SC810 datasheet(15/19 Pages) SEMTECH

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SC810

Applications Information (continued)

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 SC810 supports adapter-

current-limited charging with a low UVLO falling threshold

and with internal circuitry designed for low input voltage

operation. To operate an adapter in current limit, R

IPRGM

chosen such that the programmed fast-charge current I

exceeds the current limit of the charging adapter I

AD-LIM

Note that if I

AD-LIM

is less than 20% of I

, 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

at least until

the battery voltage exceeds the pre-charge threshold.

Failure to do so could permit charge current to exceed the

pre-charge current while the battery voltage is below the

pre-charge threshold. This is because the low input

voltage will also compress the pre-charge threshold inter-

nal reference voltage to below the battery voltage. This

will prematurely advance the charger logic from pre-

charge current regulation to fast-charge regulation, and

the charge current will exceed the safe level recom-

mended for pre-charge conditioning.

The low UVLO falling threshold (VT

UVLO-F) permits the

adapter voltage to be pulled down to just above the

battery voltage by the charging load whenever the

adapter current limit is less than the programmed fast-

charge current. The SC810 should be operated with

adapter voltage below the rising selection threshold

(VT

UVLO-R

) only if the low input voltage is the result of

adapter current limiting. This implies that the VIN voltage

first exceeds VT

UVLO-R to begin charging, and is subsequently

pulled down to just above the battery voltage by the

charging load.

Interaction of Thermal Limiting and Current Limited

Adapter Charging

To permit the charge current to be limited by the adapter,

it is necessary that the fast-charge current be programmed

greater than the maximum adapter current, (I

AD-LIM

). In this

configuration, the CC regulator will operate with its pass

device fully on (in saturation, also called “dropout”). The

voltage drop from VIN to BAT is determined by the product

of the minimum R

DS-ON

of the pass device multiplied by the

adapter supply current.

In dropout, the power dissipation in the SC810 is

ILIM

= (minimum R

DS-ON

) x (I

AD-LIM

)2. Since minimum R

DS-ON

does not vary with battery voltage, dropout power dissi-

pation is constant throughout the CC portion of the

charge cycle while the adapter remains in current limit.

The SC810 junction temperature will rise above ambient

by P

ILIM

x θ

. If the device temperature rises to the tem-

perature at which the thermal limiting control loop limits

charging current (rather than the current being limited by

the adapter), the input voltage will rise to the adapter

regulation voltage. The power dissipation will increase so

that the thermal limit regulation will further limit charge

current. This will keep the adapter in voltage regulation

for the remainder of the charge cycle.

To ensure that the adapter remains in current limit, the

internal device temperature must never rise to T

. This

implies that θ

must be kept small enough to ensure that

= T

+ (P

ILIM

× θ

) < T

Short Circuit Protection

The SC810 can tolerate a BAT pin short circuit to ground

indefinitely. The current into a ground short is approxi-

mately 10mA.

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