DS232BQ Version 1.8

© Future Technology Devices Intl. Ltd. 2005

Page 24 of 25

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

7.7 Power Switching

Figure 16

Bus Powered Circuit ( <= 100mA ) with Power Control

Ferrite Bead

470R

0.1uF

VCC

27R

USB "B"

Connector

27R

USB DM

8

FT232BM

USB DP

7

V

C

C

V

C

C

V

C

C

I

O

3

26

13

1

2

3

4

33nF

3v3OUT

6

A

V

C

C

30

10nF

PWREN#

15

0.1uF

Switched 5v Power to

External Logic

P-Channel Power

MOSFET

s

g

d

0.1uF

1K

Soft Start

Circuit

USB Bus powered circuits need to be able to power down in USB suspend mode in order to meet the <= 500uA total

suspend current requirement (including external logic). Some external logic can power itself down into a low current

state by monitoring the PWREN# pin. For external logic that cannot power itself down in that way, the FT232BQ

provides a simple but effective way of turning off power to external circuitry during USB suspend.

Figure 16 shows how to use a discrete P-Channel Logic Level MOSFET to control the power to external logic

circuits. A suitable device could be a Fairchild NDT456P, or International Rectifier IRLML6402, or equivalent. It is

recommended that a “soft start” circuit consisting of a 1K series resistor and a 0.1 uF 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 FT232BQ, or the USB host / hub controller. The values used here

allow attached circuitry to power up with a slew rate of ~12.5 V 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) MIC2025-2BL or equivalent.

Please note the following points in connection with power controlled designs –

a) 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.

b) Set the Pull-down on Suspend option in the FT232BQ’s EEPROM.

c) For USB high-power bus powered device (one that consumes greater than 100 mA, and up to 500 mA of current

from the USB bus), the power consumption of the device should be set in the max power field in the EEPROM.

A high-power bus powered device must use this descriptor in the EEPROM to inform the system of its power

requirements.

d) For 3.3V power controlled circuits VCCIO must not be powered down with the external circuitry (PWREN# 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 if appropriate power VCCIO from the 3V3OUT pin of the FT232BQ.

FT232BQ