参数资料
| 型号： | SC824ULTRT |
| 厂商： | Semtech |
| 文件页数： | 20/29页 |
| 文件大小： | 0K |
| 描述： | IC BATTERY MANAGEMENT |
| 标准包装： | 1 |
| 系列： | * |
| 其它名称： | SC824ULDKR |
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SC824
Applications Information (continued)
When thermal limiting is inactive,
T J = T A + V Δ I FQ θ JA ,
where T A is the ambient temperature, V Δ is the voltage dif-
ference between the VIN pin and the BAT pin, I FQ is the
programmed fast charge current, and θ JA is the thermal
resistance from junction to ambient. However, if T J com-
puted this way exceeds T TL , then thermal limiting will
become active and the thermal limiting junction tempera-
ture will be
T JTL = T A + V Δ I(T JTL ) θ JA ,
where
I(T JTL ) = I FQ + i T (T JTL ? T TL ).
(Note that i T is a negative quantity.) Combining these two
equations and solving for T JTL , the steady state junction
temperature during active thermal limiting is
(the VIN pin input voltage) down to the battery voltage
plus the charger dropout voltage. Power dissipation in
the SC824 will be reduced to the charger dropout voltage
multiplied by I AD-LIM .
If I AD-LIM is less than 20% of I FQ , then the adapter voltage can
be pulled down to the battery voltage while the battery
voltage is still below the precharge threshold. In this case,
ensure that the adapter will maintain its current limit
below 20% of I FQ at least until the battery voltage exceeds
the precharge threshold. Failure to do so could permit
charge current to exceed the precharge current while the
battery voltage is below the precharge threshold. This is
because the low input voltage can also compress the pre-
charge threshold internal reference voltage to below the
battery voltage. This will prematurely advance the charger
logic from precharge current regulation to fast-charge
regulation, and the charge current will be permitted, by
the charger, to exceed the safe level recommended for
pre-charge conditioning.
T JTL
T A
V I FQ i T T TL
1 V i T JA
JA
.
The low UVLO falling threshold ( VT UVLO-F ) permits the
adapter voltage to be pulled down to just above the
battery voltage only in adapter mode (MODE pin
The thermal limiting controller is able to reduce output
current to zero. However, this does not happen in prac-
tice. Output current is reduced to I(T JTL ), reducing power
dissipation 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.
See the section Input Over-Voltage Protection for an
example of thermal limiting operation.
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 SC824 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 is
chosen such that the programmed fast-charge current I FQ
exceeds the current limit of the charging adapter I AD-LIM .
grounded). In either USB mode, Under-Voltage Load
Regulation (UVLR) prevents the input being pulled down
by the charging current to below the UVLR limit of
V UVLR = 4.51V typically.
The SC824 should be operated with the 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 pin voltage first exceeds VT UVLO-R
to begin charging and is subsequently pulled down by the
charging current to just above the battery voltage.
Interaction of Thermal Limiting and CLA 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 R DS-ON of the internal pass device multi-
plied by the adapter current.
The charging load will pull the adapter output voltage
20
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