LP28292 Datasheet, PDF (8/10 Pages) Lowpower Semiconductor inc – 1.5A Single-chip Li-ion and Li-POL Charge with OVP

Preliminary Datasheet
LP28292
Operation
The LP28292 is a single cell lithium-ion battery charger
using a constant-current/constant-voltage algorithm. It can
deliver up to 800mA of charge current (using a good
thermal PCB layout) with a final float voltage accuracy of
±1%. The LP28292 includes an internal P-channel power
MOSFET and thermal regulation circuitry. No blocking
diode or external current sense resistor is required; thus, the
basic charger circuit requires only two external components.
Furthermore, the LP28292 is capable of operating from a
USB power source.
Normal Charge Cycle
A charge cycle begins when the voltage at the V_CC pin rises
above the UVLO threshold level and a 1% ISET ram
resistor is connected from the ISET pin to ground or when a
battery is connected to the charger output. If the BAT pin is
less than 2.9V, the charger enters trickle charge mode. In
this mode, the LP28292 supplies approximately 1/10 the
ISET rammed charge current to bring the battery voltage up
to a safe level for full current charging. (Note: The
LP28292X does not include this trickle charge feature).
When the BAT pin voltage rises above 2.9V, the charger
enters constant-current mode, where the ISET rammed
charge current is supplied to the battery. When the BAT pin
approaches the final float voltage (4.2V), the LP28292
enters constant-voltage mode and the charge current begins
to decrease. When the charge current drops to 1/10 of the
ISET rammed value, the charge cycle ends.
ISET ramming Charge Current
The charge current is ISET rammed using a single resistor
from the ISET pin to ground. The battery charge current is
1000 times the current out of the ISET pin. The ISET ram
resistor and the charge current are calculated using the
following equations:
RSET=880V/ICHG，ICHG= 880V/RSET
The charge current out of the BAT pin can be determined at
any time by monitoring the ISET pin voltage using the
following equation:
IBAT= VSET x 880/RSET
Charge Termination
A charge cycle is terminated when the charge current falls
to 1/10th the ISET rammed value after the final float
voltage is reached. This condition is detected by using an
internal, filtered comparator to monitor the ISET pin. When
1
the ISET pin voltage falls below 100mV for longer than
LP28292 – 02 Ver. 1.2 Datasheet
Nov.-2008
t_TERM (typically 1ms), charging is terminated. The charge
current is latched off and the LP28292 enters standby mode,
where the input supply current drops to 200µA. (Note: C/10
termination is disabled in trickle charging and thermal
limiting modes).
When charging, transient loads on the BAT pin can cause
the ISET pin to fall below 100mV for short periods of time
before the DC charge current has dropped to 1/10th the
ISET rammed value. The 1ms filter time (t_TERM) on the
termination comparator ensures that transient loads of this
nature do not result in premature charge cycle termination.
Once the average charge current drops below 1/10th the
ISET rammed value, the LP28292 terminates the charge
cycle and ceases to provide any current through the BAT
pin. In this state, all loads on the BAT pin must be supplied
by the battery.
The LP28292 constantly monitors the BAT pin voltage in
standby mode. If this voltage drops below the 4.05V
recharge threshold (V_RESTAT), another charge cycle begins
and current is once again supplied to the battery. To
manually restart a charge cycle when in standby mode, the
input voltage must be removed and reapplied, or the
charger must be shut down and restarted using the ISET pin.
Figure 1 shows the state diagram of a typical charge cycle.
Charge Status Indicator (STAT)
The charge status output has three different states: strong
pull-down (~10mA), weak pull-down (~20µA) and high
impedance. The strong pull-down state indicates that the
LP28292 is in a charge cycle. Once the charge cycle has
terminated, the pin state is determined by undervoltage
lockout conditions. A weak pull-down indicates that V_CC
meets the UVLO conditions and the LP28292 is ready to
charge. High impedance indicates that the LP28292 is in
undervoltage lockout mode: either V_CC is less than 100mV
above the BAT pin voltage or insufficient voltage is applied
to the V_CC pin. A microprocessor can be used to distinguish
between these three states—this method is discussed in the
Applications Information section.
Thermal Limiting
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 LP28292 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 LP28292. The charge current can be set according to
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