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
FT232R 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 FT232R 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:
i) 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 output of the LDO regulator. This results in the FT232R I/O pins driving out at between +1.8V and
+2.8V logic levels.
For a USB bus powered application, it is important to consider the following when selecting the regulator:
i) The regulator must be capable of sustaining its output voltage with an input voltage of +4.35V. A Low Drop Out (LDO) regulator should be selected.
ii) The quiescent current of the regulator must be low enough to meet the total current requirement of <= 2.5mA during USB suspend mode.
A suitable series of LDO regulators that meets these requirements is the MicroChip/Telecom (www.microchip.com) TC55 series of devices. These devices can supply up to 250mA current and have a quiescent current of under 1μA.