MCP2221 datasheet(17/80 Pages) MICROCHIP

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 2015 Microchip Technology Inc.

DS20005292B-page 17

MCP2221

The provided VDD voltage has a direct influence on the

voltage levels present on the GPIO and UART TX/RX

pins. When VDD is 5V, all of these pins will have a

logical ‘1’ around 5V with the variations specified in

Section 4.1 “DC Characteristics”.

For applications that require a 3.3V logical ‘1’ level,

VDD must be connected to a power supply providing

the 3.3V voltage. In this case, the internal USB

transceiver LDO cannot provide the required 3.3V

power. It is necessary to also connect the VUSB 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 VBUS 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 VBUS 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

the

allowed

suspend

current

budget

(500 µA/2.5 mA). The VUSB 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 VDD affects the voltage levels onto

the UART and GPIO pins. With VDD 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+

VDD

VUSB

D-

OUT

5V (USB Bus)

or external

power supply

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