SC813 datasheet(17/23 Pages) SEMTECH

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SC811 / SC813

Applications Information (continued)

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 voltage is greater than the

UVLO level and less than the OVP level of the selected

mode. 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 subsequently 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 will remain asserted for approximately 750μs before

being released. The STATB pin is not asserted for auto-

matic 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 SC811/3 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

simultaneous 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 influ-

ence 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 control-

ler 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 SC811/3, 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 by 5% the mode-dependent pro-

grammed fast-charge current, 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 Constant Current, Constant Voltage, “Constant”

Temperature (CC/CV/CT) 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 SC811/3. The

thermal limiting controller reduces the output current by

≈ 50mA/ºC for any junction temperature T

> T

When thermal limiting is inactive,

= T

+ V

where V

is the voltage difference between the VIN pin

and the BAT pin. However, if T

computed this way exceeds

, then thermal limiting will become active and the

thermal limiting regulation junction temperature will be

JTL

= T

+ V

I(T

JTL

) θ

where

I(T

JTL

) = I

− i

JTL

− T

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