WM8310 datasheet(107/291 Pages) WOLFSON

107 / 291 page

Pre-Production

WM8310

PP, May 2012, Rev 3.1

107

17.4 USB POWERED OPERATION

The WM8310 selects USB power via the USBVDD pin when this supply is within the normal USB

operating limits of 4.3V to 5.5V, and WALLVDD is less than 4.3V and USBVDD is the highest supply

source available. USB power is also selected below 4.3V if WALLVDD is less than 3.4V and

USBVDD is the highest supply available.

The maximum current drawn from the USB supply is determined by the USB_ILIM register field.

Currents ranging from 0mA to 1800mA may be selected. See also Section 7 for the limits of the USB

Current switch. If the system current demand is greater than the limit set by USB_ILIM, then this is

indicated via the USB_CURR_STS bit and by setting the PPM_USB_CURR_EINT interrupt (see

Section 17.5).

The USB power source will be supplemented by battery power, when available, in order to maintain

the USB current within the applicable limit. If there is no battery connected, or there is insufficient

capacity to support the system demands, then the supply rails may drop as the WM8310 attempts to

meet the USB current limit.

If a suitable WALLVDD supply becomes available during USB operation, then this will be selected as

the preferred power source.

When the USBVDD supply is selected and a Battery is connected, then battery charging is possible in

the ON or SLEEP power states, provided that sufficient current capacity is available. See

Section 17.7 for details of the Battery Charger.

Note that, when the USBVDD supply is selected by the WM8310, and an ‘ON’ state transition is

requested, the USB current limit must be set to 100mA or higher. If a lower current limit is selected,

then the ‘ON’ state transition event may fail. This requirement is also applicable when a battery is

available to provide supplementary power. There is no requirement to set USB_ILIM for start-up when

the WALLVDD supply is selected.

The user-configurable OTP memory contains the USB_ILIM register field. This allows users to

program their chosen USB current limit on start-up (Note that the current limit can still be updated

during normal operation.) If the WM8310 is powered up with USBVDD as the selected power source,

and the applicable USB current limit is 100mA, then the start-up behaviour is determined by the

USB100MA_STARTUP field, as defined in Table 55.

When starting up in 100mA USB mode, the normal or soft-start process can be selected. The soft-

start option controls the DC-DC converters and LDO Regulators in order to reduce the start-up current

demand. In 100mA USB soft-start operation, the DC-DC Converters are initially enabled in LDO mode

in order that the in-rush current does not exceed the USB limit. The LDO Regulators are also current-

limited during the soft start-up.

Care is required when using the 100mA soft-start; if the LDOs or DC-DCs present an excessive load,

then the WM8310 may be unable to power up; it must be ensured that the connected load is

compatible with the 100mA current limit. In particular, it is important that the loads on the DC-DC

Converters do not exceed the capacity of their LDO operating modes. (See Section 7.1 for the

maximum current in LDO mode.)

It is also possible to delay the USB start-up if the battery voltage is less than a selectable threshold; in

these cases, the WM8310 enables the battery trickle charge mode (provided that CHG_ENA = 1),

and delays the start-up request until the battery voltage threshold has been met. See also

Section 27.1 for specific external component requirements relating to the USB100MA_STARTUP

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