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FT230XQ-R Datasheet(PDF) 24 Page - Future Technology Devices International Ltd. |
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FT230XQ-R Datasheet(HTML) 24 Page - Future Technology Devices International Ltd. |
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24 / 44 page  Copyright © Future Technology Devices International Limited 24 FT230X USB TO BASIC UART IC Version 1.4 Document No.: FT_000566 Clearance No.: FTDI# 260 6.3 USB Bus Powered with Power Switching Configuration FT230X 1 2 3 4 5 SHIELD Ferrite Bead GND GND VCC GND VCC 3V3OUT USBDM USBDP VCCIO RESET# 100nF 100nF 10nF 4.7uF + CBUS3 PWREN# 100k 1k Switched 5V Power to External Logic 0.1uF 0.1uF P Channel Power MOSFET 27R 27R GND 47pF 47pF Figure 6.3 Bus Powered with Power Switching Configuration A requirement of USB bus powered applications, is when in USB suspend mode, the application draws a total current of less than 2.5mA. This requirement includes external logic. Some external logic has the ability to 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 FT230X provides a simple but effective method of turning off power during the USB suspend mode. Figure 6.3 shows an example of using a discrete P-Channel MOSFET to control the power to external logic. A suitable device to do this is an International Rectifier IRLML6402, or equivalent. It is recommended that a “soft start” circuit consisting of a 1kΩ series resistor and a 0.1μF capacitor is used to limit the current surge when the MOSFET turns on. Without the soft start circuit it is possible that the transient power surge, caused when the MOSFET switches on, will reset the FT230X or the USB host/hub controller. The soft start circuit example shown in Figure 6.3 powers up with a slew rate of approximaely12.5V/ms. Thus supply voltage to external logic transitions from GND to +5V in approximately 400 microseconds. As an alternative to the MOSFET, a dedicated power switch IC with inbuilt “soft-start” can be used. A suitable power switch IC for such an application is the Micrel MIC2025-2BM or equivalent. With power switching controlled designs the following should be noted: i) The external logic to which the power is being switched should have its own reset circuitry to automatically reset the logic when power is re-applied when moving out of suspend mode. ii) Set the Pull-down on Suspend option in the internal FT230X MTP memory. iii) One of the CBUS Pins should be configured as PWREN# in the internal FT230X MTP memory, and used to switch the power supply to the external circuitry. iv) For USB high-power bus powered applications (one that consumes 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 FT230X MTP memory. A high-power bus powered application uses the descriptor in the internal FT230X MTP memory to inform the system of its power requirements. v) PWREN# gets its VCC from VCCIO. For designs using 3V3 logic, ensure VCCIO is not powered down using the external logic. In this case use the +3V3OUT. |
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