MM232R Mini USB-Serial UART Development Module Datasheet Version 1.00

© Future Technology Devices International Ltd. 2005

Page 15

6.3 USB Bus Powered with Power Switching Configuration

Figure 7 - Bus Powered with Power Switching Configuration

USB Bus powered circuits need to be able to power down in USB suspend mode in order to meet the <= 500μA

total USB suspend current requirement (including external logic). Some external logic can power itself down into a

low current state by monitoring the PWREN# signal. For external logic that cannot power itself down in this way, the

FT232R provides a simple but effective way of turning off power to external circuitry during USB suspend.

Figure 7 shows how to use a discrete P-Channel Logic Level MOSFET to control the power to external logic circuits.

a “soft start” circuit consisting of a 1kΩ series resistor and a 0.1μF capacitor are used to limit the current surge when

the MOSFET turns on. Without the soft start circuit there is a danger that the transient power surge of the MOSFET

turning on will reset the FT232R, or the USB host / hub controller. The values used here allow attached circuitry to

power up with a slew rate of ~12.5V per millisecond, in other words the output voltage will transition from GND to 5V in

approximately 400 microseconds.

Alternatively, a dedicated power switch I.C. with inbuilt “soft-start” can be used instead of a MOSFET. A suitable power

Please note the following points in connection with power controlled designs –

i) The logic to be controlled must have its own reset circuitry so that it will automatically reset itself when power is re-

applied on coming out of suspend.

ii) Set the Pull-down on Suspend option in the internal EEPROM.

iii) The PWREN# pin should be used to switch the power supply to the external circuitry.

iv) For USB high-power bus powered device (one that consumes greater than 100mA, and up to 500mA of current

from the USB bus), the power consumption of the device should be set in the max power field in the internal

EEPROM. A high-power bus powered device must use this descriptor in the internal EEPROM to inform the

system of its power requirements.

v) For 3.3V power controlled circuits the VCCIO pin must not be powered down with the external circuitry (the

PWREN# signal gets its VCC supply from VCCIO). Either connect the power switch between the output of the

3.3V regulator and the external 3.3V logic or power VCCIO from the 3V3OUT pin of the FT232R.

MM232R

© FTDI 2005

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TXD

RXD

VCC

VCCIO

USBPWR

VCC50

PWREN#

s

g

d

P-Channel Power

MOSFET

0.1uF

0.1uF

1k

Soft Start

Circuit

Switched 5V Power

to External Logic