Power Supply Integrity

Proper bypassing and decoupling techniques improve overall power supply signal integrity, which is important for reliable design operation. These techniques become more significant with increased power supply current requirements as well as increased distance from the power supply to the point-of-load (generally the FPGA or CPLD device). The type of bypassing and decoupling techniques designers should consider depends on the system design and board requirements.

When an output buffer changes state, e.g. driving an output pin from a logical high to a logical low, the output structure momentarily presents a low impedance path across the structure from power supply rail to ground. This output transition causes the output to charge or discharge, requiring that current must be immediately available on the output load to reach the required voltage level. Bypass capacitors locally provide the stored energy required for this current transient.

The transient response for this energy storage system must cover a large frequency and load range. Therefore, a storage system should be comprised of a variety of capacitor types. Small capacitors with low series inductance can provide fast current for high-frequency transitions. Large capacitors continue to supply current after the high-frequency capacitors have been depleted of their energy stores. Figure 1 shows a typical energy storage system designed for large frequency and load ranges. Typical designs require capacitors with frequencies ranging from 1 KHz to 500 MHz in three ranges:

  • 0.001 to 0.1 µF
  • 47 to 100 µF
  • 470 to 3,300 µF
Figure 1. Typical Energy Storage System

The amount of logic used in the device and output switching requirements define decoupling requirements. Additional decoupling capacitance is needed as the number of I/O pins and the capacitive load on the pins increases. Designers should add as many 0.2 µF power-supply decoupling capacitors as possible to the VCCINT, VCCIO, and ground pins/planes. Ideally, these small capacitors should be located as close as possible to the device. Designers can decouple each VCCINT or VCCIO and ground pin pair with a 0.2-µF capacitor. If a design uses high-density packages such as ball grid array (BGA) packages, it may be difficult to use one decoupling capacitor per VCCINT / VCCIO and ground pin pair. In such cases, designers make every effort to use as many decoupling capacitors as allowed by the layout. Decoupling capacitors should have a good frequency response, such as monolithic-ceramic capacitors.

Capacitor Choice & Placement

Proper placement and location are very important for high-frequency capacitors (0.001 to 0.1 µF low inductance ceramic chip). Designers should minimize trace lengths when possible to reduce the inductance in the path from capacitor terminals to the device power pins. This includes paths that go through a solid ground or power plane (VCCINT or VCCIO) where the inductance of one inch of solid copper plane is about 1 nH. Bypass capacitor vias should route directly to ground, VCCINT, or VCCIO planes. Other capacitor types (47 to 100 µF medium-frequency and 470 to 3,300 µF low-frequency capacitors) are referred to as “bulk” capacitance and can be mounted anywhere on the board. Designers should, however, locate bulk capacitance as close to the device as possible. Place VCCINT or VCCIO high-frequency bypass capacitors within one centimeter of the associated VCCINT or VCCIO pin on the PCB. VCCINT or VCCIO medium-frequency bypass capacitors should be placed within 3 cm of VCCINT or VCCIO pins.

VCCINT Bypass Capacitance

In the case of Stratix® II, individual logic array structures within different architectural features conduct very small currents (picoamps or less) for very short durations (< 50 ps). Although these currents are small, when added up across the entire device they can add up to several amperes of current. Considering that these minute current transitions can occur hundreds of millions of times per second, along with the existence of millions of individual switches carrying out these transitions, bypass capacitor calculation is based on an average energy storage requirement. High-frequency capacitor values can be approximated with:

logic array power = equivalent switched logic array capacitance × VCCINT2 × clock frequency

or

equivalent switched logic array capacitance = (logic array power) / (VCCINT2 × clock frequency)

The equivalent switched logic array capacitance is the equivalent switched capacitance of the entire Stratix II logic array powered by VCCINT. In order to reduce power noise, the VCCINT power supply bypass capacitor must be significantly larger than the equivalent switched logic array capacitance. High-frequency bypass capacitors should be 25 to 100 times larger than the equivalent switched logic array capacitance. A factor of 50 will result in a 2 percent variation of VCCINT.

High-frequency bypass capacitance = <25 to 100> × equivalent switched logic array capacitance

Every VCCINT and ground pin pair should have a high-frequency bypass capacitor. To determine the optimum size of each high-frequency bypass capacitor, divide the total high-frequency bypass capacitance by the number of VCCINT pins on the device, and round up to the next commonly available value. Therefore, the minimum size of each high-frequency VCCINT capacitor is:

Capacitor size= (<25 to 100> × equivalent switched logic array capacitance) / number of VCCINT pins
= (<25 to 100> / number of VCCINT pins) × logic array power / (VCCINT2 × clock-frequency)

Consider the following example:

  • Device VCCINT power = 5 W
  • VCCINT = 1.2 V
  • System clock frequency = 150 MHz
  • High-frequency bypass capacitor multiplier = 50
  • Number of device VCCINT pins = 36
Capacitor size= (50 / 36 ) x 5W / (1.2V2 x 150MHz)
= 3.215E-08
= 0.03215E-06

The capacitor size should be at least 0.032 µF. Given this example, the designer should select individual high-frequency capacitors at least this large.

The medium-frequency capacitors should be tantalum capacitors from 47 µF to 100 µF. If tantalum is not available, low-inductance aluminum electrolytic capacitors can be used. Stratix II devices require at least four medium-frequency capacitors mounted within 3 cm of the device. In addition, at least one low-frequency capacitor (470 µF to 3300 µF) is required on the PCB.

VCCIO Bypass Capacitance

Similar to VCCINT considerations, VCCIO bypass requirements are also based on an average energy storage requirement. The loads driven by the FPGA or CPLD device determine the size of the equivalent switched capacitance. Since different I/O banks can operate at different voltages and different switching frequencies, designers should consider bypassing networks individually, using the equations below to determine high-frequency capacitor requirements.

In order to reduce the amount of VCCIO noise, bypass capacitance must be significantly greater than the total output load capacitance. High-frequency bypass capacitance should be 25 to 100 times the total load capacitance. Every VCCIO and ground pair should have a high-frequency bypass capacitor to provide immediate current needs when the device has a large current draw. The following equations determine the optimum size of each capacitor:

equivalent switched I/O capacitance (per VCCIO)

= number of loads × average load per output signal
high-frequency I/O capacitance=<25 to 100> × equivalent switched I/O capacitance
individual capacitor size

= high-frequency I/O capacitance / number of VCCIO pins in the bank

= (<25 to 100> / number of VCCIO pins) × number of loads × average load per output signal

Consider the following example:

  • Number of loads = 40 signals
  • Average load value = 10pF
  • High-frequency bypass capacitor multiplier = 50
  • Number of device VCCIO pins = 5
Capacitor size= (50 / 5) * 40 * 10pF
= 4.0E-09

= 0.004E-06

The capacitor size should be 0.004 µF. Given this example, the designer should select individual high-frequency capacitors at least this large. The next larger available capacitor size should be chosen (0.047 µF or 0.01 µF).

Medium-frequency capacitors should be tantalum capacitors from 47 µF to 100 µF. One middle-frequency capacitor is required for every two VCCIO banks. If tantalum capacitors are not available, low-inductance aluminum electrolytic capacitors can be used. These capacitors should be located within 3 cm of the VCCIO pin connections. Lastly, at least one low-frequency capacitor (470 µF to 3,300 µF) is required on the PCB for each VCCIO voltage level.