USB334X datasheet(37/92 Pages) MICROCHIP

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DS00002646A-page 37

USB334x

5.7.2.4

VBUS Pulsing with Pull-up and Pull-down Resistors

In addition to the internal VBUS comparators, the USB334x also includes the integrated VBUS pull-up and pull-down

resistors used for VBUS Pulsing during OTG Session Request Protocol. To discharge the VBUS voltage so that a Ses-

sion Request can begin, the USB334x provides a pull-down resistor from VBUS to GND. This resistor is controlled by

the DischargeVbus bit 3 of the OTG Control register. The pull-up resistor is connected between VBUS and VDD33. This

resistor is used to pull VBUS above 2.1 volts so that the A-Device knows that a USB session has been requested. The

state of the pull-up resistor is controlled by the bit 4 ChargeVbus of the OTG Control register. The Pull-Up and Pull-Down

resistor values are detailed in Table 4-9.

The internal VBUS Pull-up and Pull-down resistors are designed to include the RVBUS external resistor in series. This

external resistor is used by the VBUS Over voltage protection described below.

5.7.2.5

VBUS Input Impedance

The OTG Supplement requires an A-Device that supports Session Request Protocol to have a VBUS input impedance

less than 100kΩ and greater the 40kΩ to ground. The USB334x provides a 75kΩ resistance to ground, RVB. The RVB

resistor tolerance is detailed in Table 4-9.

5.7.2.6

VBUS Over Voltage Protection (OVP)

The USB334x provides an integrated over voltage protection circuit to protect the VBUS pin from excessive voltages

that may be present at the USB connector. The over voltage protection circuit works with an external resistor (RVBUS)

by drawing current across the resistor to reduce the voltage at the VBUS pin.

When voltage at the VBUS pin exceeds 5.5V, the Over voltage Protection block will sink current to ground until VBUS

is below 5.5V. The current drops the excess voltage across RVBUS and protects the USB334x VBUS pin. The required

RVBUS value is dependent on the operating mode of the USB334x as shown in Table 5-6.

The Over voltage Protection circuit is designed to protect the USB334x from continuous voltages up to 30V on the

RVBUS resistor.

The RVBUS resistor must be sized to handle the power dissipated across the resistor. The resistor power can be found

using the equation below:

For example, protecting a peripheral or device only application to 15V would require a 20kΩ RVBUS resistor with a power

rating of 0.05W. To protect an OTG product to 15V would require a 1kΩ RVBUS resistor with a power rating of 0.1W.

TABLE 5-6:

REQUIRED RVBUS RESISTOR VALUE

Operating Mode

RVBUS

Device only

20kΩ ±5%

OTG Host Capable of less than 100mA of current on VBUS

1kΩ ±5%

Host or OTG Host capable of >100mA

UseExternalVbusIndicator = 1

20kΩ ±5%

PRVBUS

Vprotect 5.0

–



RVBUS

--------------------------------------------

Where:

• Vprotect is the VBUS protection required.

•RVBUS is the resistor value, 1kΩ or 20kΩ.

•PRVBUS is the required power rating of RVBUS.

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