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Document Reference No.: FT_000202
UM245R USB - Parallel FIFO Development Module Datasheet Version 1.04
Clearance No.: FTDI# 124
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 (2.5mA with remote wake-up enabled) total USB suspend current requirement, including external
logic. Some external logic 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 FT245R 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 1kΩ series 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 FT245R, 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 dedica ted 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:
i)
The logic to be controlled must have its own reset circuitry so that it will automatically reset itself
when power is re-applied on coming out of suspend.
ii) Set the Pull-down on Suspend option in the internal EEPROM.
iii) The PWE# pin should be used to switch the power to the external circuitry.
iv) For USB high-power bus powered device (one that consumes greater than 100mA, and up to
500mA 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.
v) For 3.3V power controlled circuits the VCCIO pin must not be powered down with the external
circuitry (the PWREN# signal gets its supply from VCCIO). Either connect the power switch
between the output of the 3.3V regulator and the external 3.3V logic or power VCCIO from
the 3V3OUT pin of the FT245R.
? Copyright 2009 Future Technology Devices International Ltd
20
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