WM8351 datasheet(167/330 Pages) WOLFSON

167 / 330 page

Production Data

WM8351

PD, April 2012, Rev 4.5

167

18 SYSTEM MONITORING AND UNDERVOLTAGE LOCKOUT (UVLO)

The WM8351 includes several mechanisms to prevent the system from starting up, or force it to shut

down, when power sources are critically low.

The under-voltage lockout (UVLO) is a non-programmable voltage limit. When the available supplies

are below this limit, the WM8351 enters the BACKUP state. The WM8351 can only proceed from

BACKUP to the OFF state if LINE is above the UVLO level. Whenever the WM8351 is in ACTIVE,

HIBERNATE or OFF state and LINE falls below the UVLO level, the WM8351 returns to the BACKUP

state.

The UVLO limit threshold is equal to VRTC + 50mV. The precise value of VRTC may vary between

devices, within the limits defined in the Electrical Characteristics (see Section 7.5).

The startup threshold is a programmable voltage limit. The WM8351 can only proceed from OFF to

the ACTIVE state if LINE is above the startup threshold. (Note that, in the case of USB-powered

operation, there are additional voltage requirements; see Section 17.4.). The startup threshold is

determined by the PCCMP_ON_THR register field.

The shutdown threshold is determined by the PCCMP_OFF_THR register field. When LINE falls

below this threshold, the WM8351 raises a SYS_HYST_COMP_FAIL_EINT interrupt. In addition, the

WM8351 takes the action set by PCCOMP_ERRACT. If this bit is set, then the WM8351 will shut

down in response to the LINE voltage falling below the shutdown threshold.

The startup and shutdown control register fields are described in Table 112. Note that the startup

threshold should always be set higher than the shutdown threshold in order to create a hysteresis,

making the system more stable.

The SYS_HYST_COMP_FAIL_EINT interrupt is one of the second-level interrupts which triggers a

first-level System Interrupt, SYS_INT (see Section 24). This can be masked by setting the mask bit as

described in Table 113.

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

R179 (B3h)

Power Check

Comparator

PCCMP_ER

RACT

Action to take when LINE falls below

PCCMP_OFF_THR level (as well as

generating an interrupt)

0 = ignore

1 = shut down system

6:4

PCCMP_OF

F_THR [2:0]

010

Power check comparator system shutdown

threshold value

000 = 2.9V

001 = 3.0V

…

111 = 3.6V

Protected by security key.

2:0

PCCMP_ON

_THR [2:0]

101

Power check comparator system startup

threshold value

000 = 2.9V

001 = 3.0V

…

111 = 3.6V

Protected by security key.

Table 112 Battery Monitoring and UVLO Control

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