DLP Design USB-Parallel FIFO Module DLP-USB245R User Manual page 10

9.0 BUS-POWERED CIRCUIT WITH POWER CONTROL
USB bus-powered circuits need to be able to power down in USB Suspend Mode in order to
meet the <= 2.5mA total Suspend current requirement (including external logic):
Figure 4.
Figure 4 shows how to use a discrete P-Channel MOSFET to control the power to external logic
circuits. This "soft-start" circuit accommodates designs that draw more than 100mA at power
up.
Please note the following points in connection with power-controlled designs:
1. The logic to be controlled must have its own reset circuitry so that it will automatically reset
itself when power is reapplied on coming out of Suspend.
2. Set the soft pull-down option bit in the FT245R EEPROM.
3. For USB high-power, bus-powered applications (ones that consume greater than 100mA,
and up to 500mA of current from the USB bus), the power consumption of the application must
be set in the Max Power Field in the internal FT245R EEPROM. A high-power, bus-powered
application uses the descriptor in the internal FT245R EEPROM to inform the system of its
power requirements.
4. PWREN# gets its VCC from VCCIO. For designs using 3.3-volt logic, ensure that VCCIO is
not powered down during Suspend.
Rev. 1.0 (November 2008)
P-Channel
Power MOSFET
S D
G
.1uF
10K
1K
PWREN#
7
8
9
Power Controlled by
PWREN#
10
Microcontroller
VCC
© DLP Design, Inc.