LP28056 Datasheet, PDF (8/9 Pages) Lowpower Semiconductor inc – Programmable Charge Current Up to 1200mA

Preliminary Datasheet
LP28056
An internal thermal feedback loop reduces the ISET
rammed charge current if the die temperature attempts to
rise above a preset value of approximately 120°C. This
feature protects the LP28056 from excessive temperature
and allows the user to push the limits of the power handling
capability of a given circuit board without risk of damaging
the LP28056. The charge current can be set according to
typical (not worst-case) ambient temperature with the
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.
The conditions that cause the LP28056 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
LP28056 power dissipation is approximately:
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 LP28056 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:
Manual Shutdown
At any point in the charge cycle, the LP28056 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 LP28056 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 LP28056 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
LP28056 – 01 Ver. 1.1 Datasheet
FEB.-2012
Example: Consider the extreme case when an LP28056
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 LP28056
will begin to reduce the 250mA charge current is
approximately: (Correctly soldered to a 2500mm²
double-sided 1 oz. copper board, the LP28056 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 LP28056 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:
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Note: 1V = 1J/C = 1W/A
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