Figure 8
USB Self Powered Configuration
FT232BQ USB UART ( USB - Serial) I.C.
470R
USB "B"
Connector
VCC
1
27R
30
2
3
3
26
13
0.1uF
V
C
C
V
C
C
V
FT232BM
A
V
C
C
27R
4
C
C
I
6
3v3OUT
O
33nF
4k7
10k
8
7
USB DM
USB DP
FT232BQ
1k5
5
RSTOUT#
RESET#
4
VCC
A
VCC
G
N
D
G
N
D
G
N
D
14
POWERCTL
+
10uF
9
17
29
0.1uF
0.1uF
Decoupling Capacitors
Figure 8 illustrates a typical USB 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 basic rules for USB Self power devices are as
follows –
a) A Self-Powered device should not force current down the USB bus when the USB Host or Hub Controller is
powered down.
b) A Self Powered Device can take as much current as it likes during normal operation and USB suspend as it has its
own POWER SUPPLY.
c) A Self Powered Device can be used with any USB Host and both Bus and Self Powered USB Hubs
PWRCTL (pin 14) is pulled high to tell the device to use a USB Bus Power descriptor. The power descriptor in the
EEPROM should be programmed to a value of zero. The USB power descriptor option in the EEPROM should be
programmed to a value of zero (self powered).
To meet requirement a) the 1.5K pull-up resistor on USBDP is connected to RSTOUT# as per the bus-power circuit.
However, the USB Bus Power is used to control the RESET# Pin of the FT232BQ device. When the USB Host or
Hub is powered up RSTOUT# will pull the 1.5K resistor on USBDP to 3.3V, thus identifying the device as a full speed
device to USB. When the USB Host or Hub power is off, RESET# will go low and the device will be held in reset. As
RESET# is low, RSTOUT# will also be low, so no current will be forced down USBDP via the 1.5K pull-up resistor
when the host or hub is powered down. Failure to do this may cause some USB host or hub controllers to power up
erratically.
Note : When the FT232BQ is in reset, the UART interface pins all go tri-state. These pins have internal 200K pull-up
resistors to VCCIO, so they will gently pull high unless driven by some external logic.
DS232BQ Version 1.8
© Future Technology Devices Intl. Ltd. 2005
Page 17 of 25
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