LP3971

SNVS432V – JANUARY 2006 – REVISED MAY 2013

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Back-Up Charger Electrical Characteristics

appearing in boldface type apply over the entire junction temperature range for operation,

−40°C to +125°C.

Symbol

Parameter

Conditions

Min

Typ

Max

Units

Operational Voltage Range

3.3

5.5

V

Backup Battery Charging Current

190

μA

Charger Termination Voltage

2.91

3.1

V

Enabled. Programmable

Backup Battery Charger Short Circuit

Backup_Bat = 0V, Backup Battery Charger

9

mA

Current

Enabled

PSRR

Power Supply Ripple Rejection Ratio

15

dB

f < 10 kHz

Quiescent Current

25

μA

Output Capacitance

0

0.1

μF

Output Capacitor ESR

5

500

m

Ω

(1)

All voltages are with respect to the potential at the GND pin.

(2)

All limits specified at room temperature and at temperature extremes. All room temperature limits are production tested, specified

through statistical analysis or by design. All limits at temperature extremes are specified via correlation using standard Statistical Quality

Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

(3)

LP3971 Battery Switch Operation

The LP3971 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.

14

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