参数资料
| 型号： | SC811ULTRT |
| 厂商： | Semtech |
| 文件页数： | 18/23页 |
| 文件大小： | 0K |
| 描述： | IC BATTERY MANAGEMENT |
| 标准包装： | 1 |
| 系列： | * |
| 其它名称： | SC811ULDKR |
SC811 / SC813
Applications Information (continued)
Combining these two equations and solving for T JTL , the
steady state junction temperature during active thermal
limiting is
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
T JTL
T A
V I FQ _ x i T T TL
1 V i T JA
JA
first exceeds VT ADUVLO-R to begin charging and is subse-
quently pulled down to just above the battery voltage by
the charging load.
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 is
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-
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
P 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 JA . 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 TL . This
implies that θ JA must be kept small enough to ensure that
T J = T A + (P ILIM × θ JA ) < T TL .
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-
? 2008 Semtech Corporation
18
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| SC813 | 制造商:SEMTECH 制造商全称:Semtech Corporation 功能描述:Adapter/USB Tri-Mode Single-cell Li-ion Charger |
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