Document No.: FT_000052
FT245R USB FIFO IC Datasheet Version 2.10
Clearance No.: FTDI# 39
7.3 USB Bus Powered with Power Switching Configuration
P-Channel Power
MOSFET
s
d
Switched 5V Power
to External Logic
0.1uF
0.1uF
g
Soft Start
Circuit
1k
Ferrite
Bead
D0
D1
D2
D3
5V VCC
1
2
3
4
VCC
USBDM
USBDP
FT245R
VCCIO
NC
D4
D5
+
5
10nF
RESET#
NC
SHIELD
D6
OSCI
D7
OSCO
GND
TXE#
RXF#
RD#
5V VCC
5V VCC
3V3OUT
100nF
4.7uF
+
A
G
N
D
T
E
S
T
100nF
10K
G
N
D
G
N
D
G
N
D
WR#
PWREN#
GND
GND
GND
Figure 7.3 Bus Powered with Power Switching Configuration
A requirement of USB bus powered applications, is when in USB suspend mode the application draws a
total current of less than 2.5mA. This requirement includes external logic. Some external logic has the
ability to 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 FT245R provides a simple but effective method of turning
off power during the USB suspend mode.
Figure 7.3 shows an example of using a discrete P-Channel MOSFET to control the power to external
logic. A suitable device to do this is 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 is used
to limit the current surge when the MOSFET turns on. Without the soft start circuit it is possible that the
transient power surge, caused when the MOSFET switches on, will reset the FT245R or the USB host/hub
controller. The soft start circuit example shown in Figure 7.3 powers up with a slew rate of
approximaely12.5V/ms. Thus supply voltage to external logic transitions from GND to +5V in
approximately 400 microseconds.
As an alternative to the MOSFET, a dedicated power switch IC with inbuilt “soft-start” can be used. A
suitable power switch IC for such an application is the Micrel (www.micrel.com) MIC2025-2BM or
equivalent.
With power switching controlled designs the following should be noted:
i) The external logic to which the power is being switched should have its own reset circuitry to
automatically reset the logic when power is re-applied when moving out of suspend mode.
ii) Set the Pull-down on Suspend option in the internal FT245R EEPROM.
iii) The PWREN# pin should be used to switch the power to the external circuitry.
iv) For USB high-power bus powered applications (one that consumes greater than 100mA, and up
to 500mA of current from the USB bus), the power consumption of the application must be set in
the Max Power field in the internal FT245R EEPROM. A high-power bus powered application uses
the descriptor in the internal FT245R EEPROM to inform the system of its power requirements.
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24
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