FT232R Future Technology Devices Intl Ltd., FT232R Datasheet - Page 20

• Manufacturer Part Number: FT232R
• Manufacturer: Future Technology Devices Intl Ltd.

Figure 11 - 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 11 shows how to use 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.
Please note the following points in connection with power controlled designs –
i)
ii) Set the Pull-down on Suspend option in the internal EEPROM.
iii) One of the CBUS Pins should be configured as PWREN# in the internal EEPROM, and should be used to switch
iv) For USB high-power bus powered device (one that consumes greater than 100mA, and up to 500mA of current
v) For 3.3V power controlled circuits the VCCIO pin must not be powered down with the external circuitry (the
FT232R USB UART I.C. Datasheet Version 1.02
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.
the power supply to the external circuitry..
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.
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.
7.3 USB Bus Powered with Power Switching Configuration
SHIELD
5
1
2
3
4
10nF
100nF
GND
+
Ferrite
Bead
4.7uF
GND
+
5V VCC
P-Channel Power
5V VCC
s
MOSFET
g
d
1k
0.1uF
GND
100nF
Soft Start
Circuit
VCC
USBDM
USBDP
3V3OUT
VCCIO
RESET#
OSCI
OSCO
NC
NC
© Future Technology Devices International Ltd. 2005
A
G
N
D
0.1uF
FT232R
G
N
D
G
N
D
GND
Switched 5V Power
G
N
D
to External Logic
T
E
S
T
CBUS0
CBUS1
CBUS2
CBUS3
CBUS4
DSR#
DCD#
RTS#
CTS#
DTR#
RXD
TXD
RI#
PWREN#