FT232BM Datasheet, PDF (21/25 Pages) Future Technology Devices International Ltd. – USB UART ( USB - Serial) I.C.

7.6 Interfacing to 3.3v Logic
FT232BM USB UART ( USB - Serial) I.C.
Figure 14
Bus Powered Circuit with 3.3V logic drive / supply voltage
3.3v LDO Regulator
In
Out
Gnd
3.3v Power to External
Logic
0.1uF
USB "B"
Connector
1
2
3
4
Ferrite Bead
VCC
27R
27R
10nF
33nF
470R
3 26 13
VV V
CC C
CC C
6
I
3v3OUT _O
30
A
V
C
C
FT232BM
8 _{USB DM}
7 _{USB DP}
0.1uF
Figure 14 shows how to configure the FT232BM 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 UART 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 FT232BM to supply the 3.3v without any other components being required. In this case, connect
VCCIO to the 3v3OUT pin of the FT232BM.
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 the USB bus, or from the 3V3OUT pin of the FT232BM, and not from
any other source.
DS232B Version 1.8 © Future Technology Devices Intl. Ltd. 2005
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