Copyright © 2013 Future Technology Devices International Limited

24

FT240X USB 8-BIT FIFO IC Datasheet

Version 1.3

Document No.: FT_000626 Clearance No.:

FTDI# 259

6.3 USB Bus Powered with Power Switching Configuration

FT240X

1

2

3

4

5

SHIELD

Ferrite

Bead

GND

GND

VCC

GND

VCC

3V3OUT

USBDM

USBDP

VCCIO

RESET#

100nF

100nF

10nF

4.7uF

+

CBUS5

PWREN#

10k

1k

Switched 5V Power to

External Logic

0.1uF

0.1uF

P Channel Power

MOSFET

27R

27R

47pF

47pF

Figure 6.4 Bus Powered with Power Switching Configuration

A requirement of USB bus powered applications, is when in USB suspend mode the application draws a

total current of less than 2.5mA. This requirement includes external logic. Some external logic has the

ability to power itself down into a low current state by monitoring the PWREN# signal. For external logic

that cannot power itself down in this way, the FT240X provides a simple but effective method of turning

off power during the USB suspend mode.

Figure 6.4 shows an example of using a discrete P-Channel MOSFET to control the power to external

logic. A suitable device to do this is an International Rectifier (www.irf.com) IRLML6402, or equivalent. It

is recommended that a “soft start” circuit consisting of a 1kΩ series resistor and a 0.1μF capacitor is used

to limit the current surge when the MOSFET turns on. Without the soft start circuit it is possible that the

transient power surge, caused when the MOSFET switches on, will reset the FT240X or the USB host/hub

controller. The soft start circuit example shown in Figure 6.4 powers up with a slew rate of

approximaely12.5V/ms. Thus supply voltage to external logic transitions from GND to +5V in

approximately 400 microseconds.

As an alternative to the MOSFET, a dedicated power switch IC with inbuilt “soft-start” can be used. A

suitable power switch IC for such an application is the Micrel (www.micrel.com) MIC2025-2BM or

equivalent.

With power switching controlled designs the following should be noted:

i) The external logic to which the power is being switched should have its own reset circuitry to

automatically reset the logic when power is re-applied when moving out of suspend mode.

ii) Set the Pull-down on Suspend option in the internal FT240X MTP memory.

iii) The PWREN# pin should be used to switch the power to the external circuitry.

iv) For USB high-power bus powered applications (one that consumes greater than 100mA, and up

to 500mA of current from the USB bus), the power consumption of the application must be set in

the Max Power field in the internal FT240X MTP memory. A high-power bus powered application

uses the descriptor in the internal FT240X MTP memory to inform the system of its power

requirements.

v) PWREN# gets its VCC from VCCIO. For designs using 3V3 logic, ensure VCCIO is not powered

down using the external logic. In this case use the +3V3OUT.