FTDI FT8U232 Скачать руководство пользователя страница 3

FT8U2XXAM

High Speed USB Controllers for serial and FIFO applications

Future Technology Devices Intl.                      AN232-08 Rev 1.00      Page 

3

SELF POWERED OPTIONS

1. The Standard Self Powered Schematic

In this case the RESET# and RCCLK circuits are the same as for the standard bus
powered version.

The USB specification states that a peripheral device is not allowed to source power into a
USB host or hub port when the PC or hub is powered down. When a device is self
powered ( has it’s own PSU ) it would be possible for the device to force current down the
D+ line via the 1.5k resistor on USB D+  when the PC ( or hub ) is turned off and the
device is still powered up through it’s own PSU. Failure to pay attention to this can cause
unreliable enumeration on some hosts and hubs.

In the above diagram, VCC is supplied from the self-powered device’s PSU whilst
USBVCC refers to the voltage supplied by VCC on the power pin of the upstream
connector. The solution is to connect an NPN transistor in series with the 1.5k pull up
resistor on USB D+,  getting it’s bias current from USBVCC as shown above. The voltage
from the upstream connector will turn the transistor off and on. If it is unplugged or the
host PC is off, the transistor is turned off  and then there is no current supplied to the D+
line through the 1.5k pull up.

0.01uF

100K

2N3906

RESET#

vcc

GND

100K

470K

0.1uF

100K

RCCLK

1.5K

USBDP (D+)

3v3OUT

Connections for RESET#, RCCLK and USBDP in a self powered mode

2N3904

USBVCC

100K

20K

transistor is used to

prevent supplying

current upstream

when host PC is

powered off

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