SC813 datasheet(18/23 Pages) SEMTECH

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SC811 / SC813

Applications Information (continued)

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 SC811/3 adapter mode sup-

ports 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

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

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

ADUVLO-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 SC811/3 should be operated with

adapter voltage below the rising selection threshold

(VT

ADUVLO-R

) only if the low input voltage is the result of

adapter current limiting. This implies that the VIN voltage

first exceeds VT

ADUVLO-R to begin charging and is subse-

quently 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 adapter mode 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 SC811/3 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 SC811/3 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

Under-Voltage Load Regulation in USB Modes

VIN pin UVLR in either USB mode prevents the battery

charging current from overloading the USB Vbus network,

regardless of the programmed fast-charge value. When

USB High Power or USB Low Power mode is selected, the

SC811/3 monitors the input voltage (V

VIN

) and reduces the

charge current as necessary to keep V

VIN

at or above the

UVLR limit (V

UVLR

). UVLR operates like a fourth output con-

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