Document No.: FT_000329
FT232BL/BQ USB UART IC Datasheet Version 2.2
Clearance No.: FTDI# 171
6 USB Power Configurations
The following sections illustrate possible USB power configurations for the FT232B.
6.1 USB Bus Powered Configuration
Figure 6.1 Bus Powered Configuration
Figure 6.1 illustrates a typical USB bus powered configuration. A USB Bus Powered device gets its power
from the USB bus. Basic rules for USB Bus power devices are as follows:
a) On plug-in, the device must draw no more than 100mA
b) On USB Suspend the device must draw no more than 500uA (2.5mA for devices configured to
support remote wake-up).
c) A Bus Powered High Power Device (one that draws more than 100mA) should use the PWREN#
pin to keep the current below 100mA on plug-in and 500uA(2.5mA) on USB suspend.
d) A device that consumes more than 100mA cannot be plugged into a USB Bus Powered Hub
e) No device can draw more that 500mA from the USB Bus.
The power descriptor in the EEPROM should be programmed to match the current draw of the device.
PWRCTL (pin 14) is pulled low to configure the device with a USB Bus Power descriptor. A Ferrite Bead is
connected in series with USB power to prevent noise from the device and associated circuitry (EMI) being
radiated down the USB cable to the Host. The value of the Ferrite Bead depends on the total current
required by the circuit – a suitable range of Ferrite Beads is available from Steward (www.steward.com)
for example Steward Part # MI0805K400R-00, also available as DigiKey Part # 240-1035-1.
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