LP3972 datasheet(15/59 Pages) NSC

15 / 59 page

Back-Up Charger Electrical Characteristics

Unless otherwise noted, V

IN =VBATT = 3.6V. Typical values and limits appearing in normal type apply for TJ = 25˚C. Limits ap-

pearing in boldface type apply over the entire junction temperature range for operation, −40˚C to +125˚C. (Notes 2, 6) and

(Note 8).

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Operational Voltage Range

Voltage at V

3.3

5.5

OUT

Backup Battery Charging Current

IN = 3.6V, Backup_Bat = 2.5V,

Backup Battery Charger Enabled

(Note 8)

190

µA

OUT

Charger Termination Voltage

IN = 5.0V Backup Battery Charger

Enabled. Programmable

2.91

3.1

Backup Battery Charger Short

Circuit Current

Backup_Bat = 0V, Backup Battery

Charger Enabled

9mA

PSRR

Power Supply Ripple Rejection

Ratio

OUT

≤ 50 µA, V

OUT = 3.15V

OUT + 0.4

≤ V

BATT =VIN

≤ 5.0V

f < 10 kHz

Quiescent Current

OUT < 50 µA

µA

OUT

Output Capacitance

0 µA

≤ I

OUT

≤ 100 µA

0.1

µF

Output Capacitor ESR

500

Ω

LP3972 BATTERY SWITCH OPERATION

The LP3972 has provisions for two battery connections, the main battery Vbat and Backup Battery

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 a separate circuit called nBATT_FLT will warn the system. Then if no action is taken to

restore the charge on the main battery, and discharging is continued the battery switch will disconnect the input of the RTC_LDO

from the main battery and connect to the backup battery.

•

The main battery voltage at which the RTC LDO is switched over from main to backup battery is 2.8V typically.

•

There is a hysteric 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.

•

The system designer may wish to disable the battery switch 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 2.9V threshold. The

Backup battery input should also be connected to main battery.

LP3972 Datasheet(PDF) 15 Page - National Semiconductor (TI)

LP3972 Datasheet(HTML) 15 Page - National Semiconductor (TI)

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