Copyright © 2015 Future Technology Devices International Limited

20

FT234XD USB TO BASIC UART IC

Version 1.2

Document No.: FT_000753 Clearance No.: FTDI#324

6 USB Power Configurations

The following sections illustrate possible USB power configurations for the FT234XD. The illustrations

have omitted pin numbers for ease of understanding since the pins differ between the FT234XDS and

FT234XDQ package options.

All USB power configurations illustrated apply to both package options for the FT234XD device. Please

refer to Section 9 for the package option pin-out and signal descriptions.

6.1 USB Bus Powered Configuration

FT234XD

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 FT234XD 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

CBUS pin configured as PWREN# and use it 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 FT234XD 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 FT234XD

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 Laird Technologies (http://www.lairdtech.com) for example Laird Technologies Part #

MI0805K601R-10.