LP3971_16 Datasheet, PDF (38/52 Pages) Texas Instruments – Power Management Unit for Advanced Application Processors

LP3971
SNVS432V – JANUARY 2006 – REVISED MAY 2013
GPIO<1>
X
0
1
0
1
Controls
GPIO<1>
Nextchgen_sel
X
X
0
0
0
0
1
0
1
0
bucen
1
X
X
X
X
Port Function
GPIO1
X
HiZ
Input (dig)->
Output = 0
Output = 1
Reg
Gpin 1
Input
0
0
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batmonchg
Function
Enabled
GPIO<1>
0
1
0
1
GPIO<1>
0
0
1
1
Factory fm disabled
GPIO_tstiob
1
1
1
1
GPIO2
HiZ
Input (dig)->
Output = 0
Output = 1
gpin2
0
input
0
0
The LP3971 has provision for two battery connections, the main battery Vbat and Backup Battery (See
Applications Schematic Diagrams Figure 1 and Figure 2).
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 (V_BAT > 3.1V), the switch will
automatically connect the RTC LDO power to the main battery.
• As the main battery is discharged by use, the user will be warned by a separate circuit called nBATT_FLT.
Then if no action is taken to restore the charge on the main battery, and discharging is continued the battery
switch will protect the RTC LDO by disconnecting from the main battery and connecting to the backup battery.
– The main battery voltage at which the RTC LDO is switched from main to backup battery is 2.9V typically.
– There is a hysterisis 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.
• Additionally, the user may wish to disable the battery switch, such as, in the case when only a main battery is
used. This is accomplished by setting the “no back up battery bit” in the control register 8h’89 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 2.9 Volt threshold.
REGULATED VOLTAGES OK
All the power domains have own register bit (X_OK) that processor can read via serial interface to be sure that
enabled powers are OK (regulating). Note that these read only bits are only valid when regulators are settled
(avoid reading these bits during voltage change or power up).
THERMAL MANAGEMENT
Application: There is a mode wherein all 6 comparators (flags) can be turned on via the “enallflags” control
register bit. This mode allows the user to interrogate the device or system temperature under the set operating
conditions. Thus, the rate of temperature change can also be estimated. The system may then negotiate for
speed and power trade off, or deploy cooling maneuvers to optimize system performance. The “enallflags” bit
needs enabled only when the “bct<2:0> bits are read to conserve power.
Note: The thermal management flags have been verified functional. Presently these registers are accessible by
factory only. If there is a demand for this function, the relevant register controls may be shifted into the user
programmable bank; the temperature range and resolution of these flags, might also be refined/redefined.
Application Note - LP3971 Reset Sequence
INITIAL COLD START POWER ON SEQUENCE
1. The Back up battery is connected to the PMU, power is applied to the back-up battery pin, the RTC_LDO
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