STW4810BRA datasheet(39/72 Pages) STMICROELECTRONICS

39 / 72 page

STw4810

4 Functional description

Rev3

39/72

VBUS monitoring and control

The monitoring is made of three comparators that determine if the VBUS voltage is at a valid

level for operation:

●

VBUS VALID: It corresponds to the minimum level on VBUS. Any voltage on VBUS below

the threshold is considered to be a fault. During power-up, it is expected that this

comparator output is ignored.

●

VBUS SESSION VALID: This threshold is necessary for session request protocol to detect

the VBUS pulsing.

●

VBUS SESSION END: Session is ended. In this USB block, a B-device Session End

threshold is defined within the range [0.2; 0.8] V. The reason for a low 0.2 V limit is that the

leakage current could charge the VBUS up to 0.2 V (maximum).

When the A-device (default master) is power supplied and does not supply VBUS, it presents

an input impedance RA_BUS_IN on VBUS of no more than 100 k

Ω. If the A-device responds to

the VBUS pulsing method of SRP, then the input impedance RA_BUS_IN may not be lower

than 40 k

Ω.

When the A-device supplies power, the rise time TA_VBUS_RISE on VBUS to go from 0 to

4.4 V is less than 100 ms when driving 100 mA and with an external load capacitance of 10 µF

(in addition to VBUS decoupling capacitance). If VBUS does not reach this voltage within

TA_VBUS_RISE maximum time, it indicates that the B-device is drawing more current that the

A-device is capable of providing and an over-current condition exists. In this case, the A-device

turns VBUS off and terminates the session.

VBUS capacitance

A dual-role device must have a VBUS capacitance CDRD_VBUS value comprised between

1 µF and 6.5 µF (see charge pump specification). The limit on the decoupling capacitance

allows a B-device to differentiate between a powered-down dual-role device and a powered-

down standard host. The capacitance on a host is higher than 96 µF.

Data line pull-down resistance

When an A-device is idle or acting as host, it activates the pull-down resistors RPD on both DP

and DN lines.

When an A-device is acting as peripheral, it disables RPD on DP, not DN.

The A-device can disable both pull-down resistors during the interval of a packet transmission

when acting as either host or peripheral.

The two bits of USB control register, dn_pulldown and dp_pulldown (

Table 11) are used to

connect/disconnect the pull-down resistors.

When the line is not used, the pull-down is activated and the maximum level on this ball should

not exceed 0.342 V.

Data line pull-up resistance

Full-speed and low-speed devices are differentiated by the position of the pull-up resistor from

the peripheral device. A pull-up resistor is connected to DP line for a full-speed device and a

pull-up resistor is connected to DN line for a low-speed device. The pull-up resistor value is in

the range of 900

Ω to 1600 Ω when the bus is idle and 1425 Ω to 3100 Ω when the upstream

device is transmitting.

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