LP3971_16 Datasheet, PDF (14/52 Pages) Texas Instruments – Power Management Unit for Advanced Application Processors

LP3971
SNVS432V – JANUARY 2006 – REVISED MAY 2013
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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⁾ and ⁽³⁾.
Symbol
Parameter
Conditions
Min Typ Max Units
V_IN
Operational Voltage Range
I_OUT
Backup Battery Charging Current
Voltage at V_IN
V_IN = 3.6V, Backup_Bat = 2.5V, Backup
Battery Charger Enabled ⁽³⁾
3.3
5.5
V
190
μA
V_OUT
Charger Termination Voltage
V_IN = 5.0V Backup Battery Charger
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
I_OUT ≤ 50 μA, V_OUT = 3.15V
V_OUT + 0.4 ≤ V_BATT = V_IN ≤ 5.0V
f < 10 kHz
15
dB
I_Q
C_OUT
Quiescent Current
Output Capacitance
Output Capacitor ESR
I_OUT < 50 μA
0 μA ≤ I_OUT ≤ 100 μA
25
μA
0.1
μF
5
500 mΩ
(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) Back-up battery charge current is programmable via the I²C compatible interface. Refer to the Application Section for more information.
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.
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