UMFT230XA Datasheet
Version 1.4
Document Reference No.: FT_000519 Clearance No.: FTDI#269
5.5 USB Bus Powered with Power Switching Configuration
5V
P-Channel Power
MOSFET
s
d
VCC
TXD
RXD
0.1uF
0.1uF
10K
g
Soft
Start
Circuit
TXD
RTS#
CTS#
CBUSx
PWREN#
RXD
RTS#
CTS#
1K
MCU
VCCIO
VCC
3V3OUT
VBUS
GND
Bus Powered
Mode
Solder Link JP2 - Closed
Self-Sorced
3.3V TTL Mode
Solder Link JP1 - Closed
Viewing pins thourgh the board
from the top.
Figure 5.3 – Bus Powered with Power Switching Configuration
USB bus powered mode is introduced in Section 5.3. This section describes how to use this 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 FT230X is not enumerated. (Note: It is impossible to be in suspended mode when
enumerated.) (Note that Figure 5.3 is for illustration only and that the pins do not actually go all the way
through the PCB)
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 anda 100nF cap as shown in Figure 5.3. The time constant
Connecting the source of the P-Channel MOSFET to 3V3OUT instead of VBUS can allow external logic to
source 3.3V power from the FT230X 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 FT230X must not
draw more that 50mA, this is because the current limit of the internal 3.3V regulator is 50mA.
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