FT232RN datasheet(22/40 Pages) FTDI

22 / 40 page

FT232RN USB UART IC Datasheet

Version 1.0

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

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 FT232RN EEPROM.

iii) One of the CBUS Pins should be configured as PWREN# in the internal FT232RN EEPROM and used

to switch the power supply to the external circuitry. This should be pulled high through a 10 kΩ

resistor.

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 FT232RN EEPROM. A high-power bus powered application uses the

descriptor in the internal FT232RN EEPROM 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.

6.4 USB Bus Powered with Selectable External Logic Supply

Figure 6.4 USB Bus Powered with +3.3V or +5V External Logic Power Supply

Figure 6.4 illustrates a USB bus power application with selectable external logic supply. The external logic

can be selected between +3.3V and +5V using the jumper switch. This jumper is used to allow the FT232RN

to be interfaced with a +3.3V or +5V logic devices. The VCCIO pin is either supplied with +5V from the

USB bus (jumper pins1 and 2 connected), or from the +3.3V output from the FT232RN 3V3OUT pin (jumper

pins 2 and 3 connected). The supply to VCCIO is also used to supply external logic.

With bus powered applications, the following should be noted:

To comply with the 2.5mA current supply limit during USB suspend mode, PWREN# or SLEEP#

signals should be used to power down external logic in this mode. If this is not possible, use

the configuration shown in Section 6.3.

ii)

The maximum current sourced from the USB bus during normal operation should not exceed

100mA, otherwise a bus powered design with power switching (Section 6.3) should be used.

Another possible configuration could use a discrete low dropout (LDO) regulator which is supplied by the

5V on the USB bus to supply between +1.8V and +2.8V to the VCCIO pin and to the external logic. In this

case VCC would be supplied with the +5V from the USB bus and the VCCIO would be supplied from the

FT232RN Datasheet(PDF) 22 Page - Future Technology Devices International Ltd.