Copyright © Future Technology Devices International Limited

21

FT232R USB UART IC Datasheet

Version 2.16

Document No.: FT_000053 Clearance No.: FTDI# 38

6.3 USB Bus Powered with Power Switching Configuration

FT232R

GND

GND

100nF

VCC

USBDM

USBDP

VCCIO

NC

RESET#

NC

OSCI

OSCO

3V3OUT

A

G

N

D

G

N

D

G

N

D

G

N

D

T

E

S

T

TXD

RXD

RTS#

CTS#

DTR#

DSR#

DCD#

RI#

CBUS0

CBUS1

CBUS2

CBUS3

CBUS4

1

2

3

4

GND

SHIELD

GND

100nF

4.7uF

+

5

Ferrite Bead

s

d

g

P-Channel Power

MOSFET

PWREN#

1K

Switched 5V Power

To External Logic

Soft Start

Circuit

0.1uF

0.1uF

5V VCC

5V VCC

5V VCC

10K

Figure 6.3 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 FT232R provides a simple but effective method of turning

off power during the USB suspend mode.

Figure 6.3 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 FT232R or the USB host/hub

controller. The soft start circuit example shown in Figure 6.3 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 FT232R EEPROM.

iii) One of the CBUS Pins should be configured as PWREN# in the internal FT232R EEPROM, and used

to switch the power supply to the external circuitry. This should be pulled high through a 10 kΩ

resistor.