LP3972 datasheet(50/59 Pages) NSC

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LP3972 Controls (Continued)

GENERAL PURPOSE I/O FUNCTIONALITY (GPIO1 AND

GPIO2)

LP3972 has 2 general purpose I/Os for system control. I

compatible interface will be used for setting any of the pins to

input, output or hi-Z mode. Inputs value can be read via

serial interface (GPI1,2 bits). The pin 29 functionality needs

to be set to GPIO by serial interface register bit nEXTCH-

GEN. (GPIO/CHG)

Controls

Port Function

Reg

batmonchg

GPIO<1>

Nextchgen_sel

bucen

GPIO1

Gpin 1

Function

Input = 0

Enabled

Input = 1

Not Enabled

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 LP3972 has provision for two battery connections, the

main battery Vbat and Backup Battery (See Applications

Schematic Diagrams1&2ofthe LP3972 Data Sheet).

The function of the battery switch is to connect power to the

RTC LDO from the appropriate battery, depending on con-

ditions described below:

•

If only the backup battery is applied, the switch will auto-

matically connect the RTC LDO power to this battery.

•

If only the main battery is applied, the switch will auto-

matically connect the RTC LDO power to this battery.

•

If both batteries are applied, and the main battery is

sufficiently charged (V

BAT > 3.1V), the switch will auto-

matically connect the RTC LDO power to the main bat-

tery.

•

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’0B 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 en-

abled 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).

LP3972 Datasheet(PDF) 50 Page - National Semiconductor (TI)

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