EVAL232R Datasheet, PDF (13/18 Pages) Future Technology Devices International Ltd. – FT232RL USB to RS232 Evaluation Module

6. Module Configurations
Page 13
6.1 Bus Powered Configuration
The EVAL232R module is designed to be powered from the 5V supply available on the USB bus powered. VCCIO, the
supply to the FT232RL’s UART and CBUS IO pins, can be connected to VCC or VCC3O, depending on wheather 5V
or 3.3V output levels are required on the CBUS.
A USB Bus Powered device gets its power from the USB bus. Basic rules for USB Bus power devices are as follows –
i) On plug-in to USB, the device must draw no more than 100mA.
ii) On USB Suspend the device must draw no more than 500μA.
iii) A Bus Powered High Power USB Device (one that draws more than 100mA) should use PWREN# to keep the
current below 100mA on plug-in and 500μA on USB suspend.
iv) A device that consumes more than 100mA can not be plugged into a USB Bus Powered Hub.
v) No device can draw more that 500mA from the USB Bus.
6.2 Self Powered Configuration
A USB Self Powered device gets its power from its own power supply and does not draw current from the USB bus.
The EVAL232R is not designed to be used in a USB self powered configuration. However, the FT232R chip can be
used in such a configuration. See the FT232R datasheet for more details on how to do this.
6.3 USB 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.
This can be done using a discrete P-Channel Logic Level MOSFET to control the power to external logic circuits. A
suitable device would be an International Rectifier (www.irf.com) IRLML6402, or equivalent. It is recommended that
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
switch I.C. for such an application would be a Micrel (www.micrel.com) MIC2025-2BM or equivalent.
See the FT232R datasheet for more details, and an example schematic for a bus powered design with power
switching.
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
EVAL232R FT232RL USB to RS232 Evaluation Module Datasheet Version 0.90
© Future Technology Devices International Ltd. 2005