UMFT231XE Datasheet, PDF (10/18 Pages) Future Technology Devices International Ltd. – UMFT231XE USB to Full-Handshake UART Development Module

UMFT231XE
Version 1.0
Document Reference No.: FT_000652 Clearance No.: FTDI# 294
5.4 USB Bus Powered with Power Switching Configuration
5V
VCC
MCU
TXD
RXD
RTS#
CTS#
TXD
RXD
RTS#
CTS#
P-Channel Power
MOSFET
s
d
10K
g
0.1uF
CBUSx
PWREN#
1K
Soft
Start
Circuit
0.1uF
GND
GND
VCCIO
3V3OUT
VCC
VBUS
Self-Sorced
3.3V TTL Mode
Jumper JP1 - Closed
Bus Powered
Mode
Jumper JP2 - Closed
Figure 5.3 – Bus Powered with Power Switching Configuration
USB bus powered mode is introduced in Section 5.2. This section describes how to use bus-powered
mode with a power switch.
USB bus powered circuits are required by USB compliance standards to consume less than 2.5mA (and
less than 100mA when not enumerated and not suspended) when connected to a host or hub when in
USB suspend mode. The PWREN# CBUS function can be used to remove power from external circuitry
whenever the FT231X is not enumerated.
To implement a power switch using PWREN#, configure a P-Channel Power MOSFET to have a soft start
by fitting a 10K pull-up, a 1K series resistor and a 100nF cap as shown in Figure 5.3.
Connecting the source of the P-Channel MOSFET to 3V3OUT instead of VBUS can allow external logic to
source 3.3V power from the FT231X without breaking USB compliancy. In this setup it is important that
the VCCIO is not sourced from the drain of this MOSFET, this is because the power used to drive the gate
of this transistor is sourced from VCCIO. VCCIO should be connected directly to 3V3OUT for this setup to
function effectively. It is also important that the external logic must and IO core of the FT231X must not
draw more that 50mA, this is because the current limit of the internal 3.3V regulator is 50mA.
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