芯美电子
Operation
The AAP2154 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 AAP2154 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 AAP2154 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% program
resistor is connected from the PROG 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 AAP2154 supplies
approximately 1/10 the programmed charge current to
bring the battery voltage up to a safe level for full current
charging.
When the BAT pin voltage rises above 2.9V, the charger
enters constant-current mode, where the programmed
charge current is supplied to the battery. When the BAT
pin approaches the final float voltage (4.2V), the
AAP2154 enters constant-voltage mode and the charge
current begins to decrease. When the charge current
drops to 1/10 of the programmed value, the charge cycle
ends.
Programming Charge Current
The charge current is programmed using a single resistor
from the PROG pin to ground. The battery charge current
is 1000 times the current out of the PROG pin. The
program resistor and the charge current are calculated
using the following equations:
R PROG
=
AAP2154
Charge Termination
A charge cycle is terminated when the charge current
falls to 1/10th the programmed value after the final float
voltage is reached. This condition is detected by using an
internal, filtered comparator to monitor the PROG pin.
When the PROG pin voltage falls below 100mV for
longer than t
TERM
(typically 1ms), charging is terminated.
The charge current is latched off and the AAP2154 enters
standby mode, where the input supply current drops to
200µA.
When charging, transient loads on the BAT pin can cause
the PROG pin to fall below 100mV for short periods of
time before the DC charge current has dropped to 1/10th
the programmed 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 termi-
nation. Once the average charge current drops below
1/10th the programmed value, the AAP2154 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 AAP2154 constantly monitors the BAT pin voltage
in standby mode. If this voltage drops below the 4.05V
recharge threshold (V
RECHRG
), 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 PROG
pin. Figure 7 shows the state diagram of a typical charge
cycle.
Charge Status Indicator (
CHRG
)
The charge status output has three different states: strong
pull-down(~10mA), weak pull-down (~12µA) and high
impedance. The strong pull-down state indicates that the
AAP2154 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 AAP2154 is ready to
charge. High impedance indicates that the AAP2154 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.
1000V
1000V
,
I CHG
=
I CHG
R PROG
The charge current out of the BAT pin can be determined
at any time by monitoring the PROG pin voltage using
the following equation:
I BAT
=
V
PROG
•
1000
R PROG
DS2154 Ver1.0 Aug. 2007
11
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EUTECH [ EUTECH MICROELECTRONICS INC ]
