STW4810_07 Datasheet, PDF (44/78 Pages) STMicroelectronics – Power management for multimedia processors

Functional description
STw4810
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 kΩ. 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 kΩ.
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 12) 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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