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| UMFT221XE Datasheet, PDF (10/18 Pages) Future Technology Devices International Ltd. – UMFT221XE USB to 8-bit SPI/FT1248 Development Module | |||
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UMFT221XE
Version 1.0
Document Reference No.: FT_000651 Clearance No.: FTDI# 293
5.4 USB Bus Powered with Power Switching Configuration
VCC
MCU
IO0
SCLK
SS#
MISO
IO0
Optionial
IO1
Optionial
IO2
Optionial
IO3
Optionial
IO4
Optionial
IO5
Optionial
IO6
GND
VCCIO
3V3OUT
SCLK
SS#
MISO
CBUS3
PWREN#
VCC
Optionial
IO7
VBUS
P-Channel Power
MOSFET
s
d
10K
g
0.1uF
Soft
Start
Circuit
1K
0.1uF
GND
Self-Sorced
3.3V TTL Mode
Jumper JP1 - Closed
Bus Powered
Mode
Jumper JP2 - Closed
Figure 5.3 â Bus Powered with Power Switching Configuration
USB bus powered mode is introduced in Section5.3 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 FT221X is not enumerated. (Note: It is impossible to be in suspended mode when PWREN#
is low.)
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 FT221X 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 FT221X must not
draw more that 50mA, this is because the current limit of the internal 3.3V regulator is 50mA.
10
Copyright © 2012 Future Technology Devices International Limited
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