Digilent Nexys2 Reference Manual - Page 2

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Nexys2 Reference Manual

The input power bus drives a 3.3V voltage regulator

that supplies all required board current. Some devices

require 2.5V, 1.8V, and 1.2V supplies in addition to the

main 3.3V supply, and these additional supplies are

created by regulators that take their input from the

main 3.3V supply. The primary supplies are generated

by highly efficient switching regulators from Linear

Technology. These regulators not only use USB power

efficiently, they also allow the Nexys2 to run from

battery packs for extended periods.

Total board current depends on the FPGA configuration,

clock frequency, and external connections. In test

circuits with roughly 20K gates routed, a 50MHz clock

source, and all LEDs illuminated, about 200mA of

current is drawn from the 1.2V supply, 50mA from the

2.5V supply, and 100mA from the 3.3V supply. Required

current will increase if larger circuits are configured in

the FPGA, and if peripheral boards are attached. The

table above summarizes the power supply parameters.

The Nexys2 board can also receive power from (or

deliver power to) a peripheral board connected to a

Pmod connector or to the large 100-pin expansion

connector. Jumpers near the Pmod connectors and

large expansion connector (JP1 – JP5) can connect the

Nexys2's input power bus to the connector's power pins.

The Pmod jumpers can be used to route either the input

power bus or regulated 3.3V to the Pmod power pins,

while the expansion connector jumper can only make or

break a connection with the input power bus.

USB power is supplied to the USB circuitry directly, but

to the rest of the board through an electronic switch (Q1

in the Nexys2 schematic). The on-board USB controller

turns on switch Q1 only after informing the host PC that

Figure 2: Nexys2 power supply block diagram

Page 2/17

Table 1: Nexys2 Power Supplies

Supply

Device

3.3V main

IC6: LTC1765

2.5V FPGA

IC7: LTC3417

1.2V FPGA

IC7: LTC3417

1.8V SRAM

IC8: LTC1844

3.3V USB

IC5: LTC1844

Figure 3: Nexys2 power supply jumpers

Digilent

www.digilentinc.com

Amps (max/typ)

3A/100mA

1.4A/50mA

1.4A/200mA

150mA/90mA

150mA/60mA

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