SC824 datasheet(19/29 Pages) SEMTECH

19 / 29 page

SC824

Applications Information (continued)

supply to power LEDs. The conditions indicated by STAT0

and STAT1 are summarized in Table 2.

Table 2 — STATx Pin Truth Table

Condition

STAT0

STAT1

Power applied (VT

UVLO

< V

VIN

OVP

), prior to charging

(asserted)

(released)

Charging

(asserted)

Charging Done, any stage of a

recharge cycle, or No-Battery

Mode

(asserted)

(released)

Charging Fault (Pre-charge or CC

time-out, NTC temperature fault,

IPRGM pin short to ground)

(released)

(asserted)

No valid input supply

(released)

The STAT0 and STAT1 outputs indicate charging faults by

asserting STAT1 while releasing STAT0. Charging faults

include pre-charge or CC-stage timeout, a battery NTC

temperature fault (hot or cold), and the shorting of the

IPRGM pin to ground.

Charging status is indicated during precharge, CC charg-

ing, and CV charging until termination. Top-off charging

is not indicated. Upon termination, whether the timer is

enabled or disabled, charging status will be indicated for

an additional 22 seconds, either while top-off charging

with the timer enabled, or while in monitor state with the

output off if the timer is disabled. This feature ensures

that “charging” will be indicated long enough to be seen,

even if the adapter voltage is applied when the battery is

already fully charged. It informs the user that a charge

cycle was begun and completed normally. Note that the

SC824 can terminate charging of a fully charged battery in

as little as a millisecond; without this feature the “charg-

ing” indication would not be visible.

Logical CC-to-CV Transition

The SC824 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 V

(nominally 4.2V), the

voltage feedback signal begins to influence the control

loop, which causes the output current to decrease even

though the output voltage has not yet reached V

. The

output voltage limit dominates the controller when the

battery reaches V

, 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.

The SC824 uses a logical transition from CC to CV to

recover the charge current lost due to a 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 7mV higher than the CV regu-

lation voltage, which provides a sharp and clean transition

free of chatter between regulation modes. The difference

between the transition voltage and the regulation voltage

is referred to as the CC/CV overshoot. While in CV regula-

tion, the output current sense remains active. If the output

current exceeds the mode-dependent programmed fast-

charge current by approximately 5%, the controller reverts

to current regulation.

The logical transition from CC to CV results in a faster

charging cycle that is compliant with the specified 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 (TL) is the third output constraint

of the CC/CV/TL control. This feature permits a higher

input OVP threshold in the SC824, and thus the use of

higher regulation voltage or poorly regulated adapters. If

high input voltage results in excessive power dissipation,

the output current is reduced to prevent overheating of

the SC824. The thermal limiting controller reduces the

output current at the rate of i

≈ −50mA/ºC for any junc-

tion temperature T

> T

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