Copyright © 2013 Future Technology Devices International Limited

22

FT240X USB 8-BIT FIFO IC Datasheet

Version 1.3

Document No.: FT_000626 Clearance No.:

FTDI# 259

6 USB Power Configurations

The following sections illustrate possible USB power configurations for the FT240X. The illustrations have

omitted pin numbers for ease of understanding since the pins differ between the FT240XS and FT240XQ

package options.

All USB power configurations illustrated apply to both package options for the FT240X device. Please refer

to Section 0 for the package option pin-out and signal descriptions.

6.1 USB Bus Powered Configuration

FT240X

1

2

3

4

5

SHIELD

Ferrite

Bead

GND

GND

VCC

GND

VCC

3V3OUT

USBDM

USBDP

VCCIO

VCC

RESET#

100nF

100nF

10nF

4.7uF

+

27R

27R

GND

47pF

47pF

Figure 6.1 Bus Powered Configuration

Figure 6.1 illustrates the FT240X in a typical USB bus powered design configuration. A USB bus powered

device gets its power from the USB bus. Basic rules for USB bus power devices are as follows –

i)

On plug-in to USB, the device should draw no more current than 100mA.

ii)

In USB Suspend mode the device should draw no more than 2.5mA.

iii)

A bus powered high power USB device (one that draws more than 100mA) should use the

PWREN# to keep the current below 100mA on plug-in and 2.5mA on USB suspend.

iv)

A device that consumes more than 100mA cannot be plugged into a USB bus powered hub.

v)

No device can draw more than 500mA from the USB bus.

The power descriptors in the internal MTP memory of the FT240X should be programmed to match the

current drawn by the device.

A ferrite bead is connected in series with the USB power supply to reduce EMI noise from the FT240X and

associated circuitry being radiated down the USB cable to the USB host. The value of the Ferrite Bead

depends on the total current drawn by the application. A suitable range of Ferrite Beads is available from

Steward (www.steward.com), for example Steward Part # MI0805K601R-10.