FT245BQ USB FIFO ( USB - Parallel ) I.C.
Figure 10
Self Powered Circuit with 3.3 V logic drive / supply voltage
VCC3V
VCC5V
470R
USB "B"
Connector
1
2
3
4
27R
27R
0.1uF
3
26
13
30
V
C
C
V
C
C
V
C
C
I
A
V
C
C
6
O
3v3OUT
33nF
FT245BM
FT245BQ
4K7
8
7
USB DM
USB DP
VCC3V
10K
1K5
11
5
4
SI/WU
RSTOUT#
RESET#
VCC5V
A
G
N
D
G
N
D
G
N
D
+
9
17
29
0.1uF
0.1uF
10uF
Decoupling Capacitors
Figure 10 is an example of a USB self powered design with 3.3V interface. In this case VCCIO is supplied by an
external 3.3V supply in order to make the device IO pins drive out at 3.3V logic level, thus allowing it to be connected
to a 3.3V MCU or other external logic. A USB self powered design uses its own power supplies, and does not draw
any of its power from the USB bus. In such cases, no special care need be taken to meet the USB suspend current
(0.5 mA) as the device does not get its power from the USB port.
As with bus powered 3.3V interface designs, in some cases, where only a small amount of current is required (<5mA),
it may be possible to use the in-built regulator of the FT245BQ to supply the 3.3V without any other components being
required. In this case, connect VCCIO to the 3v3OUT pin of the FT245BQ.
Note that if SI/WU is not being used it should be pulled up to VCCIO.
DS245BQ Version 1.7
© Future Technology Devices Intl. Ltd. 2005
Page 20 of 24
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