LP3925 Datasheet, PDF (16/51 Pages) Texas Instruments – High Performance Power Management Unit for Handset Applications

BATTERY CHARGING FUNCTION
LP3925 has a safe and smart Li-Ion/Li-Poly battery charge
management system, keeping the number of battery charging
cycles to a minimum and thus increasing battery lifetime.
Following the correct detection of a voltage at the charger in-
put, the charger enters precharge mode. In this mode the
battery is charged with a small constant current. Precharge
settings are available in register 0x82 and these values are
remembered as long as the PMU has power. CHARGER
IPRECHARGE bits select the battery current in precharge
mode. It should be noted, that value '00' disables precharge
current completely.
CHARGER VFULLRATE bits select the level, from which the
battery can be charged with full charging current. If battery
voltage reaches that level, then the charger will move on to
full charging mode. CHARGER TPRECHARGE sets the max-
imum precharge time, after which the battery will be isolated,
protecting it from further charging.
In full charging mode full rate constant current is applied to
the battery, to raise the voltage to the termination level (se-
lected by VTERM bits). The maximum charging current is
selectable via IBATT bits, but the actual current can be lower
than this limit, depending on the load of system branch. When
termination voltage is reached, the charger enters constant
voltage mode and a constant voltage is maintained. Now
charging current is monitored to detect the end-of-charge
condition. After reaching this condition, charger disables the
battery charging branch and enters standby mode. FULL
TIMEOUT sets the maximum full charge duration, after which
the charging will be ended automatically. Then the charger
enters standby mode, where the battery is isolated and the
charger monitors battery voltage. If restart conditions have
been met, then the charging cycle is re-initiated to re-establish
the termination voltage level.
In standby mode the battery is isolated and the charger mon-
itors battery voltage.
CC TO CV MODE TRANSITION
CC to CV mode transition is shown on Figure 8. The voltage
level of charging mode change (CC to CV) is dependent on
both V_TERM (0x47, bit3-0) and I_BATT (real charging current)
settings, equation shows below:
V_CCtoCV = V_TERM – (I_BATT*0.05)
where 0.05 stands for charger’s internal resistance. This
equation is valid only when the power routing switch is in non
conducting state and the charging is done through charger’s
branch. When the switch is ON, the charging is done through
the supply branch, and the internal resistance in the equation
is replaced by the power routing switch’s ON resistance. Note
that the I_BATT current in equation is the real battery charging
current which may differ from the programmed one.
USER can use ADC to measure charging current and BATT
pin voltage.
END-OF-CHARGE AND RESTART
Ending and re-starting the charging cycle can be accom-
plished in multiple ways.
Automatic control can be enabled with AUTOMATIC START/
STOP bit. If this bit is '0', then the charging cycle can only be
controlled manually. Writing this bit to '1' enables fully auto-
matic cycle control.
Manual control is always available via STOP CHARGING and
START CHARGING bits. These control bits are write-only
and not tied to any internal monitoring, allowing to start and
stop charging at any time. Only one of these bits must be
written to '1' at once.
Automatic control utilizes internal battery monitoring, which
can be configured trough control registers.
Automatic end-of-charge requires that the battery has
reached termination level. Then the charger starts monitoring
battery charging current and comparing it to end-of-charge
current (set by ABS EOC and EOC LEVEL bits). If the charg-
ing current is below this level for the duration of EOC TIME,
then the automatic end-of-charge condition has been
reached.
Automatic restart requires that one charging cycle has al-
ready been completed and the charger is in standby mode.
Then the charger monitors battery voltage and compares it to
restart level (set by RESTART LEVEL). If the battery voltage
is below this level for the duration of RESTART TIME, then
the automatic restart condition has been reached.
In manual-only mode it is possible to get automatic cycle con-
trol information via interrupts. CHARGE STOP INT is gener-
ated, if end-of-charge condition has been reached, and
CHARGE START INT is generated, if restart condition has
been reached).
POWER ROUTING SWITCH
This switch separates the battery from the system. This helps
to conserve charge and reduce the count of charge-discharge
cycles.
If charger input is not connected, then the switch is in ON
(conducting) state.
In normal conditions the switch is OFF during charger oper-
ation. The switch will be turned ON, if the system load is
greater than the charger can supply. This event also gener-
ates a SWITCH AUTO ON interrupt. If the switch is turned ON
during charger operation, then it does not turn back OFF au-
tomatically. The only exception to that rule is, if the PMU
enters standby mode. If the charger is operating and the
switch is turned ON, then only system supply branch is work-
ing. Maximum output current is then set by I_BATT code, to avoid
too high current into the battery.
If the charger is working, then power routing switch state can
be controlled manually with SWITCH ON and SWITCH OFF
command bits. It is advisable to use the SWITCH ON com-
mand before the system enters a mode, which draws more
current than the input can provide. This will help to avoid volt-
age drop on VDD, before the internal overload detection
reacts. If the system returns to low consumption mode, the
SWITCH OFF command bit should be used to restore normal
charger operation.
OPERATION WITHOUT BATTERY
The charger is not aware of battery presence, it always be-
haves by the same configuration. If the battery is not con-
nected, then it is recommended to keep the battery charging
part disabled. This can be done by disabling the restart timer
and ending the charging with STOP CHARGING bit. Other-
wise the charger will keep trying to regulate the voltage on
BATT pin, causing frequent charging cycles.
HIGH-CURRENT MODE
If HIGH-CURRENT MODE bit is set, then the charger enters
special high load mode. In this mode the power routing switch
is set ON, input current limit is disabled, and the charger
branches are working together. All charging cycle controls do
not have any effect in high current mode.
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