LP3971 datasheet(39/57 Pages) TI1

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LP3971

www.ti.com

SNVS432U – JANUARY 2006 – REVISED OCTOBER 2008

Controls

Port Function

Reg

batmonchg

GPIO<1>

Nextchgen_sel

bucen

GPIO1

Gpin 1

Function

Enabled

HiZ

Input (dig)->

Input

Output = 0

Output = 1

GPIO<1>

Factory fm disabled

GPIO_tstiob

GPIO2

gpin2

HiZ

Input (dig)->

input

Output = 0

Output = 1

The LP3971 has provision for two battery connections, the main battery Vbat and Backup Battery (See

Applications Schematic Diagrams 1 & 2 of the LP3971 Data Sheet).

The function of the battery switch is to connect power to the RTC LDO from the appropriate battery, depending

on conditions described below:

•

If only the backup battery is applied, the switch will automatically connect the RTC LDO power to this battery.

•

If only the main battery is applied, the switch will automatically connect the RTC LDO power to this battery.

•

If both batteries are applied, and the main battery is sufficiently charged (VBAT > 3.1V), the switch will

automatically connect the RTC LDO power to the main battery.

•

As the main battery is discharged by use, the user will be warned by a separate circuit called nBATT_FLT.

Then if no action is taken to restore the charge on the main battery, and discharging is continued the battery

switch will protect the RTC LDO by disconnecting from the main battery and connecting to the backup battery.

–

The main battery voltage at which the RTC LDO is switched from main to backup battery is 2.9V typically.

–

There is a hysterisis voltage in this switch operation so, the RTC LDO will not be reconnected to main

battery until main battery voltage is greater than 3.1V typically.

•

Additionally, the user may wish to disable the battery switch, such as, in the case when only a main battery is

used. This is accomplished by setting the “no back up battery bit” in the control register 8h’89 bit 7 NBUB.

With this bit set to “1”, the above described switching will not occur, that is the RTC LDO will remain

connected to the main battery even as it is discharged below the 2.9 Volt threshold.

REGULATED VOLTAGES OK

All the power domains have own register bit (X_OK) that processor can read via serial interface to be sure that

enabled powers are OK (regulating). Note that these read only bits are only valid when regulators are settled

(avoid reading these bits during voltage change or power up).

THERMAL MANAGEMENT

Application: There is a mode wherein all 6 comparators (flags) can be turned on via the “enallflags” control

conditions. Thus, the rate of temperature change can also be estimated. The system may then negotiate for

speed and power trade off, or deploy cooling maneuvers to optimize system performance. The “enallflags” bit

needs enabled only when the “bct<2:0> bits are read to conserve power.

Note: The thermal management flags have been verified functional. Presently these registers are accessible by

factory only. If there is a demand for this function, the relevant register controls may be shifted into the user

programmable bank; the temperature range and resolution of these flags, might also be refined/redefined.

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