UM232R Просмотр технического описания (PDF) - Future Technology

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UM232R

USB-Serial UART Development Module

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6.4 USB Bus Powered with 3.3V Logic Drive / IO Supply Voltage

123

1 J1

24

Page 16

J2

12

13

Figure 7 - USB Bus Powered 3.3V Logic Drive

Figure 7 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

on 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 500μA. If this is not possible, use the configuration shown in Section 6.3.

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

powered design with power switching (Section 6.3) 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 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, 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.

UM232R USB-Serial UART Development Module Datasheet Version 1.02

© Future Technology Devices International Ltd. 2005

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