The XEM7305 must be powered externally via the two expansion connectors with several well-regulated power supplies.

To reduce cost and size, the XEM7305 is designed with only one on-board linear power supply which is used for the USB microcontroller core voltage. All other power must be provided from the attached peripheral. The additional power supplies provide DC power for the USB microcontroller, FPGA, DDR3 memory, and other peripherals. In addition to allowing reuse of existing peripheral supplies, this also facilitates the use of linear-only supplies for applications that cannot tolerate switching supplies.

The USB’s 5v supply is provided to the expansion header and power header to enable the design of bus-powered applications. The USB 3.0 specification allows for up to 4.5 W (900 mA at 5 V) to be provided to external peripherals over the USB cable. While power consumption of an unconfigured XEM7305 is quite low, due to the flexibility allowed in FPGA design, the Spartan 7 and DDR3 could potentially consume over 4.5 W during operation with a user design, thus violating the USB specification.

Before relying on USB power, you should be aware of the limitations and the fact that using USB power may render the XEM7305 a USB-noncompliant device.

The following fixed supplies are required by the XEM7305. These must be provided by the user peripheral attached to the XEM7305. Our BRK7305 may be used as a reference for the required supplies. Schematics and layout files in Altium Designer format are available for download through Pins.

The following table provides suggestions for operating currents on the various supply rails. Unless otherwise noted, these numbers do not take into account the power requirements of the FPGA. The user must determine these requirements using power estimates or empirical data once the final system design has been determined. Power requirements can vary significantly depending on operating frequencies, temperature, I/O standards, switching rates, logic, and other variables. The suggested supply guidelines are rather conservative and also reflect sizing to common LDO and switcher limits.

The recommended supply sequencing at power-up is as follows:

1. +1.0VDD
2. +1.8VDD
3. +VDD_DDR3, +3.3VDD, VCCO_MC1, and VCCO_MC2 in any order

The recommended sequence at power-down is the reverse of the power-up sequence.

When applying 3.3V to VCCO_MC1 or VCCO_MC2, the Spartan-7 requires additional timing constraints between VCCO and VCCAUX (+1.8VDD). See the Power-On/Off Power Supply Sequencing section in the Spartan-7 Data Sheet (DS189) for additional details.

The USB’s 5v supply is provided to the expansion header and power header to enable the design of bus-powered applications. The USB 3.0 specification allows for up to 4.5 W (900mA at 5v) to be provided to external peripherals over the USB cable. While power consumption of an unconfigured XEM7305 is quite low, due to the flexibility allowed in FPGA design, the Spartan-7 and DDR3 could potentially consume over 4.5 W during operation with a user design, thus violating the USB specification.

Additionally, there is a USB voltage supervisor (ON Semiconductor NCP361SNT1G) present at the USB bus power. This device has a low over-current protection limit that can trip, preventing the device from enumerating due to in-rush current requirements of the USB microcontroller.

Before relying on USB power, you should be aware of the limitations and the fact that using USB power may render the XEM7305 a USB-noncompliant device.

USB bus power is available on MC1-3 and can be used by the attached peripheral device to generate the power rails required by the XEM7305.