Datasheet

 2014 Microchip Technology Inc. DS20005292A-page 17
MCP2221
The provided VDD voltage has a direct influence on the
voltage levels present on the GPIO and UART T
X/RX
pins. When VDD is 5V, all of these pins will have a logi-
cal ‘1’ around 5V with the variations specified in
Section 4.1 “DC Characteristics”.
For applications that require a 3.3V logical ‘1’ level,
V
DD must be connected to a power supply providing
the 3.3V voltage. In this case, the internal USB trans-
ceiver LDO cannot provide the required 3.3V power. It
is necessary to also connect the V
USB pin of the
MCP2221 to the 3.3V power supply rail. This way, the
USB transceiver is powered-up directly from the 3.3V
power supply.
1.6.2.2 USB Bus Powered (5V)
In Bus Power Only mode, all power for the application
is drawn from the USB (Figure 1-3). This is effectively
the simplest power method for the device.
FIGURE 1-3: BUS POWER ONLY
In order to meet the inrush current requirements of the
USB 2.0 specifications, the total effective capacitance
appearing across V
BUS and ground must be no more
than 10 µF. If it is more than 10 µF, some kind of inrush
limiting is required. For more details on Inrush Current
Limiting, search for that subject in the latest “Universal
Serial Bus Specification”.
According to the USB 2.0 specification, all USB devices
must also support a Low-Power Suspend mode. In the
USB Suspend mode, devices must consume no more
than 500 µA (or 2.5 mA for high-powered devices that
are remote wake-up capable) from the 5V V
BUS line of
the USB cable.
The host signals the USB device to enter Suspend
mode by stopping all USB traffic to that device for more
than 3 ms.
The USB bus provides a 5V voltage. However, the USB
transceiver requires 3.3V for the signaling (on D+ and
D- lines).
During USB Suspend mode, the D+ or D- pull-up
resistor must remain active, which will consume some
of the allowed suspend current budget
(500 µA/2.5 mA). The V
USB pin requires an external
bypass capacitor with a value between 0.22 and
0.47 µF (ceramic cap).
Figure 1-4 shows a circuit where MCP2221’s internal
LDO is used to provide 3.3V to the USB transceiver.
The voltage on the V
DD affects the voltage levels onto
the UART and GPIO pins. With V
DD at 5V, these pins
will have a logic ‘1’ of 5V with the variations specified in
Section 4.1 “DC Characteristics”.
FIGURE 1-4: TYPICAL POWER SUPPLY
OPTION USING THE 5V
PROVIDED BY THE USB
VDD
VUSB
VSS
VBUS
LDO
3.3V
USB
Transceiver
D+
V
DD
VUSB
D-
IN
OUT
5V (USB Bus)
or external
power supply