UMFT240XE Datasheet, PDF(10/18 Page) Future Technology Devices International Ltd. – UMFT240XE USB to 8-bit 245 FIFO Development Module

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UMFT240XE Datasheet, PDF (10/18 Pages) Future Technology Devices International Ltd. – UMFT240XE USB to 8-bit 245 FIFO Development Module

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UMFT240XE

Version 1.0

Document Reference No.: FT_000653 Clearance No.: FTDI# 295

5.4 USB Bus Powered with Power Switching Configuration

VCC

MCU

DATA0-7

RXF#

TXE#

RD#

WR#

GND

P-Channel Power

MOSFET

10K

0.1uF

CBUS5

PWREN#

Soft

Start

Circuit

0.1uF

VCCIO

3V3OUT

VCC

VBUS

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 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 FT240X 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 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 FT240X 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 FT240X must not

draw more that 50mA, this is because the current limit of the internal 3.3V regulator is 50mA.

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