LP3972 datasheet(14/59 Pages) TI1

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LP3972

SNVS468J – SEPTEMBER 2006 – REVISED AUGUST 2011

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Back-Up Charger Electrical Characteristics

Unless otherwise noted, VIN = VBATT = 3.6V. Typical values and limits appearing in normal type apply for TJ = 25°C. Limits

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

−40°C to +125°C.

(1) (2) and (3).

Symbol

Parameter

Test Conditions

Min

Typ

Max

Unit

VIN

Operational Voltage Range

Voltage at VIN

3.3

5.5

IOUT

Backup Battery Charging Current

VIN = 3.6V, Backup_Bat = 2.5V, Backup

190

µA

Battery Charger Enabled (3)

VOUT

Charger Termination Voltage

VIN = 5.0V Backup Battery Charger Enabled.

2.91

3.1

Programmable

Backup Battery Charger Short

Backup_Bat = 0V, Backup Battery Charger

Circuit Current

Enabled

PSRR

Power Supply Ripple Rejection

IOUT ≤ 50 µA, VOUT = 3.15V

Ratio

VOUT + 0.4 ≤ VBATT = VIN ≤ 5.0V

f < 10 kHz

Quiescent Current

IOUT < 50 µA

µA

COUT

Output Capacitance

0 µA

≤ IOUT ≤ 100 µA

0.1

µF

Output Capacitor ESR

500

Ω

(1)

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

(2)

All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits

are production tested, guaranteed through statistical analysis or guaranteed by design. All limits at temperature extremes are

guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing

Quality Level (AOQL).

(3)

Back-up battery charge current is programmable via the I2C compatible interface. Refer to the Application Section for more information.

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 LDO_RTC from the appropriate battery, depending

on conditions described below:

•

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

•

If only the main battery is applied, the switch will automatically connect the LDO_RTC 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 LDO_RTC 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 LDO_RTC from the main battery and connect to the backup battery.

•

The main battery voltage at which the LDO_RTC is switched over from main to backup battery is 2.8V

typically.

•

There is a hysteric voltage in this switch operation, thus the LDO_RTC 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 LDO_RTC 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.

Logic Inputs and Outputs DC Operating Conditions

(1)

Logic Inputs (SYS_EN, PWR_EN, SYNC, nRSTI, PWR_ON, nTEST_JIG, SPARE and GPI's)

Symbol

Parameter

Test Conditions

Min

Max

Unit

VIL

Low Level Input Voltage

0.5

VIH

High Level Input Voltage

VRTC − 0.5V

ILEAK

Input Leakage Current

−1

µA

(1)

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

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