Preliminary Datasheet
assurance that the charger will automatically reduce the
current in worst-case conditions. TDFN power consid-
erations are discussed further in the Applications Informa-
tion section.
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors the input
voltage and keeps the charger in shutdown mode until V
CC
rises above the undervoltage lockout threshold. The UVLO
circuit has a built-in hysteresis of 200mV. Furthermore, to
protect against reverse current in the power MOSFET, the
UVLO circuit keeps the charger in shutdown mode if V
CC
falls to within 30mV of the battery voltage. If the UVLO
comparator is tripped, the charger will not come out of
shutdown mode until V
CC
rises 100mV above the battery
voltage.
Manual Shutdown
At any point in the charge cycle, the LP28011 can be put
into shutdown mode by removing R
ISET
thus floating the
ISET pin. This reduces the battery drain current to less than
2µA and the supply current to less than 50µA. A new
charge cycle can be initiated by reconnecting the ISETram
resistor.
In manual shutdown, the STAT pin is in a weak pull-down
state as long as V
CC
is high enough to exceed the UVLO
conditions. The STAT pin is in a high impedance state if
the LP28011 is in undervoltage lockout mode: either V
CC
is
within 100mV of the BAT pin voltage or insufficient
voltage is applied to the V
CC
pin.
Automatic Recharge
Once the charge cycle is terminated, the LP28011 continu-
ously monitors the voltage on the BAT pin using a com-
parator with a 2ms filter time (t
RECHARGE
). A charge cycle
restarts when the battery voltage falls below 4.05V (which
corresponds to approximately 80% to 90% battery capacity).
This ensures that the battery is kept at or near a fully
charged condition and eliminates the need for periodic
charge cycle initiations. STAT output enters a strong
pull-down state during recharge cycles.
Power Dissipation
The conditions that cause the LP28011 battery charger
to reduce charge current through thermal feedback can
be approximated by considering the total power
dissipated in the IC. For high charge currents, the
LP28011 power dissipation is approximately:
LP28011
Where PD is the total power dissipated within the IC,
ADP is the input supply voltage,
VBAT
is the battery
voltage,
IBAT
is the charge current and
PD_BUCK
is the
power dissipation due to the regulator.
PD_BUCK
can be
calculated as:
Where
VOUTB
is the regulated output of the switching
regulator,
IOUTB
is the regulator load and is the regulator
efficiency at that particular load.
It is not necessary to perform worst-case power
dissipation scenarios because the LP28011 will
automatically reduce the charge current to maintain the
die temperature at approximately 125°C. However, the
approximate ambient temperature at which the thermal
feedback begins to rotect the IC is:
Example: Consider the extreme case when an LP28011
is operating from a 6V supply providing 250mA to a 3V
Li-Ion battery, the switching regulator and the LDO are
off. The ambient temperature above which the LP28011
will begin to reduce the 250mA charge current is
approximately: (Correctly soldered to a 2500mm
2
double-sided 1 oz. copper board, the LP28011 has a
thermal resistance of approximately 43°C/W.)
T=
If there is more power dissipation due to the switching
regulator or the LDO, the thermal regulation will kick in
at a somewhat lower temperature than this. In the above
circumstances, the LP28011 can be used above 82.75°C,
but the charge current will be reduced from 250mA. The
approximate current at a given ambient temperature can
be calculated:
o
o
o
Note: 1V = 1J/C = 1W/A
Furthermore, the voltage at the ISET pin will change
proportionally with the charge current as discussed in
the ISET ramming Charge Current section.
LP28011 – 01 Ver. 1.1 Datasheet
Nov.-2007
Page 9 of 10
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