Nexys2 Reference Manual
Digilent
www.digilentinc.com
Copyright Digilent, Inc.
Page 2/17
Doc: 502-134
Figure 3: Nexys2 power supply jumpers
Figure 2: Nexys2 power supply block diagram
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
Table 1: Nexys2 Power Supplies
Supply
Device
Amps (max/typ)
3.3V main
IC6: LTC1765
3A/100mA
2.5V FPGA
IC7: LTC3417
1.4A/50mA
1.2V FPGA
IC7: LTC3417
1.4A/200mA
1.8V SRAM IC8: LTC1844
150mA/90mA
3.3V USB
IC5: LTC1844
150mA/60mA
