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Following are recommendations for a 1.1V power supply implementation, as outlined in the Virtex-4 XC4VFX20CES2/3 and XC4VFX60CES2/3 Errata, available at:

http://www.xilinx.com/support/documentation/virtex-4.htm

NOTE: Xilinx also recommends these regulators for a 1.2V power supply implementation, as outlined in the Virtex-4 Data Sheet (production device requirements).

**Introduction**

The "Virtex-4 XC4VFX20CES2/3 and XC4VFX60CES2/3 Errata" requires that AVCCAUXTX and AVCCAUXRX be sourced at 1.1V +/-3%, a deviation from the data sheet recommendation of 1.2V +/-5%.

**Devices**

Table 1 describes the Maxim MAX8556/MAX8557*, Linear Technologies LTC3026, and ON Semiconductor NCP5663 Low Drop-Out (LDO) voltage regulators.

Table 1 MAX8556/8557, LTC3026, and NCP5663 Device Descriptions

The following three sections provide implementation details for these three device types.

**Maxim MAX8556/8557**

An example of a typical MAX8556/8557 operating circuit for 1.2V output is shown in Figure 1.

Figure 1 MAX8556/8557 Operating Circuit Example

In the 1.2V circuit shown in Figure 1, R2 and R3 are modified to produce 1.1V output as follows:

R2 = 1.2 * R3

Consequently:

R3 = 1 kOhm

R2 = 1.2 kOhm

For optimum accuracy, resistor tolerances of 0.1% are used.

Operation in dropout (VIN = VOUT + VDO) is not recommended. A minimum 100 mV additional guardband above VDO should be maintained to ensure accurate regulation.

*Output Voltage Selection*

The MAX8556 and MAX8557 regulators feature an adjustable output voltage from 0.5V to 3.4V. The output voltage is set using an external resistor-divider from the output to GND with FB connected to the center tap, as shown in Figure 1 (as well as the example circuits of the other two device types).

R3 =< 1 kOhm is chosen for light-load stability. R2 is determined using the following equation:

R2 = R3*(VOUT/VFB - 1)

In this equation, VOUT is the desired output voltage and VFB is 0.5V.

**Linear Technology LTC3026**

An example of the LTC3026 configured to operate at 1.2V output is shown in Figure 2.

Figure 2 LTC3026 1.2V Output Example Circuit

In the LTC3026 operating circuit for 1.2V output, shown in Figure 2, R1 and R2 are modified to produce 1.1V output as follows:

R2 = 1.75 * R1

Consequently:

R1 = 4.3 kOhm

R2 = 7.5 kOhm

For optimum accuracy, resistor tolerances of 0.1% are used.

Operation in dropout (VIN =< VOUT + VDO) is not recommended. A minimum 100 mV *additional* guardband above VDO should be maintained to ensure accurate regulation.

**ON Semiconductor NCP5663**

An example of a typical NCP5663 operating circuit is shown in Figure 3.

Figure 3 NCP5663 Voltage Regulator Circuit

For VOUT = 1.1V, R1 = 1.222 * R2. Use 0.1% tolerance resistors for optimum accuracy.

The maximum rated dropout voltage of this LDO regulator is 1.3V at maximum current. In dropout, the output regulation specification is not guaranteed. Operation in dropout will violate the +/-3% tolerance and is not recommended. Consequently, a minimum input voltage of 2.6V is required when operating with 1.1V output. When operating with a 1.2V output, the minimum input voltage is 2.7V.

Since these nonstandard voltages are not normally available, the closest standard voltage (3.3V) should typically be used. This provides plenty of overhead for the output, but also increases power loss through the regulator. When operating from 3.3V, the efficiency of the regulator circuit drops to 33% for a 1.1V output. Proper thermal dissipation must be designed into the board to ensure that the regulator operates properly.

Linear regulators turn excess voltage into heat. The amount of power dissipated by the linear regulator die is P = ILOAD * (VIN - VOUT).

The maximum rated load current can be achieved only if it does not violate the maximum allowable total power dissipation. For a design to function, neither max load nor max power dissipation can be violated.

The maximum power a package can dissipate is given by PMAX = (MaxTj - Ta) / theta-ja.

- The LTC3026 exhibits theta-ja of 40 degrees C/W of power dissipation and can operate up to 125 degrees C MaxTj. That translates into a 2.5W overall power dissipation to meet the maximum junction temperature of 125 degrees C at 25 degrees C Ta.

- The MAX8556/8557 exhibits theta-ja of 30 degrees C/W. The package can dissipate 3.3W at 25 degrees C with a junction temperature of 125 degrees C.

- The NCP5663 exhibits theta-ja of 45 degrees C/W. The package can dissipate 2.2W when operating with a junction temperature of 125 degrees C.

Consequently, Xilinx recommends that VIN for any of these devices be limited to 1.5V under near-maximum current loads. It is also very important that you follow the guidelines of the manufacturer in laying out devices for proper heat dissipation.

Data sheets for these two devices can be downloaded from the manufacturer Web sites.

Maxim MAX8556/8557:

http://pdfserv.maxim-ic.com/en/ds/MAX8556-MAX8557.pdf

Linear Technologies LTC3026:

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1040,C1055,P9891,D7038

ON Semiconductor NCP5663: