Document No.: FT_000329
FT232BL/BQ USB UART IC Datasheet Version 2.2
Clearance No.: FTDI# 171
6.3 Interfacing to 3.3V Logic
6.3.1 Bus Powered Circuit with 3.3V logic drive / supply voltage
Figure 6.3 Bus Powered Circuit with 3.3V logic drive / supply voltage
Figure 6.3 shows how to configure the FT232B 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 MCP1700 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 FT232B to supply the 3.3v without any other components being required. In this
case, connect VCCIO to the 3v3OUT pin of the FT232B.
Note: It should be emphasized 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
FT232B, and not from any other source.
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