WM8352
Production Data
17.4 USB POWERED OPERATION
The WM8352 selects USB Slave mode by default. In USB Slave Mode, the USB pin can be used as
one of the sources of power for the WM8352. In USB Master Mode (selected using the USB_MSTR
register bit) the WM8352 can provide power to an external USB device.
In USB Slave mode, the WM8352 selects USB power if the Wall (LINE) supply is outside its normal
operating limits and the USB supply is the highest supply source available. For a transition from OFF
to ACTIVE state to occur under USB power, the USB supply must be no less than 4.0V.
The maximum current drawn from the USB supply can be set to 100mA (USB low power mode) or
500mA (USB high power mode). The default is set according to the selected Config Mode (see
Section 14).
When the WM8352 is in the ACTIVE state, USB high power mode can be selected using the register
bits USB_MSTR_500MA (in USB Master Mode) or USB_SLV_500MA (in USB Slave Mode) as
defined in Table 101. If a USB current higher than the applicable threshold is demanded, then
internal protection circuits will limit the USB current, and the USB_LIMIT_EINT interrupt will be
asserted.
Short term currents higher than 500mA can also be supported. This may be necessary for supporting
transient demands (eg. for a hard drive starting up). When the USB_NOLIM register field is set, the
internal protection circuits are disabled, and the current limit interrupt threshold is raised to double
the normal value. In 500mA mode, the current limit interrupt threshold is raised to approximately 1A.
This feature must be used with caution, as the internal protection circuits are disabled when
USB_NOLIM is set. The maximum steady-state current supported is 500mA; higher currents can
only be supported for short term transients.
USB power may be supplemented by battery power if available and if necessary to maintain the USB
current within the applicable limit. If a suitable Wall (LINE) supply becomes available during USB
operation, then the Wall (LINE) supply will be selected as the preferred power source. Note that,
when hot-swapping from USB to Wall supply, a usable Battery must be present on the BATT pin.
In USB low power mode, trickle charging is enabled by default. Trickle charging is suspended if
necessary to keep within the 100mA USB limit.
In USB high power mode, fast charging is possible (subject to other conditions - see Section 17.7.4).
The fast charge current is controlled dynamically as necessary to keep the overall USB current within
the 500mA limit.
Note that Battery Charging from the USB source is only possible in USB Slave Mode.
USB power may be suspended by writing to the USB_SUSPEND register bit. Setting this bit to ‘1’
disconnects the WM8352 from the USB supply, resulting in the selection of Battery as the power
source. USB Suspend mode is invoked under software control, by writing to the USB_SUSPEND bit.
Suspend mode should be invoked whenever the USB connection is not used.
To comply with the USB 2.0 specification, the host processor should initially invoke USB Suspend
mode after the WM8352 has successfully started up, and whenever the USB connection is not in
use. If the USB connection is active and USB enumeration has been completed, the host processor
may (but is not required to) switch the WM8352 into USB low-power mode or USB high-power mode.
However, if wall adaptor power is available, it is recommended to remain in USB Suspend mode.
PD, February 2011, Rev 4.4
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
