MM232R Mini USB-Serial UART Development Module Datasheet Version 1.00

© Future Technology Devices International Ltd. 2005

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

Figure 8 - Bus Powered with 3.3V Logic Drive

Figure 8 shows a configuration where the MM232R module has been configured to be interfaced with a 3.3V MCU. To

do this the MM232R’s VCCIO pin is connect to VCC3O. The MM232R’s VCC3O 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

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

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. The LDO would be supplied with the 5V

from the USB bus (available from the module’s USBPWR pin). VCCIO would be connected to the output of the low

dropout regulator, would in turn will cause the MM232R 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

MM232R

© FTDI 2005

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TXD

RXD

VCC3O

VCCIO

VCC50

RTS#

CTS#

CLK12M

GND

RTS#

TXD

CTS#

RXD

CLK_IN

I/O

MCU

VCC3O

USBPWR

PWREN#