SC810_1 Datasheet, PDF (14/20 Pages) Semtech Corporation – Single Input/Single Mode Single-cell Li-ion Charger

SC810
Applications Information (continued)
voltage. ENB should be held low for at least 1ms to ensure
a successful forced re-charge.
Forced re-charge can be requested at any time during the
charge cycle, or even with no charging source present,
with no detrimental effect on charger operation. This
allows the host processor to schedule a forced re-charge
at any desired interval, without regard to whether a charge
cycle is already in progress, or even whether a charging
source is present. Forced re-charge will neither assert nor
release the STATB output.
Status Output
The STATB pin is an open-drain output. It is asserted
(driven low) as charging begins after a valid charging
input is applied and the VIN pin is greater than the input
UVLO level and less than the OVP level. STATB is also
asserted as charging begins after the ENB input returns to
either of the enable voltage ranges (mid or low voltage)
from the disable (high voltage) range. STATB is subse-
quently released when the termination current is reached
to indicate end-of-charge, when the ENB input is driven
high to disable charging, or when the input voltage is
removed. If the battery is already fully charged when a
charge cycle is initiated, STATB is asserted, and remains
asserted for approximately 750μs before being released.
The STATB pin is not asserted for automatic re-charge
cycles.
The STATB pin may be connected to an interrupt input to
notify a host controller of the charging status or it can be
used as an LED driver.
Logical CC-to-CV Transition
The SC810 differs from monolithic linear single cell Li-ion
chargers that implement a linear transition from CC to CV
regulation. The linear transition method uses two simul-
taneous feedback signals — output voltage and output
current — to the closed-loop controller. When the output
voltage is sufficiently below the CV regulation voltage, the
influence of the voltage feedback is negligible and the
output current is regulated to the desired current. As the
battery voltage approaches the CV regulation voltage
(4.2V), the voltage feedback signal begins to influence the
control loop, which causes the output current to decrease
although the output voltage has not reached 4.2V. The
output voltage limit dominates the controller when the
battery reaches 4.2V and eventually the controller is
entirely in CV regulation. The soft transition effectively
reduces the charge current below that which is permitted
for a portion of the charge cycle, which increases charge
time.
In the SC810, a logical transition is implemented from CC
to CV to recover the charge current lost due to the soft
transition. The controller regulates only current until the
output voltage exceeds the transition threshold voltage.
It then switches to CV regulation. The transition voltage
from CC to CV regulation is typically 5mV higher than the
CV regulation voltage, which provides a sharp and clean
transition free of chatter between regulation modes. The
difference between the transition voltage and the regula-
tion voltage is termed the CC/CV overshoot. While in CV
regulation, the output current sense remains active. If the
output current exceeds the programmed fast-charge
current by 5%, the controller reverts to current
regulation.
The logical transition from CC to CV results in the fastest
possible charging cycle that is compliant with the speci-
fied current and voltage limits of the Li-ion cell. The output
current is constant at the CC limit, then decreases abruptly
when the output voltage steps from the overshoot voltage
to the regulation voltage at the transition to CV control.
Thermal Limiting
Device thermal limiting is the third output constraint of
the CC/CV/TL control. This feature permits a higher input
OVP threshold, and thus the use of higher voltage or
poorly regulated adapters. If high input voltage results in
excessive power dissipation, the output current is reduced
to prevent overheating of the SC810. The thermal limiting
controller reduces the output current by i ≈ –50mA/ºC for
T
any junction temperature T > T .
J TL
When thermal limiting is inactive,
T =T +V I θ ,
JA
Δ FQ JA
where V is the voltage difference between the VIN pin
Δ
and the BAT pin. However, if T computed this way exceeds
J
T , then thermal limiting will become active and the
TL
thermal limiting regulation junction temperature will be
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