Copyright © 2013 Future Technology Devices International Limited

23

FT240X USB 8-BIT FIFO IC Datasheet

Version 1.3

Document No.: FT_000626 Clearance No.:

FTDI# 259

6.2 Self Powered Configuration

FT240X

1

2

3

4

5

SHIELD

GND

GND

VCC

GND

VCC

3V3OUT

USBDM

USBDP

VCCIO

VCC(3.3-5.25V)

RESET#

100nF

100nF

100nF

4.7uF

+

GND

4k7

10k

27R

27R

GND

47pF

47pF

VBUS_SENSE

Figure 6.2 Self Powered Configuration

Figure 6.2 illustrates the FT240X in a typical USB self powered configuration. A USB self powered device

gets its power from its own power supply, VCC, and does not draw current from the USB bus. The basic

rules for USB self powered devices are as follows –

i)

A self powered device should not force current down the USB bus when the USB host or hub

controller is powered down.

ii)

A self powered device can use as much current as it needs during normal operation and USB

suspend as it has its own power supply.

iii)

A self powered device can be used with any USB host, a bus powered USB hub or a self

powered USB hub.

The power descriptor in the internal MTP memory of the FT240X should be programmed to a value of

zero (self powered).

In order to comply with the first requirement above, the USB bus power (pin 1) is used to control the

VBUS_Sense pin of the FT240X device. When the USB host or hub is powered up an internal 1.5kΩ

resistor on USBDP is pulled up to +3.3V, thus identifying the device as a full speed device to the USB

host or hub. When the USB host or hub is powered off, VBUS_Sense pin will be low and the FT240X is

held in a suspend state. In this state the internal 1.5kΩ resistor is not pulled up to any power supply

(hub or host is powered down), so no current flows down USBDP via the 1.5kΩ pull-up resistor. Failure to

do this may cause some USB host or hub controllers to power up erratically.

Figure 6.3 illustrates a self powered design which has a +3.3V to +5.25V supply.

Note:

1. When the FT240X is in reset, the interface I/O pins are tri-stated. Input pins have internal 200kΩ

pull-up resistors to VCCIO, so they will gently pull high unless driven by some external logic.