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Document Reference No.: FT_000367
UM232H Single Channel USB Hi-Speed FT232H Development Module Error!
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Datasheet Version 1.3
Clearance No.: FTDI# 198
7.2 USB Bus Powered with Power Switching Configuration
Figure 7.2 : 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 total USB suspend current requirement (including external logic). Some external hardware 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 FT232H provides a simple but effective way of turning off power to
external circuitry during USB suspend.
Figure 7.2 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 FT232H, 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.
A 100kΩ resistor t o VBUS creates a week pull up on the gate, this can prevent current from flowing through
the transistor during a power up or power down of the FT232H.
Alternatively, a dedicated 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 applied on coming out of suspend.
ii) Set the Pull-down on Suspend option in the internal EEPROM.
iii) One of the ACBUS pins should be configured as PWREN# in the internal EEPROM, and should be used
to switch the power supply to the external circuitry.
iv)
v)
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.
For 3.3V power controlled circuits the FT232H’s VIO pin must not be powered down with the external
circuitry (the PWREN# signal gets its VCC supply from VIO). Either connect the power switch between
17
? Copyright 2012 Future Technology Devices International Ltd
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