DS232B Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 21 of 25

FT232BM USB UART ( USB - Serial) I.C.

7.6 Interfacing to 3.3v Logic

Figure 14

Bus Powered Circuit with 3.3V logic drive / supply voltage

Ferrite Bead

470R

0.1uF

VCC

27R

USB "B"

Connector

27R

USB DM

8

FT232BM

USB DP

7

V

C

C

V

C

C

V

C

C

I

O

3

26

13

1

2

3

4

33nF

3v3OUT

6

A

V

C

C

30

10nF

0.1uF

In

Out

Gnd

3.3v LDO Regulator

3.3v Power to External

Logic

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.