SC820 datasheet(17/22 Pages) SEMTECH

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SC820

Applications Information (continued)

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 de-selection

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 de-selection falling threshold (VT

ADsel-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 SC820 should be operated with

adapter voltage below the rising selection threshold

(VT

ADSel-R

) only if the low input voltage is the result of

adapter current limiting. This implies that the VAD voltage

first exceeds VT

ADsel-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 adapter input 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 VAD 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 SC820 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 SC820 junction temperature will rise above ambient

by P

ILIM

x θ

. If the device temperature rises to the tem-

perature at which the TL control loop limits charging

current (rather than the current being limited by the

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

tion voltage. The power dissipation will increase so that

the TL regulation will further limit charge current. This will

keep the adapter in voltage regulation for the remainder

of the charge cycle. In this case, the SC820 will continue

to charge with thermal limiting until charge current

decreases while in CV regulation (reducing power dissipa-

tion sufficiently), resulting in a slow charge cycle, but with

no other negative effect.

To ensure that the adapter remains in current limit, the

internal device temperature must not rise to T

. This

implies that θ

must be kept small enough, through

careful layout, to ensure that T

= T

+ (P

ILIM

× θ

) < T

VUSB Under-Voltage Load Regulation

VUSB pin UVLR prevents the battery charging current from

overloading the USB Vbus network, regardless of the pro-

grammed fast-charge value. When the VUSB input is

selected, the SC820 monitors the input voltage (V

VUSB

) and

reduces the charge current as necessary to keep V

VUSB

at or

above the UVLR limit of V

UVLR

= 4.57V typically. UVLR oper-

ates like a fourth output constraint (along with CC, CV, and

CT constraints), but it is active only when the VUSB input

is selected.

If the VUSB voltage is externally pulled below V

UVLR

while

the VAD input is absent, the UVLR feature will reduce the

charging current to zero. This condition will not be inter-

preted as termination and will not result in an end-of-

charge indication. The STATB pin will remain asserted as if

charging is continuing. This prevents repetitive indica-

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