FT245BL Datasheet, PDF (21/24 Pages) List of Unclassifed Manufacturers – FT245BL USB FIFO ( USB - Parallel ) I.C.

Figure 11
FT245BL USB FIFO ( USB - Parallel ) I.C.
Bus Powered Circuit with Power Control _{P-Channel Power}
MOSFET
s
d
Switched 5v Power to
External Logic
g
0.1uF
Soft Start
1K ^Circuit
0.1uF
USB "B"
Connector
1
2
3
4
Ferrite Bead
VCC
27R
27R
10nF
33nF
470R
3 26 13
VV V
CC C
CC C
6
I
3v3OUT _O
30
A
V
C
C
FT245BM
8 _{USB DM}
7 _{USB DP}
PWREN# ¹⁵
0.1uF
USB Bus powered circuits need to be able to power down in USB suspend mode in order to meet the <= 500uA total
suspend current requirement (including external logic). Some external logic can power itself down into a low current
state by monitoring the PWREN# pin. For external logic that cannot power itself down in that way, the FT245BL
provides a simple but effective way of turning off power to external circuitry during USB suspend.
Figure 11 shows how to use a discrete P-Channel Logic Level MOSFET to control the power to external logic
circuits. A suitable device could be a Fairchild NDT456P, or International Rectifier IRLML6402, or equivalent. It is
recommended that a “soft start” circuit consisting of a 1K series resistor and a 0.1 uF 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 FT245BL, or the USB host / hub controller. The values used here
allow attached circuitry to power up with a slew rate of ~12.5 V per millisecond, in other words the output voltage will
transition from GND to 5 V in approximately 400 microseconds.
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) MIC2025-2BL or equivalent.
Please note the following points in connection with power controlled designs –
a) 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.
b) Set the Pull-down on Suspend option in the FT245BL’s EEPROM.
c) For USB high-power bus powered device (one that consumes greater than 100 mA, and up to 500 mA of current
from the USB bus), the power consumption of the device should be set in the max power field in the EEPROM.
A high-power bus powered device must use this descriptor in the EEPROM to inform the system of its power
requirements.
d) For 3.3V power controlled circuits VCCIO must not be powered down with the external circuitry (PWREN# gets
its VCC supply from VCCIO). Either connect the power switch between the output of the 3.3V regulator and the
external 3.3V logic OR if appropriate power VCCIO from the 3v3OUT pin of the FT245BL.
DS245BL Version 1.6 © Future Technology Devices Intl. Ltd. 2005
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