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Document Reference No.: FT_000214
MM232R USB - Serial UART Development Module Incorporating Clock Generator
Datasheet Version 1.1
Clearance No.: FTDI# 132
6.4 Bus Powered with 3.3V Logic Drive / IO Supply Voltage
Figure 6.4 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 show n) the supply to MM232R?s 3V3 pi n 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)
ii)
PWRE N# 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
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.
? Copyright 2010 Future Technology Devices International Ltd
16
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