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21

FT232RN USB UART IC Datasheet

Version 1.0

Document No.: FT_001521 Clearance No.: FTDI#571

pulled up to +3.3V (generated using the 4K7 and 10k resistor network), thus identifying the device as a

full speed device to the USB host or hub. When the USB host or hub is powered off, RESET# will be low

and the FT232RN is held in reset. Since RESET# is low, the internal 1.5kΩ resistor is not pulled up to any

power supply (hub or host is powered down), so no current flows down USBDP via the 1.5kΩ pull-up

resistor. Failure to do this may cause some USB host or hub controllers to power up erratically.

Figure 6.2 illustrates a self-powered design which has a +3.3V to +5.25V supply.

Note:

1. When the FT232RN is in reset, the UART interface I/O pins are tri-stated. Input pins have internal

200kΩ pull-up resistors to VCCIO, so they will gently pull high unless driven by some external logic.

2. Any design which interfaces to +3.3 V or +1.8V would be having a +3.3V or +1.8V supply to

VCCIO.

6.3 USB Bus Powered with Power Switching Configuration

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 could 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 FT232RN 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 (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 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 FT232RN 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 (www.micrel.com) MIC2025-2BM or

equivalent.