting the best power architectu
Page 7 and 8: Stratix II FPGAs Stratix II power r
Page 9 and 10: Stratix FPGAs Stratix power require
Page 11 and 12: Cyclone FPGAs Cyclone power require
Page 13: MAX II CPLDs MAX II power requireme
Page 17 and 18: Stratix II & Stratix FPGAs Stratix
Page 19 and 20: Stratix II & Stratix FPGAs Stratix
Page 21 and 22: MAX II CPLDs MAX II Complete Pow
Page 23 and 24: High-speed data conversion ICs Prod
Page 25 and 26: Select voltage supervisors/power-on
Page 27 and 28: Featured power management solutions

Power Management Design Guide for AlteraÂ® FPGAs and CPLDs ...

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