Datasheet

Document Reference No.: FT_000051

UM232R USB - Serial UART Development Module

Datasheet Version 1.1

Clearance No.: FTDI# 125

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

2 1

1 2 3

UM232R © FTDI 2008

FT232RL

FTDI

12 13

Figure 7.4 USB Bus Powered 3.3V Logic Drive

Figure 7.4 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 FT232R‟s VCCIO pin is either supplied with 5V from the USB bus (connect

together pins 2 and 3 in J1), or with 3.3V from the FT232R‟s 3V3OUT pin (connect together pins 1 and 2 on

J1 as shown) the supply to UM232R‟s 3V3 pin can also be used to supply up to 50mA to 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 500 μA. 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 regulation which is supplied by the

5V on the USB bus to supply 2.8V – 1.8V to the VIO pin and to the external logic. VCC would be supplied

with the 5V from the USB bus (available from the module‟s USB pin). With VIO connected to the output of

the low dropout regulator, this will cause the FT232R I/O pins to drive out at 2.8V – 1.8V logic levels.

For USB bus powered circuit some considerations have to be taken into account when selecting the

regulator:

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.

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

requirement of <= 500 μA 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 1 μA.