DS232BQ Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 16 of 25

FT232BQ USB UART ( USB - Serial) I.C.

7.3 USB Bus Powered and Self Powered Configuration

Figure 7

USB Bus Powered Configuration

VCC

27R

USB "B"

Connector

27R

USB DM

RSTOUT#

8

FT232BM

USB DP

7

5

V

C

C

V

C

C

V

C

C

I

O

3

26

13

1

2

3

4

Ferrite Bead

1k5

33nF

3v3OUT

6

470R

A

V

C

C

30

0.1uF

POWERCTL

14

G

N

D

G

N

D

A

G

N

D

9

17

29

10nF

VCC

RESET#

4

Decoupling Capacitors

10uF

VCC

0.1uF

0.1uF

+

Figure 7 illustrates a typical USB bus powered configuration. A USB Bus Powered device gets its power from the USB

bus. Basic rules for USB Bus power devices are as follows –

a) On plug-in, the device must draw no more than 100mA

b) On USB Suspend the device must draw no more than 500uA.

c) A Bus Powered High Power Device (one that draws more than 100mA) should use the PWREN# pin to keep the

current below 100mA on plug-in and 500uA on USB suspend.

d) A device that consumes more than 100mA can not be plugged into a USB Bus Powered Hub

e) No device can draw more that 500mA from the USB Bus.

PWRCTL (pin 14) is pulled low to tell the device to use a USB Bus Power descriptor. The power descriptor in the

EEPROM should be programmed to match the current draw of the device.

A Ferrite Bead is connected in series with USB power to prevent noise from the device and associated circuitry (EMI)

being radiated down the USB cable to the Host. The value of the Ferrite Bead depends on the total current required by

the circuit – a suitable range of Ferrite Beads is available from Steward (www.steward.com) for example Steward Part

#

MI0805K400R-00 also available as DigiKey Part # 240-1035-1.

FT232BQ