UM232R FTDI, Future Technology Devices International Ltd, UM232R Datasheet - Page 20

• Manufacturer Part Number: UM232R
• Description: MODULE USB-SER UART FT232 24-DIP
• Manufacturer: FTDI, Future Technology Devices International Ltd

Specifications of UM232R

• Main Purpose: Interface, USB 2.0 to UART (RS232) Bridge
• Embedded: No
• Utilized Ic / Part: FT232R
• Primary Attributes: Full Speed, Rates of 300 ~ 3 MBaud, Security ID Number
• Secondary Attributes: 6 ~ 48MHz Clock Generator Output, Royalty-Free Drivers
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 768-1019

7.3 USB Bus Powered with Power Switching Confiduration
Figure 7.3 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
<=500μA total USB suspend current requirement (including external logic). Some external can 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 way of turning off power to external circuitry
during USB suspend.
Figure 7.3 shows how to use a discrete P-Channel Logic Level MOSFET to control the power to external logic
circuits. A suitable device would be an International Rectifier (www.irf.com) IRLML6402, or equivalent. It is
recommended that a “soft start” circuit consisting of a 1kΩseries resistor and a 0.1μF 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 FT232R, or the USB host / hub controller.
The values used here allow attached circuitry to power up with a slew rate of ~12.5V 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:
© Copyright 2009 Future Technology Devices International Ltd
iv) For USB high-power bus powered device (one that consumes greater than 100mA, and up to 500mA
iii) One of the CBUS Pins should be configured as PWE# in the internal EEPROM, and should be used to
ii) Set the Pull-down on Suspend option in the internal EEPROM.
i) The logic to be controlled must have its own reset circuitry so that it will automatically reset itself
when power is applied on coming out of suspend.
switch the power supply to the external circuitry.
of current from the USB bus), the power consumption of the device should be set in the max power
field in the internal EEPROM. A high-power bus powered device must use this descriptor in the
internal EEPROM to inform the system of its power requirements.
MCU
TXD
RXD
RTS#
CTS#
UM232R USB - Serial UART Development Module Incorporating Clock Generator
CTS#
GND
TXD
RTS# 3
RXD
10
12
7
1
5
J1
UM232R
1
J2
©
2
FTDI 2008
2
3
1
Document Reference No.: FT_000051
(PWREN#)
14
13
24
18
USB
CB3
Clearance No.: FTDI# 125
P-Channel Power
Datasheet Version 1.04
MOSFET
s
g
1K
d
Circuit
Start
Soft
0.1uF
0.1uF
19