SC820 datasheet(16/21 Pages) SEMTECH

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SC820

Applications Information (continued)

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 SC820, 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 programmed fast-

charge current, the controller rever ts 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 SC820. 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

Combining these two equations and solving for T

JTL

, the

steady state junction temperature during active thermal

limiting is

JTL

Although the thermal limiting controller is able to reduce

output current to zero, this does not happen in practice.

Output current is reduced to I(T

JTL

), reducing power dissi-

pation such that die temperature equilibrium T

JTL

reached.

While thermal limiting is active, all charger functions

remain active and the charger logical state is preserved.

Operating a Charging Adapter in Current Limit

In high charging current applications, charger power dis-

sipation can be greatly reduced by operating the charging

adapter in current limit. The SC820 VAD input supports

adapter-current-limited charging with a low deselection

falling threshold and with internal circuitry designed for

low input voltage operation. To operate an adapter in

current limit, R

IPRGM

is chosen such that the adapter input

programmed fast-charge current I

FQ_AD

exceeds the current

limit of the charging adapter I

AD-LIM

Note that if I

AD-LIM

is less than 20% of I

FQ_AD

, then the adapter

voltage can be pulled down to the battery voltage while

the battery voltage is below the pre-charge threshold. In

this case, care must be taken to ensure that the adapter

will maintain its current limit below 20% of I

FQ_AD

at least

until the battery voltage exceeds the pre-charge thresh-

old. Failure to do so could permit charge current to exceed

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