Document No.: FT_000329
FT232BL/BQ USB UART IC Datasheet Version 2.2
Clearance No.: FTDI# 171
6.2 Self Powered Configuration
Figure 6.2 Self Powered Configuration
Figure 6.2 illustrates a typical USB self powered configuration. A USB Self Powered device gets its power
from its own POWER SUPPLY and does not draw current from the USB bus. The basic rules for USB Self
power devices are as follows:
a) A Self-Powered device should not force current down the USB bus when the USB Host or Hub
Controller is powered down.
b) A Self Powered Device can take as much current as it requires during normal operation and USB
suspend as it has its own POWER SUPPLY.
c) A Self Powered Device can be used with any USB Host and both Bus and Self Powered USB Hubs
PWRCTL (pin 14) is pulled high to configure the device with a USB Bus Power descriptor. The power
descriptor in the EEPROM should be programmed to a value of zero. The USB power descriptor option in
the EEPROM should be programmed to a value of zero (self powered).
To meet requirement, the 1.5 K pull-up resistors on USBDP is connected to RSTOUT# as per the bus-
power circuit. However, the USB Bus Power is used to control the RESET# Pin of the FT232B device.
When the USB Host or Hub is powered up RSTOUT# will pull the 1.5K resistor on USBDP to 3.3V, thus
identifying the device as a full speed device to USB. When the USB Host or Hub power is off, RESET# will
go low and the device will be held in reset. As RESET# is low, RSTOUT# will also be low, so no current
will be forced down USBDP via the 1.5K pull-up resistor when the host or hub is powered down. Failure to
do this may cause some USB host or hub controllers to power up erratically.
Note: When the FT232B is in reset, the UART interface pins all go tri-state. These pins have internal 200K
pull-up resistors to VCCIO, so they will gently pull high unless driven by some external logic.
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