DS232B Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 22 of 25

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

VCC5V

27R

USB "B"

Connector

27R

USB DM

RSTOUT#

8

FT232BM

USB DP

7

5

V

C

C

I

O

13

1

2

3

4

1K5

33nF

3v3OUT

6

0.1uF

G

N

D

G

N

D

A

G

N

D

9

17

29

RESET#

4

10K

4K7

Decoupling Capacitors

10uF

VCC5V

0.1uF

0.1uF

+

VCC3V

VCC3V

PWRCTL

V

C

C

V

C

C

3

26

470R

A

V

C

C

30

14

Figure 15

Self Powered Circuit with 3.3V logic drive / supply voltage

Figure 15 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 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 that in this case PWRCTL is pulled up to VCCIO, not VCC.