FT245BL USB FIFO ( USB - Parallel ) I.C.
7.3 USB Bus Powered and Self Powered Configuration
Figure 7
USB Bus Powered Configuration
Ferrite Bead
470R
USB "B"
Connector
VCC
1
27R
30
2
3
3
26
13
0.1uF
V
C
C
V
C
C
V
FT245BM
A
V
C
C
27R
4
C
C
I
6
10nF
3v3OUT
O
33nF
8
7
USB DM
USB DP
FT245BL
1k5
5
RSTOUT#
RESET#
VCC
VCC
4
A
G
N
D
G
N
D
G
N
D
+
10uF
9
17
29
0.1uF
0.1uF
Decoupling Capacitors
Figure 7 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.
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 on USB suspend.
d) A device that consumes more than 100mA can not 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.
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.
DS245BL Version 1.7
© Future Technology Devices Intl. Ltd. 2005
Page 17 of 24
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