Production Data
WM8352
17.7.4 FAST CHARGING
Fast charging provides a faster way to charge the battery. This is only possible under certain
conditions. Fast charging must be initiated by the system controller, and can never start
autonomously.
Fast charging is normally possible in the ACTIVE state when the selected power source is Wall
(LINE) or when USB high power mode is selected. The battery charger determines whether the
conditions for fast charging are satisfied; these conditions include a suitable selected power source
voltage (see Section 17.7.2) and a suitable battery voltage (greater than 3.1V).
If the conditions for fast charging are satisfied, this is indicated by the WM8352 setting the
CHG_FAST_RDY_EINT register bit, as described in Table 106. Providing that the conditions for fast
charging are satisfied, then fast charging is enabled by setting the CHG_FAST bit. If the conditions
are not satisfied, then CHG_FAST will be held at 0.
The maximum fast charge current is set by the CHG_ISEL register field, as described in Table 106.
During fast charging, the current may be dynamically controlled by the WM8352 in order to achieve
optimum battery charging. It is recommended that the charge current limit should not be set higher
than 400mA when charging from a USB power rail.
A throttle circuit is provided to enhance the fast charge current control. This allows the charge current
to be modified according to temperature conditions or according to the USB current limit restrictions.
If the WM8352 temperature is above 115°C, then charging is interrupted for at least 8 seconds and
until the temperature has fallen below the threshold. Temperature control of the battery charger is
always enabled during Fast Charging.
If the USB current limit is reached during Fast Charging, then the charge current must be reduced. If
USB current throttling is enabled, then the charge current will be controlled dynamically in order to
continue charging. If USB current throttling is not enabled, then the charging will be terminated. (Note
that this may give rise to an erroneous indication of ‘End of Charge’ as the charging may have
terminated prematurely.) If USB current throttling is enabled, then ‘End of Charge’ will not be
indicated, even if the throttle circuit causes the charger current to fall below the End of Charge
current
threshold.
Fast
charge
USB
current
throttling
is
controlled
by
the
CHG_FAST_USB_THROTTLE register bit. The time constant for the charger’s attempts to increase
the current after USB throttling can be controlled by CHG_THROTTLE_T.
The WM8352 will revert to Trickle charging if the conditions for fast charging are no longer satisfied.
This includes selection of the OFF or HIBERNATE states, or selection of USB low power mode. The
WM8352 will also revert to Trickle charging if it detects a low battery voltage condition (see
Section 17.7.8).
PD, February 2011, Rev 4.4
161
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
