FT232R USB UART I.C. Datasheet Version 1.04

© Future Technology Devices International Ltd. 2005

Page 22

7.4 USB Bus Powered with 3.3V / 5V Supply and Logic Drive / IO Supply Voltage

Figure 14 - Bus Powered with 3.3V / 5V Supply and Logic Drive

Figure 14 shows a configuration where a jumper switch is used to allow the FT232R to be interfaced with a 3.3V or

5V logic devices. The VCCIO pin is either supplied with 5V from the USB bus, or with 3.3V from the 3V3OUT pin. The

supply to VCCIO is also used to supply external logic.

Please note the following in relation to bus powered designs of this type -

i) PWREN# or SLEEP# signals should be used to power down external logic during USB suspend mode, in order to

comply with the limit of 500μA. If this is not possible, use the configuration shown in Section 7.3.

ii) The maximum current source from USB Bus during normal operation should not exceed 100mA, otherwise a bus

powered design with power switching (Section 7.3) should be used.

Another possible configuration would be to use a discrete low dropout regulator which is supplied by the 5V on the

USB bus to supply 2.8V - 1.8V to the VCCIO pin and to the external logic. VCC would be supplied with the 5V from

the USB bus. With VCCIO connected to the output of the low dropout regulator, would in turn will cause the FT232R

I/O pins to drive out at 2.8V - 1.8V logic levels.

For USB bus powered circuits some considerations have to be taken into account when selecting the regulator –

iii) The regulator must be capable of sustaining its output voltage with an input voltage of 4.35V. A Low Drop Out

(L.D.O.) regulator must be selected.

iv) The quiescent current of the regulator must be low in order to meet the USB suspend total current requirement of

<= 500μA during USB suspend.

An example of a regulator family that meets these requirements is the MicroChip / Telcom TC55 Series of devices

(www.microchip.com). These devices can supply up to 250mA current and have a quiescent current of under 1μA.

FT232R

A

G

N

D

G

N

D

G

N

D

G

N

D

T

E

S

T

100nF

3V3OUT

VCCIO

NC

RESET#

NC

10nF

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

CBUS0

CBUS3

CBUS2

CBUS1

USBDP

USBDM

VCC

1

2

3

4

5

OSCI

OSCO

CBUS4

Ferrite

Bead

+

SHIELD

GND

GND

GND

3.3V or 5V Supply

to External Logic

100nF

+

100nF

Vcc

4.7uF

GND

1

Jumper

SLEEP#

PWREN#

2

3

Vcc