Copyright © 2011 Future Technology Devices International Limited

21

Document No.: FT_000329

FT232BL

Clearance No.: FTDI#

171

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

Figure 6.5 Bus Powered with Power Switching Configuration

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). This limit is 2.5mA for devices that

support remote wake-up. 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 FT232B

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

Figure 8.3 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 FT232B, 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) MIC2025-2BM 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 FT232B’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 FT232B.