Datasheet

Document Reference No.: FT_000202

UM245R USB - Parallel FIFO Development Module Datasheet Version 1.04

Clearance No.: FTDI# 124

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

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 FT245R to be interfaced with

a 3.3V or 5V logic devices. The FT245R‟s VCCIO pin is either supplied with 5V from the USB bus (connect

together pins 2 and 3 on J1), or with 3.3V from the FT245R‟s 3V3OUT pin (connect together pins 1 and 2

on J1 as shown). The supply to UM245R‟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) The PWREN# signal should be used to power down external logic during USB suspend mode, in

order to comply with the limit of 500μA (2.5mA with remote wake-up enabled). 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 LDO 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. With VIO connected to the output of the low dropout regulator would, in turn, will

cause the FT245R 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 as low as

4.35V. A Low Drop Out (LDO) 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/2.5mA during USB suspend.

An example of a regulator family that meets these requirements is the MicroChip MCP17xx Series of

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

current of under 1μA.