参数资料
| 型号： | SC810ULTRT |
| 厂商： | Semtech |
| 文件页数： | 15/20页 |
| 文件大小： | 0K |
| 描述： | IC BATTERY MANAGEMENT |
| 标准包装： | 1 |
| 系列： | * |
| 其它名称： | SC810ULDKR |
SC810
Applications Information (continued)
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 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 pin first
exceeds VT UVLO-R to begin charging, and is subsequently
pulled down to just above the battery voltage by the
T JTL
T A
V I FQ i T T TL
1 V i T JA
JA
charging load.
Interaction of Thermal Limiting and Current Limited
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.
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.
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 is
chosen such that the programmed fast-charge current I FQ
exceeds the current limit of the charging adapter I AD-LIM .
Note that 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 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 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 happens because the low
input voltage will also compress the pre-charge threshold
internal 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-
In dropout, the power dissipation in the SC810 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 SC810 junction temperature will rise above ambient
by P ILIM x θ JA . 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 SC810 will continue
to charge with thermal limiting until charge current
decreases while in CV regulation (reducing power dissipa-
tion suffi ciently). This results 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 TL . This
implies that θ JA must be kept small enough, through
careful layout, to ensure that T J = T A + (P ILIM × θ JA ) < T TL .
mended for pre-charge conditioning.
15
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