Datasheet

LP3972
www.ti.com SNVS468K –SEPTEMBER 2006–REVISED MAY 2013
Back-Up Charger Electrical Characteristics
Unless otherwise noted, V
IN
= V
BATT
= 3.6V. Typical values and limits appearing in normal type apply for T
J
= 25°C. Limits
appearing in boldface type apply over the entire junction temperature range for operation, −40°C to +125°C
(1)(2)(3)
Symbol Parameter Test Conditions Min Typ Max Unit
V
IN
Operational Voltage Range Voltage at V
IN
3.3 5.5 V
I
OUT
Backup Battery Charging Current V
IN
= 3.6V, Backup_Bat = 2.5V, Backup 190 µA
Battery Charger Enabled
(3)
V
OUT
Charger Termination Voltage V
IN
= 5.0V Backup Battery Charger Enabled. 2.91 3.1 V
Programmable
Backup Battery Charger Short Backup_Bat = 0V, Backup Battery Charger 9 mA
Circuit Current Enabled
PSRR Power Supply Ripple Rejection I
OUT
≤ 50 µA, V
OUT
= 3.15V 15 dB
Ratio V
OUT
+ 0.4 ≤ V
BATT
= V
IN
≤ 5.0V
f < 10 kHz
I
Q
Quiescent Current I
OUT
< 50 µA 25 µA
C
OUT
Output Capacitance 0 µA ≤ I
OUT
≤ 100 µA 0.1 µF
Output Capacitor ESR 5 500 mΩ
(1) All voltages are with respect to the potential at the GND pin.
(2) All limits specified at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are
production tested, ensured through statistical analysis or ensured by design. All limits at temperature extremes are ensured 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 I
2
C 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 V
BAT
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 (V
BAT
> 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.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Links: LP3972