6.2 Self Powered Configuration
FT240X USB 8-BIT FIFO IC Datasheet
Document No.: FT_000626 Clearance No.: FTDI# 259
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.
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.
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