NCP1800
Operation Descriptions
The NCP1800 is a linear lithium ion (Li−ion) battery
charge controller and provides the necessary control
functions for charging Li−ion batteries precisely and safely.
It features the constant current and constant voltage method
(CCCV) of charging.
Conditioning and Pre−charge Phase
The NCP1800 initiates a charging cycle upon toggling the
COMP/DIS to LOW or application of the valid external
power source (i.e. V
UVLO
t
V
CC
t
V
OVLO
) with the
Li−ion battery present or when the Li−ion battery is inserted.
Before a charge cycle can begin, the battery conditions are
verified to be within safe limits. The battery will not be
charged when its voltage is less than 0.9 V or higher than
V
SOVLO
.
Li−ion batteries can be easily damaged when fast charged
from a completely discharged state. Also, a fully discharged
Li−ion battery may indicate an abnormal battery condition.
With the built−in safety features of the NCP1800, the Li−ion
battery pre−charges (Pre−Charge Phase) at 10% of the full
rated charging current (I
REG
) when the battery voltage is
lower than V
PCTH
and the CFLG pin is HIGH. Typically, the
battery voltage reaches V
PCTH
in a few minutes and then the
Full−Charge phase begins.
Full−Charge (Current Regulation) Phase
Since the external P−channel MOSFET is used to regulate
the current to charge the battery and operates in linear mode
as a linear regulator, power is dissipated in the pass
transistor. Designing with a very well regulated external
adaptor (e.g. 5.1 V
±1%)
can help to minimize the heat
dissipation in the pass transistor. Care must be taken in
heatsink designing in enclosed environments such as inside
the battery operated portables or cellular phones.
The Full−Charge phase continues until the battery voltage
reaches V
REG
. The NCP1800 comes in two options with
V
REG
thresholds of 4.1 and 4.2 V.
Final Charge (Voltage Regulation) Phase
Once the battery voltage reaches V
REG
, the pass transistor
is controlled to regulate the voltage across the battery and the
Final Charge phase (constant voltage mode) begins. Once
the charger is in the Final Charge phase, the charger
maintains a regulated voltage and the charging current will
begin to decrease and is dependent on the state of the charge
of the battery. As the battery approaches a fully charged
condition, the charge current falls to a very low value.
Trickle Charge Phase
When the battery voltage reaches V
PCTH
, the NCP1800
begins fast charging the battery with full rate charging
current I
REG
. The NCP1800 monitors the charging current
at the I
SNS
input pin by the voltage drop across a current
sense resistor, R
SNS
, and the charging current is maintained
at I
REG
by the pass transistor throughout the Full−Charge
phase.
I
REG
is determined by R
SNS
and R
ISEL
with the following
formula:
(1.19
IREG
+
(RISEL
12 k)
RSNS)
During the Final Charge phase, the charging current
continues to decrease and the NCP1800 monitors the
charging current through the current sense resistor R
SNS
.
When the charging current decreases to such a level that I
SNS
< 0.1 X I
REG
, the CFLG pin is set to LOW and the Trickle
Charge phase begins. The charger stays in the Trickle
Charge phase until any fault modes are detected or the
COMP/DIS pin is pulled low to start over the charging cycle.
And with R
ISEL
= 60 k and R
SNS
= 0.4
W,
I
REG
= 0.6 A.
http://onsemi.com
10
ONSEMI [ ON SEMICONDUCTOR ]
