FT245BQ USB FIFO ( USB - Parallel ) I.C.
Figure 9
Bus Powered Circuit with 3.3V logic drive / supply voltage
3.3v LDO Regulator
3.3v Power to External
Logic
In
Out
Gnd
0.1uF
Ferrite Bead
27R
470R
USB "B"
Connector
VCC
1
2
3
4
30
3
26
13
0.1uF
V
C
C
V
C
C
V
A
V
C
C
27R
C
C
I
6
10nF
3v3OUT
O
33nF
FT245BQ
FT245BM
8
7
USB DM
USB DP
Figure 9 shows how to configure the FT245BQ to interface with a 3.3V logic device. In this example, a discrete
3.3V regulator is used to supply the 3.3V logic from the USB supply. VCCIO is connected to the output of the 3.3V
regulator, which in turn will cause the FIFO interface IO pins to drive out at 3.3V level. For USB bus powered circuits
some considerations have to be taken into account when selecting the regulator –
a) The regulator must be capable of sustaining its output voltage with an input voltage of 4.35 volts. A Low Drop Out
(LDO) regulator must be selected.
b) The quiescent current of the regulator must be low in order to meet the USB suspend total current requirement of
<= 500uA during USB suspend.
An example of a regulator family that meets these requirements is the MicroChip (Telcom) TC55 Series. These
devices can supply up to 250mA current and have a quiescent current of under 1uA.
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 : It should be emphasised that the 3.3V supply for VCCIO in a bus powered design with a 3.3V logic interface
should come from an LDO which is supplied by USB bus, or from the 3V3OUT pin of the FT245BQ, and not from any
other source.
DS245BQ Version 1.7
© Future Technology Devices Intl. Ltd. 2005
Page 19 of 24
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