WM8352
Production Data
17 POWER SUPPLY CONTROL
17.1 GENERAL DESCRIPTION
The WM8352 can take its power supply from a Wall adaptor, a USB interface or from a single-cell
lithium battery. The WM8352 autonomously chooses the most appropriate power source available,
and supports hot-swapping between sources (ie. the system can remain in operation while different
sources are connected and disconnected).
Comparators within the WM8352 identify which power supplies are available and select the power
source in the following order of preference:
.
.
.
Wall adaptor (LINE pins)
USB power rail (USB pins)
Battery (BATT pins)
Note that the Wall supply is always the first choice of supply, (providing that it is within required
limits), even if the Wall supply voltage is lower than the USB voltage.
When Wall or USB is selected as the power source, this may be used to charge the Battery, using
the integrated battery charger circuit. For battery charging to occur, the USB or LINE supply voltage
must be no less than 4.0V.
Figure 74 illustrates the WM8352 connections associated with the WALL, USB and Battery supplies.
Figure 74 WM8352 Power Supply Connections
The Wall Adaptor supply connects to LINE via a FET switch as illustrated in Figure 74. The FET
switch is necessary in order to provide isolation between the Wall supply and the Battery/USB
supplies; this is vital in the event of the USB voltage being greater than the Wall supply voltage.
The Wall Adapter voltage is sensed directly on the WALL_FB pin; this allows the WM8352 to
determine the preferred supply, including when the FET is switched off.
The gate connection to the external FET is controlled by LINE_SW, which is an alternate function
that can be enabled on GPIO12 (see Section 20). Note that, if the USB connection is not used, then
the FET may not be required and the Wall supply may be connected directly to LINE.
LINE is primarily an output from the WM8352; this output is the preferred supply, where the WM8352
has arbitrated between the Wall, Battery and USB connections. This output is suitable for supplying
power to the other blocks of the WM8352, including the DC-DC Converters and LDO Regulators.
LINE is also an input under some conditions, such as battery charging from Wall or providing power
at the USB connection.
PD, February 2011, Rev 4.4
152
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
